WORLD METEOROLOGICAL ORGANIZATION
INSTRUMENTS AND OBSERVING METHODS
REPORT No. 46
The WMO Automatic Digital Barometer Intercomparison
Final Report
by
Dr. J.P. van der Meulen
(de Bilt, the Netherlands, 1989-1991)
March 1992
WMO/TD – No. 474
The designations employed and the presentation of material in this
document do not imply the expression of any opinion whatsoever on the part
of the Secretariat of the World Meteorological Organization concerning the
legal status of any country, territory, city or area or of its authorities,
or concerning the delimitation of its frontiers or boundaries.
This report has been produced without editorial revision by the WMO
Secretariat, it is not an official WMO publication and its distribution in
this form does not imply endorsement by the Organization of the ideas
expressed.
FOREWORD
The WMO intercomparison of Automatic Digital Barometers was
carried out at the kind invitation of the Royal Netherlands
Meteorological Institute in De Bilt from 1989 to 1991.
An International Organizing Comittee (IOC) was set up to agree
on the rules for the Comparison, for discussion of organizational
matters, confirmation of the methods of evaluation of the measured
data and for presentation of the results in this WMO publication. The
main tasks of this WMO Automatic Digital Barometer intercomparison
have been the determination of the calibration curve, the influence
of different parameters on the accuracy and the long-term stability
of the sophisticated barometers.
The results of the comparison shows impressively the different
performance characteristics of the instruments and will certainly
help the users of barometers especially those to be used as sensors
in an Automatic Weather Station (AWS), in the selection of adequate
sensors for the required purpose. In addition, they will provide
producers of digital barometers with valuable information for
improving their products. I am confident that Members of WMO and
instrument specialists will find this report very useful.
0n behalf of CIMO I express my heartfelt thanks to members of
the
international
organizing
committee
for
their
important
contribution and to the management and staff of the Royal Nether1ands
Meteorological Institute for the very large workload in organizing
and carrying out this comparison. I am also very pleased to be able
to thank the project Leader and author of this report for his work.
(Jaan Kruus)
President of CIMO
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Contents
0. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Accuracy requirements for the measurements of atmospheric
pressure.
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1.3 Objectives of the intercomparison . . . . . . . . . . . .
1.4 Procedures for the intercomparison . . . . . . . . . . . .
1.4.1 Calibrations. . . . . . . . . . . . . . . . . . .
1.4.2 Temperature tests . . . . . . . . . . . . . . . .
1.4.3 Reliability . . . . . . . . . . . . . . . . . . .
1.4.4 Transport and other environmental tests . . . . .
1.4.5 Special features
. . . . . . . . . . . . . . . .
1.5 Test programme . . . . . . . . . . . . . . . . . . . . . .
1.6 General remarks . . . . . . . . . . . . . . . . . . . . .
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3
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2. Description of References and Facilities
2.1 Reference instruments . . . . . . .
2.2 Basic test facilities . . . . . . .
2.3 Data acquisition . . . . . . . . . .
2.4 Temperature test facilities . . . .
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3. Instruments and their Operating Principles
3.1 Operating principles . . . . . . . . .
3.2 Temperature dependency . . . . . . . .
3.3 Hysteresis . . . . . . . . . . . . . .
3.4 Table of participating instruments . .
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4. Summary of Data Availability and Equipments Faults . . . . . . .
4.1 Remarks on instrument performance . . . . . . . . . . . . .
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5. Data Analysis and Overall Results .
5.1 General performance . . . . .
5.2 Objectives and and the methods
barometer: . . . . . . . . .
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used for analysis
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6. Description of Instruments and Results by
6.1 Aanderaa . . . . . . . . . . . . . .
6.2 A.I.R.
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6.3 Casella . . . . . . . . . . . . . .
6.4 Druck . . . . . . . . . . . . . . .
6.5 L.E.E.M. . . . . . . . . . . . . . .
6.6 MeteoLabor . . . . . . . . . . . . .
6.7 Nakaasa . . . . . . . . . . . . . .
6.8 Paroscientific . . . . . . . . . . .
6.9 Ruska . . . . . . . . . . . . . . .
6.10 Setra . . . . . . . . . . . . . . .
6.11 Solartron . . . . . . . . . . . . .
6.12 Vaisala . . . . . . . . . . . . . .
Instruments
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7. Conclusions and Recommendations . . . . . . . . . . . . . . . . .
7.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . .
7.2 Recommendations . . . . . . . . . . . . . . . . . . . . . .
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8. Acknowledgements
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9. References
Appendix A.
The International Organizing Committee.
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Appendix B.
Terminology. . . . . . . . . . . . . . . . . . . . . . .
95
Appendix C.
Fitting mathematics. . . . . . . . . . . . . . . . . . .
97
Appendix D.
Questionnaire. . . . . . . . . . . . . . . . . . . . . .
99
iv
0. Summary
The first WMO Automatic Digital Barometer Intercomparison was held in De
Bilt, the Netherlands at the Royal Netherlands Meteorological Institute
from begin 1989 to July 1991. Thirty-three instruments of 14 different
designs were entered by nine Members of WMO 1).
This report contains a description of the procedures followed, the data
acquisition, the data analysis, and a technical description of the
principles of the instruments. Finally the results of the intercomparison
are given in a general table and the result per instrument in detail.
The report also includes recommendations for operational use. Also possible
use of digital barometers as portable standards is discussed.
1: A table with the participating instruments is presented on page 22.
1
2
1. Introduction
1.1
Background
Atmospheric pressure is one of the most important parameters in
meteorology. Since Torricelli has demonstrated in 1643 the existence of
atmospheric pressure, various experiments are carried out using mercury
barometers and later on also the aneroid barometers, demonstrating its
usefulness for investigations of the atmosphere. The strong correlation
between both the absolute value and the trend of the atmospheric
pressure and the change of the weather, is for more than a century the
motive to develop suitable barometers to be used for meteorology. Today
the mercury barometer, as designed by Torricelli almost 350 years ago,
is still the most popular instrument for operational measurements by
professional observers. Nevertheless, in the past ten years both modern
developments of sensor technology and the need for remote measurements
have given new impulses for developing a new generation of automatic
digital barometers.
Automatic digital barometers make use of a combination of sensor and
microprocessor techniques. In fact these instruments are composed of
sensors, transducers, a micro-processor and digital electronics.
Electrical signals generated by the pressure sensor(s) are interpreted
by the micro-processor using a more or less sophisticated algorithm.
The outcome of most of the sensors, however, is temperature dependent.
So inside most of the devices the temperature of the pressure sensor is
determined by a temperature sensor. Consequently the pressure indicated
by the barometer is a result of calculations in which parameters
related to both the pressure sensor and the temperature sensor are
used. Off course, these parameters are a result of an appropriate
calibration where both pressure and temperature variations are
considered. In other words the corrections due to temperature effects
with mercury barometers or the mechanical compensation with aneroid
barometers is taken over by arithmetics using a microprocessor.
In practice these modern types of sophisticated barometers have a lot
of benefits compared to the mercury and aneroid barometers. As was
demonstrated already (see ref. [1]) the accuracy and stability of these
modern barometers are of the same order or even better than the
traditional barometers. One of the great advantages of automatic
barometers is the feature that these devices can be read out easily
with a digital display. Moreover modern digital barometers are suited
for automatic weather stations, because reliable results can be
obtained.
Another important issue is the ability to transport automatic digital
barometers with much less precautions than mercury barometers. Portable
automatic digital barometers with low drift, shock proof and the same
accuracy as a reference standard mercury barometer, may be as a matter
3
of fact an ideal portable standard barometers.
Because of these new developments and the appearing on the market of a
large number of instruments, an intercomparison of automatic digital
barometers was felt to be necessary. Moreover the results of such
intercomparison should also answer the important question if digital
barometers can replace in the near future mercury barometers as station
barometers and as travelling standards.
An International Organizing Committee (IOC) was established by the
President of CIMO. The list of members of the IOC is given in
Appendix A.
The intercomparison was carried out at the Royal Netherlands
Meteorological Institute (KNMI), De Bilt, Netherlands, from 4 April
1990 until 11 July 1991.
Nine Members entered 33 barometers of 14 different types. Although
serious failures happened during the intercomparison, no barometers had
to be rejected. A list of the participating instruments is presented in
chapter 3 (table 2, page 22)
1.2
Accuracy requirements for the measurements of atmospheric
pressure.1)
The results of an intercomparison will be only relevant if conclusions
are related to the stated accuracy requirements. The accuracy
requirements, for surface measurements of the atmospheric pressure and
tendency, as stated by various technical commissions of the WMO are
given in table 1 of Annex 1A of the Guide to Meteorological Instruments
and Methods of Observation (WMO - No. 8, 1983).
In summary:
1 pressure
2 tendency
climatology
aeronautical
meteorology
synoptic and
marine
meteorology
± 0.3 hPa
-
± 0.5 hPa
-
± 0.1 hPa
± 0.2 hPa
Accuracy requirements for meteorological surface measurements and
suggested related sensor performance characterisations for automatic
weather stations are given in the final report of CIMO-X (Brussels
1989) (WMO - No. 727, table in Annex to Recommendation 1).
Concerning atmospheric pressure:
1: See appendix B for definitions of used terminology.
4
(1)
(2)
variable:
range:
(3)
(4)
(5)
(6)
(7)
required target accuracy:
reporting resolution:
sensor constant:
output averaging time:
achievable observing accuracy:
Atmospheric pressure
200 hPa in range 500 hPa to
1100 hPa.
± 0.1 hPa
0.1 hPa
20 s
1 min.
± 0.3 hPa
The required target accuracy (3) is the most stringent accuracy taken
from WMO Guides and Manuals for application to climatology, synoptic,
aeronautical, agricultural and marine meteorology and hydrology.
The achievable observing accuracy (7) is an estimate of the best
currently obtainable accuracy with automatic sensing systems.
CIMO-X recommends that the table in Annex to Recommendation 1 be
continually reviewed by other technical commissions and updated by CIMO
based on the developments in new technology, the results of instrument
intercomparisons and changing requirements aw stated by relevant
technical commissions.
1.3
Objectives of the intercomparison
On its first session, the IOC agreed on the following objectives of the
intercomparison:
To evaluate the overall performances of automatic digital barometers
(automatic barometers with digital read-out), namely:
a
b
c
d
e
f
g
h
i
repeatability, accuracy and hysteresis
long-term stability
dependence on environmental impacts (temperature)
mechanical effects (shocks and transportability, pressure
differences during shipment by air)
other influences (electrical)
signal output (digital read-out (hPa), input to data acquisition
systems)
unattended operation (maintenance and serviceability)
calibration (and adjustment) procedures
handling of instruments for operational purposes (easy
replacement, recalibration, transport by road, by air).
To determinate the possible use as travelling standard.
1.4
Procedures for the intercomparison
The IOC has given these nine objectives, to bear in mind, for the
intercomparison of automatic digital barometers. Before the
intercomparison started these objectives were clustered in 5
procedures. Each procedure is detailed via a number of goals. Below the
procedures and the relevant measurements are given.
5
1.4.1
Calibrations.
For 12 months each instrument was compared with the KNMI primary
standard, (i.e. one calibration/month).
Goals:
a) determination
study,
b) determination
c) determination
d) determination
and c) ).
of the calibration curve of the barometer under
of the amount of hysteresis,
of the repeatability,
of the long term stability (with respect to a), b)
Measurements:
A calibration consists of four runs of measurements with each 11
equidistant pressure points within 950 hPa … 1050 hPa. Two runs are
carried out with pressure values in a upwards direction and the
other two runs with pressure in a downwards direction. At each
pressure point Pi the positive correction Ci is calculated for each
barometer, defined by:
C(Preference) ≡ Preference - P.
These correction values Ci are the principle data points used for
further analysis of the results.
As a consequence each calibration resulted in 22 data points Ci in
upwards and 22 data points Ci in downwards direction. All barometers
were read out successively within 2½ minutes with constant pressure.
One calibration took two days for all instruments and all runs.
To get informed about the performances of the barometers at
pressures below 950 hPa, one calibration over an extended range from
800 to 1050 hPa was performed at the end of the year.
Examples of results of a calibration with two different types of
instruments are shown in the figs. 1 and 2. The symbols ">" and "<"
indicate the direction of the pressure steps, upwards and downwards
respectively. The number in the label indicates the number of the
day (first day or second day).
Through the 44 datapoints Ci a polynome fit C(P) of the second
degree is drawn, representing the calibration curve. A description
of this fitting method is given in Appendix C. The figs. 1a and 3a
show typical examples of such a fit.
Repeatability.
The deviations of the 44 measured datapoints and the fitting curve
is used to calculate the so-called "estimated error of Y". This
error is a direct measure for determination of the repeatability of
a barometer.
6
Hysteresis.
Information on the hysteresis is derived by fitting the upwards and
downwards pressure points separately. The difference between both
fitting curves can be designated to be representative for the
hysteresis. Figs. 1 and 2 demonstrate the correlation between
succeeding datapoints.
Long term stability.
Information on the long term stability can be derived by considering
the trend of the calibration curve with time. Other relevant
information concerning the long term stability can be derived from
the trend of the repeatability and of the hysteresis of any
barometer.
The drift of a barometer may be different for each pressure value.
Previous experiments [1] demonstrate that such differences usually
are small compared to the drift itself. For that reason the
correction values for three pressure points (Pref = 950, 1000 and
1050 hPa) are plotted as a function of time. In these plots also the
repeatability at Pref = 1000 hPa is presented.
In summary, to get informed about the long term stability of an
instrument the trends are considered of:
1) the calibration (or correction) curve
2) the repeatability
3) the hysteresis
1.4.2
Temperature tests
Compare each instrument, at different temperatures, with the
reference. Make at each temperature a calibration curve.
Goals:
a) To investigate the appropriate working of devices (sensors and
electronics) at a temperature range form O°C to 40°C and at an
environmental relative humidity of 95%.
b) To control the temperature compensation or to measure any
temperature dependency of the barometers.
c) To control any effect on the calibration curve as result of this
test.
Measurements:
Investigations were carried out within a climate chamber (see par.
2.4). The temperature value at which the barometers were compared to
the reference were chosen at 0°, 10°, 20°, 30° and 40°C. At each
temperature data-acquisition was carried out identically as
described above, under calibration measurements, except that for
each temperature in stead of four runs, only one upwards and one
downwards run was carried out and with pressure steps of 20 hPa in
stead of 10 hPa, providing 12 datapoints for each temperature.
7
Fig. 1 and 2. Typical examples of calibration results for two different
barometers. In the figs. results of the determined data Ci = Pi,reference Pi in hPa are shown. Any correlation between the succeeding datapoints
inform about hysteresis (’>’ and ’<’ denote upwards and downwards
respectively; the days are indicated with ’1’ and ’2’). In the figs. 1a
and 2a a second order calibration fitting curve is drawn through these
points.
8
fig. 1
fig. 1a
fig. 2
fig. 2a
The temperature test was very time consuming. In one batch only a
limited number of sensors could be tested and temperature
stabilisation took long periods. The whole test lasted 1 month.
1.4.3
Reliability
During the intercomparison year all instruments were observed for
break down and discontinuities.
Goals:
a) To investigate the overall performance of the instrument for a
relative long term (unattended operation by continuous operation
of sensor and data communication)
b) To detect discontinuities or main failures.
Data acquisition:
During the whole intercomparison the barometers where on power,
measuring and presenting the atmospheric pressure 24 hours a day.
Also some tests with continuous pressure variations (950 to 1050 hPa
continuous rising and falling) were carried out to detect any
discontinuities in the output. All data (amount of data each day:
150 kbytes) were temporarily archived on disk. This procedure
implies that as well as the barometers as the data output were
controlled on appropriate working permanently and any
discontinuities or failures were registered.
1.4.4
Transport and other environmental tests
Testing of the effects caused by external influences like shocks due
to transport. Also the influence of low pressure (600 hPa), as in
air-transport, was investigated. Finally power supply influences
were tested.
Goals:
a) To investigate if the barometers can be transported easily and
without changing any calibration by cause of chock or low
pressures.
b) To investigate the effect of noisy or interrupted power supply.
Actions carried out:
a) During the last phase of the intercomparison the barometers were
removed from the calibration facility, placed in a KNMI service
car and transported over a distance of 400 km. Afterwards the
instruments were re-calibrated and the results of both
calibrations were taken as input for further analysis. This
"transport test" was carried out under normal circumstances,
like staff personnel usually will undertake transporting such
9
devices. Off course such a test does not inform about under what
circumstances the devices will get into trouble nor that this
kind of testing is analogue to any procedure prescribed by an
ISO-standard test procedure. It was found important to learn
about transportability under "normal circumstances".
b) Intended was to investigate the influences of transporting the
barometers by aeroplane. Problems were expected due to low
pressure circumstances. Since it was found not to be practicable
to arrange such a transport by air, it was decided to carry out
an alternative test.
In principle it is of interest to detect any effect due to a
situation with low pressure to be expected. For this purpose a
test was chosen where each barometer as whole instrument, and
not cut off by any cap, was placed in a chamber with low
pressure during a sufficient long period. A Negretti Zambra
pressure chamber was used in which the pressure was kept at
about 600 hPa for two hours. After this action the barometers
where recalibrated.
c) A relatively small amount of barometers require mains current
power supply (e.g. 220 VAC/50 Hz). All other barometers need DC
power supply. Most of these instruments may be offered any
voltage within a large range (eg. 5 to 40 V DC). Only Setra
model 470 requires 5 V DC ± 1%. To test any influences due to
instable power supply two actions were carried out:
1) For the 220 VAC instruments:
To simulate disturbed mains supply a Schaffner HSG222A
interference simulator was used with the following
conditions:
- Duration and rise time: 100 ns and 5 ns respectively
- max. amplitude: 1500 V.
2) For the DC instruments:
Continuous variation within the range as stated by the
manufacturer (with AC components well suppressed).
1.4.5
Special features
Documentation was studied and procedures were tested to find out
extra features and possibilities of the tested instruments.
Goals:
a) to learn about the digital output type and format
b) calibration and adjustment routines: easiness, possibilities
c) to recognize certain special features of any device.
Actions carried out:
a) Investigation of digital output performance. Also the different
types of output were tested.
10
b) Experiences for calibrating the devices were gained during the
monthly calibration routines. Besides of this the devices were
investigated how to implement any correction curves into the
barometer (how to adjust them) Automatically or manually.
c) Based on performance and documentation each instrument was
investigated for specific unique features (if relevant).
1.5
Test programme
The intercomparison started at April 4, 1990 and ended at July 10,
1991. During this period the following tests were performed:
-
test
period
monthly calibration:
continuous data logging:
extended range calibration:
temperature tests:
transport test:
low pressure test:
April 4,
April 4,
June 18,
May
2,
June 6,
July 10,
1990
1990
1991
1991
1991
1991
-
July
July
June
June
June
July
10,
10,
21,
6,
18,
11,
1991
1991
1991
1991
1991
1991
In table 1, the time table is presented for all actions carried out
during the intercomparison.
Table 1.
Time table of actions carried out.
reception, installation
long term stability test
temperature dependency test
transport test
low pressure and power supply test
return
- 1989 -
1.6
- 1990 -
- 1991 -
General remarks
During the intercomparison a number of general rules were applied:
a) The instruments were treated gently and on a way, as such devices
normally would be handled. No special actions, like shocks or
maltreatment were taken.
b) Activities and tests (e.g. temperature tests) for which one could
expect that any failure would happen or would generate an excess
11
offset on the calibration curve were carried out at the end of the
intercomparison. Consequently the long term stability investigation
was not influenced by these activities.
c) Calibrating the devices do not imply that these barometers were
adjusted. The instruments were forwarded back to the Members
identically as they were received by KNMI.
Principles of measurement:
a) Any reading of a barometer is taken only if the pressure, as offered
to such barometer, is stable. Using the set-up as will be described
in paragraph 2, this pressure was stable to approx. 0.005 hPa. Most
pressure fluctuations are due to temperature variations in the
system, which are caused after a change of the pressure setting.
Therefore after each new pressure setting a minimum period of 5
minutes waiting time was taken before all devices were read out.
In all cases was waited until each barometer showed a constant readout.
b) Pressure readings at the various pressure levels were performed
within runs with rising pressure as well as with falling pressure.
c) To ensure that no temperature gradient would exist inside any
barometer during the temperature tests (due to a change of the
temperature setting of the climate chamber), a minimum period of one
hour waiting time was taken before the devices were read out.
d) Care was taken to place the devices at the same level of altitude.
(Note that displacement in altitude will give deviations of
0.1 hPa/m).
e) Most of the barometers can display their pressure reading in a
number of different units (psi, kgf/cm2, mm Hg or inch Hg, etc.).
All devices, except for AN1 1) and AN2, were configured to the
hPa-unit. The readings from AN1 and AN2 were converted to hPa using
parameters delivered by the manufacturer.
f) All barometers were read with the largest amount of decimal places
which could be offered by the device.
1: Acronyms are used throughout the text for easy identification of the
barometers. See table 2 on page 22 for a list of these acronyms.
12
2. Description of References and Facilities
In this chapter information is given on the reference barometers used
during the intercomparison. The test facilities are described as well as
the data collection and data logging systems.
2.1
Reference instruments
As reference barometer the Fuess Müller reference barometer type 2k was
used (acronym: FM2k). This working standard is the principle working
standard of the calibration facility of the KNMI. The purity of the
mercury and the quality of the vacuum as well as the interaction
between glass and mercury is controlled regularly. At certain intervals
(approx. 3-5 years) this reference standard is cleaned and filled with
purified mercury with well known characteristics and density. For this
reason it is assumed that this reference barometer was stable enough
during the intercomparison to guarantee an absolute accuracy to be
within ± 0.05 hPa.
Based upon repeatedly readings at the same pressure and temperature we
state that the standard deviation of the Fuess Müller 2k used in this
intercomparison was 0.04 hPa.
Besides the Fuess Müller reference standard another working standard,
namely a Paroscientific Digiquartz model 760 Portable standard, was
used (acronym: PA2).
This instrument is calibrated and adjusted to the reference standard
every four weeks and enables us to determine pressure values with a
accuracy of 0.01 hPa.
The combination of both working standards makes it possible to
determine the standard error of a reading from any barometer with an
precision of 0.01 hPa.
2.2
Basic test facilities
All instruments were installed at laboratory tables in an air
conditioned room (21° ± 2°C). Power supply and data collection-systems
were situated in the same room
All barometers were connected together with flexible tubes to a 1.5 m3
tank, providing a very stable pressure. The pressure could be regulated
to any pressure in the range 800 - 1050 hPa. The air offered to the
barometers was clean dry air (approx. 30% RH).
Special care was taken to place each sensor on the same altitude to
prevent any offset due to difference in height.
A scheme of the total test set-up is given in fig. 3.
13
Figure 3.
Pressure set-up at the basic calibration facility
submitted barometers
AN1,2
AI1,2,3 CA1,2
reference system
DR1,2 LE1,2 ME1,2,3
NA1,2 PA1,3,4
RU1,2 S31,2 S41,2 SO1,2
VA0,1,2,3,4
IN
1.5 m² TANK
constant temperature
[see table 2, page 22, for a complete list of participating instruments]
PA2
CONTROL
FM2k
OUT
In photographs 1 and 2 (page 16) an impression is given of the test
installation and the barometers under test.
2.3
Data acquisition
All the barometers were connected to a data acquisition system, able to
read out the instruments and store the data. As microprocessor to
handle this acquisition a IBM PC AT was chosen. With the help of a
number of controllable data switches, data buffers and other self made
interfaces and amplifiers it was possible to read out and to store the
data within 2.5 minute. Since stabilization of the pressure of a new
chosen pressure value takes about 10 minutes this amount of time is
short enough. The flow diagram for the data logging is shown in fig. 4.
In
1)
2)
3)
practice three types of data communication took place:
IEEE 488 (or HP-IB)
RS 232C (with or without handshaking)
counting after interrogation
The calibration measurements were performed by staff personal of the
KNMI calibration laboratory, especially by mr. J. Middelkoop,
experienced in the calibrating barometers (see photograph 3).
2.4
Temperature test facilities
To get a clear impression of the temperature influences on the
barometers, these instruments where placed in a climate chamber (Weiss,
Reiskirchen, Germany; model 500 ABT/30JU; stability ± 0.2°C). The
barometers were measured at five temperatures (from 0° to 40°C).
Because the climate chamber has a limited amount of space, not all the
barometers could be measured in only one batch. It turned out necessary
to organize five sessions to be able to measure all instruments.
In order not to reorganize the data acquisition set-up it was decided
not to install the automatic data-acquisition system. In stead of this
the barometers were read out manually from its display, or (by lack of
any display) by using a terminal.
Since the distance of a tube connection from the climate chamber to the
Fuess Müller reference standard would be too long (approx. 15m) it was
decided to use as a reference barometer the KNMI portable working
standard Paroscientific, model 760, only.
The test pressures to the instruments inside the temperature chamber
were provided by a Pressure Controller/Calibrator (DRUCK DPI 510)
together with a 40 litre buffer for stabilization and reducing pressure
fluctuations to within 0.005 hPa. The pressure controller, portable
standard and buffer were placed outside the climate chamber.
15
______________________________________________________________________________
Photographs 1 and 2. Overview of set-up (basic calibration site)
______________________________________________________________________________
16
______________________________________________________________________________
Photograph 3. Reference system. The two larger mercury barometers on the
foreground are Fuess Müller reference barometers, type 2k. Left on the
foreground the working standard (PA2) with computer facility for pressure
control.
______________________________________________________________________________
______________________________________________________________________________
17
Figure 4.
Dataflow diagram (automatic data acquisition at basic
calibration site)
Barometers:
Code activated
asynchronous switches
Barometers:
ME1 ME2 ME3 DR1
D C B A
PA1
PA2
PA3
1
Databox
CAA-4
2
Databox
CAA-4
PA4 [1]
AI1 AI2 AI3
NA1 NA2 NA3 DR2
H G F E
3
S31 S32
4
5
20
[2] [2]
L K J I
3
Databox
CAA-4
SO1 SO2 [4]
CAMPBELL
CR10
VA1 VA2 VA3 VA4
Buffering
RS232-422 to
IEEE488
Converter
RU1 RU2 VA0
P O N M
4
Databox
CAA-4
CA1 CA2 [3] AN1 AN2 [4]
4 3 2 1
[POWER SUPPLY CONTROL]
[TEMPERATURE]
[RELATIVE HUMIDITY]
AANDERAA
3010
8
IO tech
Serial488/4
IBM PC/AT
IEEE488
LE1 [2]
S41 S42
T S R Q
5
Databox
CAA-4
LE2 [2]
A/D DASH-16
RS232 (COM1:)
NOTES:
[1]
[2]
[3]
[4]
18
The Paroscientific barometers can be interconnected within a ’ring’ network structure, which is
used for datacommunication purposes.
To start output of a puls array from LE1 or LE2, a trigger is necessary. This was performed by
sending short ’breaks’ to these devices. The pulses were counted using a Metrabyte Dash-16 board
inside the PC. Both displays of LE1 and LE2 were manually read out too.
From the barometers CA1 and CA2 the RS232 port was used for tests only (collecting data buffered
inside CA1 or CA2). For calibrations the display of both devices were read out.
The Aanderaa and the Solartron sensors need interfaces for measurement and serial output.
The effect of humidity on the electronics was tested by selecting a
relative humidity of the climate chamber of 95% RH at all temperatures.
The relative humidity of the air as offered to the inlet of each
barometer was less than 30% RH at 21°C.
The set-up is shown in fig. 5.
Figure 5.
Pressure set-up at the temperature test facility
(temperature dependency test).
t = 0° 40°C
U = 95%RH
barometers under test:
reference system:
subset of {AN1…VA4}
PA2 (working reference)
pump
computer
computer controlled
Weiss Climate Chamber
model 500 ABT/30JU
Druck DPI 510
Pressure controller/
calibrator
set of cylinders
to prevent pressure
fluctuations
19
3. Instruments and their Operating Principles
A list of all the participating instruments, their type, form of output and
the identifying acronym to be used throughout this report is presented in
table 2 (page 22).
More details of each of the instruments are given in chapter 6, where the
results for each type of barometers are presented.
3.1
Operating principles
In principle there are two main categories of sensor-types used inside
the automatic digital barometers:
I
Sensors measuring any displacement or deformation due to the force
of air pressure to a vacuum-cell.
Barometers of this category use very different sensor technologies.
For instance the Casella barometers W2112, CA1 and CA2, use
integrated circuit sensors while the Vaisala PA11 use three
independent aneroid capsules with capacitive elements inside.
The sensors used inside digital barometers made by Paroscientific do
not use any capacitance technology like most others do, but measure
the tension on a very thin piezo-electric quartz crystal by
measuring its resonance frequency.
II Sensors measuring the density and temperature of the air from which
pressure is calculated.
The barometers, which measure in fact the density of the offered
air, mostly use resonators. The resonance frequency (and higher
order frequencies) of such resonators depends on the density of the
offered air. Assuming clean dry air it is possible to calculate
pressure from this density and the temperature. Special care must be
taken to be sure that only dry air is offered (and not for instance
nitrogen). For that reason these sensors are connected via an small
bottle with humidity absorbing silicon gel.
Note that especially sensors from both categories measuring (resonance)
frequencies are "real" digital barometers since the electronics
measures the output value digitally.
3.2
Temperature dependency
All sensors are sensitive to temperature influences. To compensate for
this influence different precautions are taken. Principally the next
three groups can be distinguished.
20
•
mechanical compensation: Differential measurements or by using
special materials, temperature influences are minimised.
•
measuring the temperature and feeding this information to the
internal microprocessor where the calibrated temperature influence
is compensated.
•
thermostating the whole sensor, if the whole sensor is kept at a
stable temperature, independent of the room temperature, then the
temperature effect is cancelled.
3.3
Hysteresis
Hysteresis is a common problem for mercury and aneroid barometers. With
mercury barometers the interaction between glass and mercury causes the
meniscus to depend on the trend of the pressure variation. Due to
friction on movable mechanical parts, aneroid barometers may show
significant hysteresis. Since most of the modern digital automatic
barometers use new techniques, hysteresis might be of no importance for
these type of devices but still a subject of study.
3.4
Table of participating instruments
21
Table 2.
List of participating instruments.
abbreviation:
manufacturer and type
serial
AN1
AN2
Aanderaa Air Pres. Sensor 2810 [1]
AI1
AI2
AI3
AIR Intellisensor
CA1
CA2
Casella µP Digital Barometer W 2116
DR1
DR2
Druck
LE1
LE2
LEEM Barometre a ruban vibrant
ME1
ME2
ME3
MeteoLabor barometer GB 1
NA1
NA2
NA3
Nakaasa Digital Barometer F-451-Z4
PA1
Paroscientific Digiquartz 1015 A
participating
country
maunufactured in:
form of
output
[10]
display
[12]
resolution
(in hPa)
stated
range
(in hPa)
power
supply
Norway
Norway
Norway
Norway
RS232 [1]
[1]
∼ 0.2
920…1080
-6 and -9 VDC
Netherlands
Netherlands
Netherlands
USA
USA
USA
RS232,
TTL serial,
or Parallel.
No
0.01
800…1060
600…1060
800…1060
+11…+16 VDC and
-11…-16 VDC
20556
20942
United Kingdom
United Kingdom
UK
UK
RS232 [4]
Yes
0.1
900…1100
110(120)/
220(240) VAC
14027
14065
United Kingdom
United Kingdom
UK
UK
RS232
Yes
RS232, IEEE488 [5]
0.001
800…1150
110/220 VAC
France
France
France
France
Binairy [6]
Yes
0.1
900…1050
220 VAC
60
72
85
Netherlands
Switzerland
Switzerland
Switzerland
Switzerland
Switzerland
Binairy [7]
Yes
0.1
630…1060
12 VDC ±20%
0324
8840
8841
Netherlands
Japan
Japan
Japan
Japan
Japan
IEEE488 [8]
Yes [9]
0.1
800…1050
470…1050
470…1050
85…264 VAC
12 VDC ±10% [13]
12 VDC ±10% [13]
31991
Netherlands
USA
RS232
No
0.001
0…1050
5…25 VDC
195-34364
219-34267
220-34260
Netherlands
Norway
Norway
USA
USA
USA
RS232
Yes
0.001
800…1100
0…1250
0…1250
220 VAC [14]
39952
39953
United Kingdom
United Kingdom
USA
USA
RS232
Yes
0.1
0…1310
220 VAC
529
530
DB-1A
DB-2A
DB-1A
0200
0641
0762
DPI 140
PA2 [2] Paroscientific Digiquartz 760-15 A
PA3
760-15 A
PA4
760-15 A
225
226
RU1
RU2
Ruska portable pressure gauge 6200-861
S31
S32
Setra dig. pressure gauge model 370
model 370
188847
188848
United Kingdom
United Kingdom
USA
USA
RS232
RS232
Yes
Yes
0.001
0.001
800…1100
800…1100
110/220 VAC
110/220 VAC
S41
S42
Setra dig. pr. transducer model 470
model 470
210667
210668
USA
USA
USA
USA
RS232
RS232
No
No
0.01
0.01
600…1100
600…1100
5 VDC ±1%
5 VDC ±1%
Table 2. List of participating instruments.
abbreviation:
instrument and
type
serial
(continued)
participating
country
maunufactured in:
form of
output
[10]
display ?
[12]
resolution
stated
range
(in hPa)
power
supply
(in hPa)
SO1 [3] Solartron air pr. transduc. 3088
SO2 [3]
100693
100697
United Kingdom
United Kingdom
UK
UK
[3]
[3]
0.01
35…1300
+15, -15 VDC
VA0
VA1
VA2
VA3
VA4
88148
136593
136649
136597
136597
Netherlands
Finland
Finland
Denmark
Denmark
Finland
Finland
Finland
Finland
Finland
RS232 [11]
Yes
0.1
500…1060
10…28 VDC
Vaisala digital barometer PA 11
NOTES:
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
Together with Aanderaa sensor scanning unit, type 3010 with display and with RS232 and binairy output (220 VAC
power supply); data to be converted into hPa-units using device specific formula.
ONLY used as working reference; not submitted to be intercompared with.
Together with interfacing unit (Campbell Scientific CR10 measurement module) with display and RS232 output
(provided by the Participant). Both sensors are combined with one interface to one measuring system.
Mono-directional RS232: As print out only (for control, archive purposes), manually triggered.
Option IEEE488 is used in the intercomparison for datacommunication.
Puls-array upon request (trigger); opto-coupling interface required for datacommunication. Conversion of puls
count into hPa units using device specific formula necessary.
Three types of codes (bit array, BCD and ASCII); for RS232 (in ASCII mode) an amplifier is necessary
(0…0.5 V → 0…5V).
IEEE Address 3 only. (The Participant provided new EPROMs with different IEEE addresses for this
intercomparison).
Second display with ’sea level pressure’
For all devices (unless otherwise indicated): Baudrate selectable (RS232) or address selectable (IEEE488).
Some devices have a simple RS232 output form (e.g. no handshake and TX only).
Limited baudrate: only 300 baud.
For most devices a separate display is available as an option.
Provided with separate adapter to convert 220 VAC to 12 VDC.
To charge batteries only.
4. Summary of Data Availability and Equipments Faults
After reception the barometers were investigated on appropriate working
(i.e. data-output, display).
From the moment, the data acquisition system was ready to archive that data
(01-April-1990) to the end of the intercomparison (11-July-1991) all
barometers were observed regularly and failures were detected within a day.
The data communication as well as the display were checked.
If any malfunction happened it was registered. If possible such malfunction
was investigated to understand the cause of it. The logging equipment did
handle all data as expected during the days when the barometers were
calibrated. One automatic data switch (databox) malfunctioned some time.
After resetting this switch data collection was continued. In practice
failures on hard and software did not influence the results of the
intercomparison at all.
4.1
Remarks on instrument performance
CA1 and CA2:
The Casella instruments were not able to produce data automatically.
Data requests (buffer dumps) had to be performed by hand. As a
result data had to be read out manually from the display and fed
manually into the data logging system.
LE1 and LE2:
The L.E.E.M. instruments had problems with the provided calibration
factors. The display did give correct figures, but the results from
the digital output using the calibration factors were completely out
of range.
For the calibrations the display was used as output and the results
were fed manually into the data logging system.
The LE2 sensor showed from the beginning of the intercomparison a
very large deviation from the current pressure (as well display as
from the digital output), i.e. approx. 54 hPa. Reason for this
deviation is unknown.
ME1:
Serious breakdown during the temperature test at 40°C. It took a day
before the instrument did operate properly again. The test was
repeated at 40°C a couple of days later, but no breakdown happened.
RU1:
Very unstable readings. At constant pressure the read-out varied by
approx. 0.5 hPa. This instability got worse during the
intercomparison.
24
S41 and S42:
From S41 and S42 time-to-time improper data communication. Sensitive
to power drops. After a power off/on (hard reset) both instruments
got new baud rate settings which were chosen randomly by the
instrument.
Data acquisition interface (Campbell CR10) used for S01 and S02:
At low baud rates (e.g. 300 baud) the start of a new output string
is merged into the old string. Therefore a baudrate of 9600 bits/sec
was chosen. During the temperature dependency test the internal 12
VDC power supply broke down. With an external power supply
continuation of the test was possible.
VA0 and VA3:
VA0 suffered from regular breakdowns, caused by the malfunction of
the serial data communication electronics. (This problem did not
occur with the VA1 to VA4, due to improvements by the manufacturer.)
To reset the instrument it was necessary to switch the power off and
on (hard reset).
VA3 had a breakdown during the temperature dependency test at 40°C.
After reducing the temperature to 20°C the device went to normal
operation within approx. 5 min.
All other instruments:
No significant failures occurred.
25
5. Data Analysis and Overall Results
In paragraph 1.4 the procedures are described to reach the stated goals of
this intercomparison. In this paragraph the results are presented based on
the analysis of the data as acquired by carrying out these procedures. In
order to express any goal related with e.g. repeatability1), hysteresis or
temperature dependency these issues will be defined in terms of the
measured data. In paragraph 5.2 the goals or objectives are presented with
the methods of analysis used to be able to present results in terms of
representative values. A summary of the general results is given in table 3
(page 28).
In Chapter 6 information, data and graphic results are presented concerning
the more specific items related to each type of barometer. In some cases in
stead of values a horizontal bar is given (e.g. for hysteresis, column f).
In these cases the effect or value was "not significant" or "not observable" with respect to the reference or the repeatability of the barometer
under study. If necessary notes are added to data results presented in
table 3. Some of these notes are explained in more detail in Chapter 6.
5.1
General performance
A good impression of the accuracy is given by a graph presenting the
correction values Ci(P) and its fitting curve as it is determined for
each of the pressure points within the stated pressure range.
Such graphs are shows in Chapter 6 for each submitted barometer as fig.
a and b. Fig. a shows the data and curve as determined at the beginning
of the intercomparison (04-april-1990), whereas fig. b shows the
results for the extended calibration range measured on 19 and 20-june1991.
5.2 Objectives and and the methods used for analysis of each
barometer:
In table 3 six collumns are presented related to the objectives of the
intercomparison. The presented values are based on methods of
calculation described below.
Offset correction: From the calibration curve C(P) obtained by fitting
a second order polynome through the data points Ci(P) =
Preference,i - Pi, i = 1…44, the value of C at 1000 hPa, C(1000 hPa), is
calculated. This value should be added to the reading of a barometer
to obtain a correct pressure (when P = 1000 hPa).
In table 3, column (a) this correction is presented as determined at
the beginning of the intercomparison (04-april-1990). These data
give an overall impression of the mutual differences in readings of
the various barometers when the intercomparison was started.
1: See appendix B for definitions of used terminology.
26
Long term stability: For each calibration, carried out every month a
calibration curve of the second order is calculated. The values of
these curves taken at the pressure of 950, 1000 and 1050 hPa (i.e.
C(950 hPa), C(1000 hPa) and C(1050 hPa) respectively) are calculated
and put in a graph as a function of time to get informed about any
drift at the various pressures (see Chapter 6, figs. (c) ).
In table 3, column (b) the overall long term stability is
represented by the difference between the maximum and minimum values
of C(1000 hPa) as determined during the intercomparison (04-april1990 to 10-july-1991).
Temperature dependency: For each calibration, carried out within the
range from 0° to 40°C a calibration curve of the second order is
calculated. The values of these curves taken at the pressure of 950,
1000 and 1050 hPa are calculated and put in a graph as a function of
temperature to get informed about the temperature dependency at the
various pressures. The overall temperature dependency of each
barometer is reported in table 3, column (c) by the maximum
difference between the maximum and minimum correction at P = 1000
hPa (i.e. Cmax(1000 hPa) - Cmin(1000 hPa) ) within the temperature
range from 0° to 40°C throughout the intercomparison.
Impressions of the temperature dependencies are presented in
Chapter 6, figs. (d).
Pressure sensitive error: The pressure sensitive error is defined as
the pressure related part of C(P), which remains after adjusting a
barometer to the offset only.
The reported pressure sensitive error is calculated as the
difference between the maximum and the minimum value of C(P) within
the range from 950 to 1050 hPa, i.e. Cmax - Cmin.
In table 3, column (d) the overall pressure sensitive error is
reported by the highest value of the pressure sensitive error as
determined throughout the intercomparison.
Repeatability: Repeatability is numerically determined to as two times
the measured experimental standard deviation s of the data points to
the calibration curve, i.e. ±2s. This quantity is, in principle,
equal to the overall uncertainty1) of measurement at the moment of
calibration.
In table 3, column (e) the overall repeatability of each barometer
is reported by the highest value of the repeatability, as determined
throughout the intercomparison. In Chapter 6, graph (c), the
repeatability is indicated by vertical bars.
Hysteresis: The hysteresis of a barometer is expressed as the average
difference between the values of Ci(P) for the upwards runs and of
Ci(P) for the downwards runs of a calibration.
In table 3, column (f) the overall hysteresis is reported by the
highest value of hysteresis as determined throughout the
intercomparison.
1: Guidelines for the expression of the uncertainty of measurement in
calibrations are given in WECC Doc. 19 - 1990 (WECC = Western European
Calibration Cooperation).
27
Table 3. Overall results.
Instrument
AN1
AN2
AI1
AI2
AI3
correction at
beginning of
intercomparison
[1, 2]
(hPa)
Long term
stability
temperature
dependency
[3] (hPa)
[4]
(a)
(b)
(c)
+ 0.7
+ 0.7
0.00 [9]
0.00 [9]
0.00 [9]
(hPa)
pressure
sensitive
error
(hPa)
repeatability
(d)
(e)
(hPa)
0.6 [6]
0.4 [8]
0.3
0.5
0.4
0.8
± 0.3
± 0.3
0.3
0.1
1.4 [10]
0.1
0.3
3
0.05
0.35
0.1
± 0.07
± 0.09
± 0.14
3.6 [10]
0.5
0.8
1.1
0.1
0.4
± 0.4
± 0.13
< 0.1 [11]
< 0.1 [11]
0.3
0.4
0.08
0.09
± 0.07
± 0.05
0.3
2.3 [12]
0.3
0.4
0.2
2.7
± 0.16
± 0.18
CA1
CA2
- 3.6
- 0.5
DR1
DR2
- 0.07
- 0.02
LE1
LE2
+ 1.2
+53.6
ME1
ME2
ME3
0.0
- 0.9
+ 0.3
[9]
0.5 [13]
0.4
0.7 [14]
0.6
0.9
0.6
0.06
0.07
0.10
± 0.10
± 0.16
± 0.09
NA1
NA2
NA3
0.0
- 0.2
- 0.2
[9]
0.2
< 0.1 [11]
< 0.1 [11]
0.4
0.0
0.1
0.11
0.09
0.08
± 0.11
± 0.08
± 0.08
PA1
1.5 [15]
0.15
0.04
± 0.05
PA3
PA4
- 0.03
- 0.01
< 0.1 [11]
< 0.1 [11]
0.04
0.07
0.11
0.05
± 0.05
± 0.05
RU1 [5]
RU2
- 0.07
0.00
0.3
< 0.1 [11]
0.4
0.4
0.4
0.08
± 0.7
± 0.05
S31
S32
- 0.02
- 0.02
0.2
0.3
0.23
0.18
0.10
0.10
± 0.05
± 0.05
S41
S42
- 0.02
- 0.06
0.2
0.15
0.19
0.13
0.10
0.15
± 0.09 [16]
± 0.18 [16]
SO1
SO2
- 0.03
0.00
0.2
0.45
0.11
0.12
± 0.07
± 0.07
VA0
VA1
VA2
VA3
VA4
0.0
0.0
0.0
0.0
+ 0.1
0.4
0.35
0.8
0.8
0.5
0.19
0.13
0.11
0.12
0.18
±
±
±
±
±
Notes:
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
28
0.00 [9]
0.1
< 0.1 [11]
[9]
0.1
0.1
0.2
0.1
0.1
0.11
0.09
0.09
0.10
0.10
for P = 1000 hPa and on 04-APR-1990.
With respect to sensitivity of each barometer (i.e. number of decimal digits)
Period: 04-APR-1990 to 10-JUL-1991.
Temperature range: 0° to 40°C
" " indicates: not observed (with respect to accuracy of reference system).
Until transport test: 0.3 hPa.
Not to be determined within accuracy limits of barometer.
Until transport test: 0.2 hPa.
Adjusted at the same location just before calibration.
Continuous strong drifting.
No significant deviation observed within limits of accuracy of reference system.
Until low pressure test: 0.3 hPa.
Until low pressure test: 0.2 hPa.
Until low pressure test: 0.3 hPa.
Significantly drifting as from september 1990.
After 17-APR-1990 both S41 and S42 typically ± 0.05 hPa
Hysteresis
[5]
(hPa)
(f)
[7]
[7]
0.25
0.15
∼ 0.03
∼ 0.02
[7]
6. Description of Instruments and Results by Instruments Types
In this Chapter the general technical data and the intercomparison results
are presented. The results, specific for each submitted device are grouped
together per type of instrument. For each type of instrument the relevant
technical data are presented as it is copied from the questionnaires 1)
returned by the participating Members. In cases when the questionnaire was
not filled in completely, the concerning documentation supplied with the
instrument was used as a source of information.
Each type of instrument will be regarded in a sub-paragraph. Each subparagraph is divided in two sections. Section one deals with the general
technical data and section two with the intercomparison results for each
specific instrument. In section two four graphs are shown:
a
The initial calibration plot, measured on the first day of automatic
data acquisition (4 april 1990).
b
The extended calibration plot for the range 800 to 1050 hPa, measured
from 18 to 21 june 1991.
c
The long term stability plot, presenting the pressure correction values
C(P) for P = 950, 1000 and 1050 hPa respectively. Note that differences
between those curves inform about the pressure dependency error of a
device. The determined repeatability is visualized by vertical bars
through the data points C(1000 hPa), providing an idea about a possible
trend (decreasing or increasing) of this repeatability. Note that these
bars do not reflect the accuracy of the data points themselves, but
just only the repeatability.
In general the scale used to present C(P) is chosen to be within -1 and
+1 hPa. However in some cases it was necessary to modify this scale
because of for instance too large offsets. In those particular cases,
three asterisks (***) are added to the header of the graph.
Note that the data points are linked by straight lines, which was done
for convenience only and not as a result of interpolation.
The two last data points (18JUN91 and 10JUL91) are results of
calibrations just after the transport test and the low pressure test
respectively.
d
The temperature dependency plot, presenting the pressure correction
values C(P) for P = 950, 1000 and 1050 hPa as determined during the
temperature dependency test.
In the second section of each sub-paragraph results of the other tests are
reported too. In case any instrument specific effect was observed it is
reported as comment.
Note that general overviews of the technical data as well as the results
are already presented in the previous paragraphs in table 2 (page 22) and
table 3 respectively.
1: See appendix D for a copy of this questionnaire.
29
6.1
Aanderaa
Type of instrument:
Air pressure sensor 2810
Number of instruments:
2 (AN1, AN2)
Participating member(s):
Norway
Manufacturer:
Aanderaa Instruments, Bergen, Norway
General technical data
1
Principle of operation:
The sensor utilizes a small silicon chip of 4 x 4 mm as sensing
element. In the central area of this chip is a thin membrane that is
exposed to atmospheric pressure on one side and to a vacuum on the
other. The membrane is furnished with four diffused resistors that
form a Wheatstone bridge. The output signal is proportional to the
atmospheric pressure. Four heating resistors and a temperature
sensing resistor are also diffused onto the chip. In conjunction
with an external control circuit, these resistors allow the chip to
be held at a constant temperature of 47 C during measurement.
2
Power requirements:
For sensor scanning unit 3010: -6.5 … -12
VDC or 220 VAC; (sensor: -6 VDC)
3
Operating pressure range:
920 … 1080 hPa
4
Operating temperature range:
-40
… +47 C
5
Storage temperature range:
-50
… +70 C
6
Dimensions:
7
Output:
99 mm, Ø 50 mm
a) type of reading:
Using Aanderaa sensor scanning unit 3010:
display and digital output
b) type of output:
From scanning unit: Binary words "PDC 4"
or RS232C (ASCII), 300 baud only
c) example of output string:
1023
1023
1023
1023
1023
0443
0289
0285<CR><LF>
{for N=1…8}
- pressure information:
"0289" and "0285" for AN1 and AN2
respectively. These values have to be converted to hPa units
using a calibration table provided with each instrument. To be
able to calculate the measured pressure in hPa units, a
calculation formula is provided with each instrument:
P[hPa] = A + B×N, where N stands for the reading of the sensor
and where the following values for the parameters were used as
recommended in the calibration table: AN1: A = 900.36, B =
0.19079; AN2: A = 900.72, B = 0.18987. In this example we have
therefore for AN1: "0289" ˆ
= 955.5 hPa and for AN2: "0285" ˆ
=
954.8 hPa.
- extra information:
The value "0443" is obtained by
connecting a test sensor with constant output to the scanner and
is used for controlling the stability of the scanning unit.
30
8
Other features:
With the help of the sensor scanning unit it is possible to read out
successively 12 sensors. The sensor has not a nice inlet to connect
to a calibration facility to offer any pressure different than the
current atmospheric pressure (for this intercomparison a special
housing was made, similar to the standard but with an inlet)
Intercomparison results:
1
The initial calibration plots, the extended calibration plots, the long
term stability plot and the temperature dependency plot are shown in
the figs. a, b, c and d respectively and for each barometer apart. (An
explanation of these plots is given on page 29)
Comment:
(no comment).
2
Transport test:
No significant effect observed.
3
Low pressure test:
No significant effect observed.
4
Power supply test:
No significant effect observed.
5
High humidity test:
No significant effect observed.
6
Adjustment:
Not applicable: Pressure values have to be calculated externally
using a formula with calibration parameters.
7
Comment:
The effective resolution in hPa units is equal to the calibration
parameter B, which is about 0.2 hPa. The power consumption of
sensors and scanning unit is low. The temperature dependency effects
must be regarded with respect to the relative poor repeatability.
31
fig. a
fig. b
fig. c
fig. d
AN1
32
AN1
fig. b
fig. c
fig. d
AN2
fig. a
AN2
33
6.2
A.I.R.
Type of instrument:
Intellisensor AIR-DB
Number of instruments:
3 (AI1, AI2, AI3)
Participating member(s):
the Netherlands
Manufacturer:
Atmospheric Instrumentation Research,
Inc.; Boulder, Colorado, U.S.A.
General technical data
1
Principle of operation:
The transducer is a capacitive type, aneroid pressure sensor made of
two NiSpan-C diaphragms which are bonded to opposite sides of a
ceramic disc. The volume enclosed by the diaphragms is evacuated.
The sensor consists of two variable capacitors whose capacitance
varies with the applied pressure. With the help of an oscillator
circuit, a frequency is generated which varies with the capacitance
and consequently with pressure. For compensation of temperature and
circuit drift a temperature sensing capacitor and a reference
capacitor are employed. A microprocessor controls the operation of
the barometer, samples signals from the sensing capacitors and
calculates the pressure by using special formula using calibration
parameters from a ROM.
2
Power requirements:
+11 … +16 VDC, -11 … -16 VDC (in RS232
mode) or +8 … +16 VDC (TTL or Parallel
mode)
3
Operating pressure range:
800 … 1060 hPa (AIR-DB-…A), 600 …
1100 hPa (B), 0 … 1100 hPa (C); Maximum:
1300 hPa.
4
Operating temperature range:
+5° … +40°C (AIR-DB-1…), -25° … +50°C
(2), -55° … +85°C (3).
5
Storage temperature range:
-
6
Dimensions:
7
Output:
a) type of reading:
89 mm, Ø 89 mm
digital output
b) type of output:
RS232C or TTL serial (110, 300, 1200,
9600 baud) or Parallel (8-bit with handshaking)
c) example of output string:
8
34
955.56<CR><LF>
- pressure information:
" 955.56" (in hPa)
- extra information:
-
Other features:
Selectable operating modes: Barometer, altimeter (incl. setting);
low power consumption due to CMOS circuitry. Data format is ASCII or
binary; in "stand by" mode low power consumption; various pressure
and altimeter units selectable: hPa, inHg, mmHg, psia, ft, m,
ft/inHg, m/hPa. All settings have to be done manually be
rearrangement of a jumper configuration on the electronics board.
Intercomparison results:
1
The initial calibration plots, the extended calibration plots, the long
term stability plot and the temperature dependency plot are shown in
the figs. a, b, c and d respectively and for each barometer apart. (An
explanation of these plots is given on page 29)
Comment:
(no comment).
2
Transport test:
No significant effect observed.
3
Low pressure test:
No significant effect observed.
4
Power supply test:
No significant effect observed.
5
High humidity test:
No significant effect observed.
6
Adjustment:
To calculate the pressure, the processor uses parameters stored in a
ROM. Modification of these parameters can only be done externally by
A.I.R. or with the AIR-DB calibration adjustment system, model CAS1000. This EPROM programmer makes it possible to reprogram these
parameters with respect to the correction formula C(P) = A + B×P.
7
Comment:
The accuracy of the barometers depends on the width of the pressure
range and temperature range. Notice the difference in terms of long
term stability of the three devices. Barometer AI3 may be labelled
as "defective" (reason unknown, presumably leakage of vacuum).
35
fig. a
fig. b
fig. c
fig. d
AI1
36
AI1
fig. b
fig. c
fig. d
AI2
fig. a
AI2
37
fig. a
fig. b
fig. c
fig. d
AI3
38
AI3
39
6.3
Casella
Type of instrument:
Digital barometer, W2112
Number of instruments:
2 (CA1, CA2)
Participating member(s):
United Kingdom
Manufacturer:
Casella London Limited, London, England
General technical data
1
Principle of operation:
The measuring system is based on piezoresistive ion implanted
integrated circuit diaphragm, which incorporates active chip
temperature control. One side of the diaphragm is evacuated and
sealed to form an absolute pressure measuring device. The sensor is
excited with a DC signal which is processed by electronics. The
transducer temperature is controlled using chip sensing elements.
2
Power requirements:
220 … 240 VAC or 110 … 120 VAC.
3
Operating pressure range:
900 … 1100 hPa.
4
Operating temperature range:
0
5
Storage temperature range:
-
6
Dimensions:
8×18×28 cm
7
Output:
… 50 C
a) type of reading:
Display; optional: digital output of
buffered data upon manual triggered request (using an internal
logger board and an external programming unit).
b) type of output:
B.C.D.)
(Optional: Current loop 4…20 mA, RS232C,
c) example of output string: -
8
- pressure information:
-
- extra information:
-
Other features:
Status information on display. The optional logger board can be used
for control purposes.
Intercomparison results:
1
The initial calibration plots, the extended calibration plots, the long
term stability plot and the temperature dependency plot are shown in
the figs. a, b, c and d respectively and for each barometer apart. (An
explanation of these plots is given on page 29)
Comment:
(no comment).
2
Transport test:
No significant effect observed.
3
Low pressure test:
No significant effect observed.
40
4
Power supply test:
No significant effect observed.
5
High humidity test:
No significant effect observed.
6
Adjustment:
By readjustment of two potentiometers on the backside, one for 900
hPa and the other for 1100 hPa.
7
Comment:
Both barometers show significant hysteresis, drift and temperature
dependency. The instruments have no facility for automatic data
communication and are therefore not suitable for use in automatic
weather stations. As a consequence the readings of the display had
to be used for data analysis.
41
fig. a
fig. b
fig. c
fig. d
CA1
42
CA1
fig. b
fig. c
fig. d
CA2
fig. a
CA2
43
6.4
Druck
Type of instrument:
Precision Digital Pressure Indicator, DPI
140
Number of instruments:
2 (DR1, DR2)
Participating member(s):
United Kingdom
Manufacturer:
Druck Limited, Groby, Leicestershire,
England
General technical data
1
Principle of operation:
The DPI 140 utilises a vibrating cylinder pressure sensor to provide
absolute pressure readout between 0 to 35 Pa absolute. (For a
detailed description of this sensor see section 6.11, "Solartron",
page 80.) Notice that this sensor measures in principle the air
density and temperature from which the air pressure is calculated
based on the Gas Constant for dry air. As a consequence the use of
moist air or Nitrogen (for calibration purposes) must be avoided in
cases where a high absolute accuracy is required (e.g. air at 20 C
with U = 70%RH will give an error at 1000 hPa of 0.08 hPa).
2
Power requirements:
200…260 VAC, 100…130 VAC
3
Operating pressure range:
Barometric version: 800 … 1150 hPa.
(Standard version: 35 … 1150 hPa; max.
pressure: 1 MPa)
4
Operating temperature range:
0 … 50 C standard (Calibration
temperature range: 10 … 30 C)
5
Storage temperature range:
-
6
Dimensions:
192×144×250 mm
7
Output:
a) type of reading:
Display (res.: 0.01 hPa), Serial
communication(RS232) and IEEE 488 (option)
b) type of output:
RS232 V24 (with ASCII) with software and
hardware handshaking and baud rate selectable (110…9600 baud);
option: IEEE488, address selectable. There are two modes:
Reading on demand (i.e. after reception of a <CR>) or
continuously (approx. 5 per second)
c) example of output string:
8
44
+955.571<CR>[<LF>]
- pressure information:
"955.571" in hPa (6 digits pressure
reading with floating point)
- extra information:
(no status information)
Other features:
Pressure units: mbar, kPa, psi, insHg and mmHg; Option: Altitude
display for direct readout of altitude in feet or metres based on
ICAO law.
Intercomparison results:
1
The initial calibration plots, the extended calibration plots, the long
term stability plot and the temperature dependency plot are shown in
the figs. a, b, c and d respectively and for each barometer apart. (An
explanation of these plots is given on page 29)
Comment:
(no comment).
2
Transport test:
No significant effect observed.
3
Low pressure test:
No significant effect observed.
4
Power supply test:
No significant effect observed.
5
High humidity test:
No significant effect observed.
6
Adjustment:
Automatically by entering two to ten calibration data points, while
offering pressure of the same value at the same time. The instrument
calculates from these data points and by a least squares fit the
corrections for zero and span and saves it into memory (non volatile
RAM). As explained before, the use of dry air (not nitrogen) is
recommended.
7
Comment:
A very accurate and stable instrument when used under stable
temperature conditions.
A significant temperature related offset is found, identical for
both instruments. The use of a more appropriate set of parameters
for temperature compensation calculations may reduce this effect
largely.
45
fig. a
fig. b
fig. c
fig. d
DR1
46
DR1
fig. b
fig. c
fig. d
DR2
fig. a
DR2
47
6.5
L.E.E.M.
Type of instrument:
Barometre a ruban vibrant P04-2160
Number of instruments:
2 (LE1, LE2)
Participating member(s):
France
Manufacturer:
Laboratoire d’Etudes Electroniques et
Mécaniques, Versailles, France.
General technical data
1
Principle of operation:
A set of vibrating strings is utilised to measure the elastic
deformation of an barometric capsule. The signal from the transducer
is a frequency function of the applied atmospheric pressure. The set
is mechanically temperature compensated. The use of dry air is
recommended; inside the instrument a bottle with silicagel is
connected to ensure measurement of air with a sufficient low %RH
2
Power requirements:
220 VAC ±10%
3
Operating pressure range:
900 … 1050 hPa
4
Operating temperature range:
0 … 40 C (temperature compensated within
+15 …+40 C
5
Storage temperature range:
-
6
Dimensions:
490×260×110 mm
7
Output:
a) type of reading:
display and frequency output
b) type of output:
Array of pulses; the number of pulses is
a measure for the pressure. The output is activated by an
external supplied trigger pulse. For standard counting devices
an interface using optocouplers is recommended.
c) example of output string: (No ASCII or other type of output
presenting a string of characters)
8
- pressure information:
to be calculated from the number of
counts and by using a conversion formula
- extra information:
-
Other features:
-
Intercomparison results:
1
48
The initial calibration plots, the extended calibration plots, the long
term stability plot and the temperature dependency plot are shown in
the figs. a, b, c and d respectively and for each barometer apart. (An
explanation of these plots is given on page 29)
Comment:
Notice the pressure dependent temperature effect with the LE1. From
the start of the intercomparison the LE2 showed very large
deviations from the reference pressure, which remained stable during
the intercomparison. These deviations may be due to an incorrect
"gain" setting. Both instruments showed an excess offset due to the
low pressure test (compare 10JUL91 with 18JUN91).
2
Transport test:
No significant effect observed.
3
Low pressure test:
Low pressure situations may affect the
sensor or transducer (see fig. c,
10JUL91).
4
Power supply test:
No significant effect observed.
5
High humidity test:
No significant effect observed.
6
Adjustment:
Based on calibration results the determined offset can be reduced by
means of three rotating switches in steps of 0.1 hPa.
7
Comment:
The provided calibration tables, which gives the relation between
transducer frequency output and atmospheric pressure may not be used
for calculating the pressure from the pulse counts. Any relation
between pulse counts and pressure was not provided by the
Participant. As a consequent the reading of the display had to be
used for data analysis. Although a frequency output may be
considered as digital output, it differs very much from digital
outputs like RS232C with ASCII.
49
fig. a
fig. b
fig. c
fig. d
LE1
50
LE1
fig. b
fig. c
fig. d
LE2
fig. a
LE2
51
6.6
MeteoLabor
Type of instrument:
Barometer GB1
Number of instruments:
3 (ME1, ME2, ME3)
Participating member(s):
Switzerland (ME2, ME3); the Netherlands
(ME1)
Manufacturer:
Meteolabor AG, Wetzikon, Switzerland
General technical data
1
Principle of operation:
The heated barometer consists of a measuring module and a module
with a microprocessor. The measuring section of the barometer is
contained in a housing which is well insulated against heat and
maintained at a constant temperature. The expansion of an aneroid
capsule made of a special steel alloy influences inductance and
therefore the frequency of a LC oscillator. The heating is
thermistor controlled, the influence of ambient temperature on the
sensor and the oscillator is eliminated. The pressure is calculated
from the measured frequency
2
Power requirements:
+12 VDC ±20% (optionally: 220 VAC with or
without floating battery)
3
Operating pressure range:
630…1060 hPa
4
Operating temperature range:
-10…+35 C
5
Storage temperature range:
-
6
Dimensions:
14×22×30 cm
7
Output:
a) type of reading:
various formats.
display and two digital outputs with
b) type of output:
Clocked: 11-bits binary; serial: binary
(13 bit), BCD or ASCII; seven baudrates from 110 to 9600 baud.
External amplifier necessary to convert serial output to RS232
(in ASCII code setting)
c) example of output string:
954.9 -58.2<CR><LF>
- pressure information:
"954.9" (in hPa); thousands of hPa are
not presented, thus "013.5" must be interpreted as 1013.5 hPa.
- extra information:
"-58.2" (fluctuation); It indicates the
difference in hPa between the actual pressure and a defined
fixed reference pressure value.
8
52
Other features:
With a rotating switch several modes of operation are possible
(periodic measurement, measurement upon request, display on/off).
Intercomparison results:
1
The initial calibration plots, the extended calibration plots, the long
term stability plot and the temperature dependency plot are shown in
the figs. a, b, c and d respectively and for each barometer apart. (An
explanation of these plots is given on page 29)
Comment:
Notice the significant fluctuations in the stability as well as the
temperature dependency effect. Also notice the excess offset for all
instruments during the temperature dependency measurements (at
20 C), which may be due to an instability effect.
2
Transport test:
No significant effect observed.
3
Low pressure test:
Small effect observed (-2…-3 hPa)
4
Power supply test:
No significant effect observed.
5
High humidity test:
No significant effect observed.
6
Adjustment:
Manually in calibration adjustment mode. Using +/- buttons the
offset value is readjusted in steps of ±0.01 hPa and then stored in
memory. The barometer is not protected against disadjustment by
unauthorized personnel.
7
Comment:
Although the temperature of the sensor is stabilized, the barometers
showed large temperature related offsets (see fig. d).
53
fig. a
fig. b
fig. c
fig. d
ME1
54
ME1
fig. b
fig. c
fig. d
ME2
fig. a
ME2
55
fig. a
fig. b
fig. c
fig. d
ME3
56
ME3
57
6.7
Nakaasa
Type of instrument:
Digital Barometer (Cylindrical Resonator
Type), F-451
Number of instruments:
2 (NA1, NA2, NA3)
Participating member(s):
Japan (NA2, NA3), the Netherlands (NA1)
Manufacturer:
Nakaasa Instruments Co., Ltd., Chuo-Ku,
Tokyo, Japan.
General technical data
1
Principle of operation:
The atmospheric pressure is measured on basis of the change of the
resonance frequency of a thin wall cylindrical resonator, which
varies with the pressure inside. Outside the resonator there is
vacuum. Two piezo electric elements are fitted to the resonator as
activators. Another two piezo electric elements fitted on the
resonator are utilised as transducers to measure two resonance
frequencies. By taking the ratio of both frequencies as input for
calculating the pressure, any temperature dependency can be ignored.
Notice that this sensor measures in principle the temperature
reduced air density from which the air pressure is calculated. As a
consequence the use of moist air or Nitrogen (for calibration
purposes) must be avoided in cases when a high absolute accuracy is
required. The instrument is provided with a capsule with silicagel
to ensure dry air measurements.
2
Power requirements:
85…264 VAC (NA1); 12 VDC ±10% (NA2, NA3)
3
Operating pressure range:
800 … 1050 hPa (NA1); 470 … 1050 hPa
(NA2, NA3).
4
Operating temperature range:
0
5
Storage temperature range:
"normal"
6
Dimensions:
480×149×400 mm
7
Output:
… 50 C
a) type of reading:
Two displays, digital and analog.
b) type of output:
IEEE 488 (GP IB), address #3 only.
c) example of output string:
C0009548 C1009548 C2-00311 C3-00020 C4098066 C5000000 CZ001626
(for NA1)
C0009553 C1009592 C2-00574 C3000000 C4097978 C5000358 C6@/////
(NA2, NA3)
- pressure information:
First block, "C0009548" (unit: 0.1 hPa):
954.8 hPa
- extra information:
From second block: Sea level pressure
(959.2 hPa from "C1009592"); third block: 3-hours variation data
of station pressure (-31.1 hPa from "C2-00311"); fourth block:
Offset to be set in 0.01 hPa units; fifth block: g in 0.1 mm/s2;
sixth block: elevation to be set in dm. The sea level pressure
is calculated by taking into account a value for the air
58
temperature, g and the elevation, all to be entered in the
instrument manually.
8
Other features:
Also presents sea level pressure (on second display). Other modes
may be chosen to display pressure data with 10 min. interval or the
3-hours variation. After a variation larger than 100 hPa a buzzer
will go off to warn for a shift in the analog output signal. The
barometer is protected for resetting the parameters by unauthorized
personnel with the help of a lock and key.
Intercomparison results:
1
The initial calibration plots, the extended calibration plots, the long
term stability plot and the temperature dependency plot are shown in
the figs. a, b, c and d respectively and for each barometer apart. (An
explanation of these plots is given on page 29)
Comment:
(no comment).
2
Transport test:
No significant effect observed.
3
Low pressure test:
No significant effect observed.
4
Power supply test:
No significant effect observed.
5
High humidity test:
No significant effect observed.
6
Adjustment:
By manually readjustment of the offset value stored in RAM (in steps
of 0.01 hPa)
7
Comment:
The NA2 and NA3 showed a significant better stability and
temperature independency than the NA1. On manned stations the second
display with sea level pressure can be helpful.
59
fig. a
fig. b
fig. c
fig. d
NA1
60
NA1
fig. b
fig. c
fig. d
NA2
fig. a
NA2
61
fig. a
fig. b
fig. c
fig. d
NA3
62
NA3
63
6.8
Paroscientific
Type of instrument:
PA1: Digiquartz Intelligent Transmitter,
model 1015A; PA3 and PA4: Digiquartz
Portable Field Standard, model 760-15a;
all with the Digiquartz Pressure
Transducer.
Number of instruments:
3 (PA1, PA3, PA4)
Participating member(s):
Norway (PA3, PA4); the Netherlands (PA1)
Manufacturer:
Paroscientific, Inc., Redmond,
Washington, U.S.A.
General technical data
1
Principle of operation:
The transmitter is developed around a "Piezoelectric Force
Transducer" consisting of a resonator with thin quartz crystal. A
set of four electrodes is deposited on the quartz to effect
piezoelectric excitation. An alternating electric field causes
vibration of the quartz. With a bridge type oscillator circuit the
resonator is tuned into its resonant frequency. The frequency, is a
steep function of the forces on the quartz crystal. With bellows and
a miniature balance, atmospheric pressure is passed to the
resonator. The resonant frequency is also temperature dependent. By
using a transducer with two layers excitated one in the resonant and
the other in a higher order resonant frequency, it is possible to
calculate from both frequencies a parameter, representative for the
temperature. Based on calibrations where the resonant frequency as
well as the temperature parameter is determined as functions of
pressure and temperature it is possible by means of a least squares
fit to produce a set of calibration parameters, to be used to
calculate the pressure from a higher order polynome function of
resonant frequency and temperature parameter. These parameters are
stored in an EEPROM and can be altered on base of new calibration
results. Microelectronics inside the transmitter process the
measurement and manage the data communication.
2
Power requirements:
model 1015A (PA1): 5…25 VDC; model 760
(PA3, PA4): 220 VAC with battery capable
of over 200 hours of continuous
operation.
3
Operating pressure range:
0 … 1250 hPa
4
Operating temperature range:
Depends on calibration range; standard
-55 … +100 C (PA3, PA4: 0 … 40 C)
5
Storage temperature range:
-55
6
Dimensions:
model 1015A (PA1): 75×75×130 mm; model
760 (PA3, PA4): 191×229×152 mm
7
Output:
a) type of reading:
64
… +107 C
all models: serial digital output; model
760 (PA3, PA4): 6 digits display
b) type of output:
RS-232 (ASCII), seven baudrates to be
selected (300 … 19200 baud), no handshaking. Option: 16 bit
parallel binary output.
c) example of output string:
- pressure information:
*0001955.5680<CR><LF>
"955.5680" (in hPa)
- extra information:
"*" indicates ’start of string’; "00" is
target address (terminal); "01" is source address (barometer #1)
8
Other features:
The transmitters can be operated as a single standard output device
or as an addressable chain of up to 98 transmitters on one RS-232
port. possible pressure units are: psi, hPa, bar, kPa, Mpa, inches
of Hg, torr, meters of water, or user definable. Sample integration
time is selectable. All parameters are stored in EEPROM to be
altered by a simple terminal connection. The transmitter can send
measured temperature too (for control purposes). Various modes of
data transmission: Continuously, sample and send, sample and hold
(to be send upon request later on).
Intercomparison results:
1
The initial calibration plots, the extended calibration plots, the long
term stability plot and the temperature dependency plot are shown in
the figs. a, b, c and d respectively and for each barometer apart. (An
explanation of these plots is given on page 29)
Comment:
Notice the strong drift of the PA1, starting july 1990.
2
Transport test:
No significant effect observed.
3
Low pressure test:
No significant effect observed.
4
Power supply test:
No significant effect observed.
5
High humidity test:
No significant effect observed.
6
Adjustment:
Although all parameters can be altered in the EEPROM it is
recommended to adjust the transmitters by changement of two
parameters, especially defined for adjustment purposes (Offset and
gain). This can be performed by hand with the help of an terminal
with RS232 interface, or automatically with a computerized
calibrating system.
7
Comment:
The drift of the PA1 is caused by leakage of the high vacuum case in
which the transducer is placed. Based on the results of PA3 and PA4
(stability, temperature independency), model 760 looks very suitable
as a travelling standard.
65
fig. a
fig. b
fig. c
fig. d
PA1
66
PA1
fig. b
fig. c
fig. d
PA3
fig. a
PA3
67
fig. a
fig. b
fig. c
fig. d
PA4
68
PA4
69
6.9
Ruska
Type of instrument:
Portable Pressure Gage, model 6200-801
(19 psia)
Number of instruments:
2 (RU1, RU2)
Participating member(s):
United Kingdom
Manufacturer:
Ruska Instrument Corporation; Houston,
Texas, USA.
General technical data
1
Principle of operation:
The instrument utilises a vibrating cylinder pressure sensor to
provide absolute pressure readout between 0 to 1300 hPa absolute.
(For a detailed description of this sensor see section 6.11,
"Solartron", page. 80) Notice that this sensor measures in principle
the air density and temperature from which the air pressure is
calculated based on the Gas Constant for dry air. As a consequence
the use of moist air must be avoided.
2
Power requirements:
115/230 VAC ±10%
3
Operating pressure range:
0 … 1300 hPa
4
Operating temperature range:
0
5
Storage temperature range:
-25
6
Dimensions:
234×231×150 mm
7
Output:
70
… 85 C
a) type of reading:
Two displays (pressure and rate of
change) and serial digital output
b) type of output:
RS 232C (ASCII), no handshake, four
selectable baudrates: 1200…19200 baud.
c) example of output string:
8
… 50 C
955.38<CR><LF>
- pressure information:
"955.38" (in hPa)
- extra information:
-
Other features:
Selectable modes: Only one pressure on request, start or stop
continuous output; selection of transmission interval to be
programmed remotely. Output may be pressure or rate of change
(remotely selectable based on commands send to device) or tare mode
(difference between current pressure and a fixed reference
pressure). Units are selectable by touch keys on the front panel or
remotely via the serial interface. The units to be chosen are: inHg,
psi, hPa or mbar, kPa, mmHg, inH20, mmH20, kg/cm2, alt. feet, alt.
meters. The barometer may be applied to dry air or to nitrogen
(switch selectable).
Intercomparison results:
1
The initial calibration plots, the extended calibration plots, the long
term stability plot and the temperature dependency plot are shown in
the figs. a, b, c and d respectively and for each barometer apart. (An
explanation of these plots is given on page 29)
Comment:
Notice the remarkable difference between RU1 and RU2. This
difference in response quality is due to the very noisy readings of
the RU1. During the temperature dependency test the observer
averaged the readings from the display to calculate the pressure
output, as during all other calibrations the data acquisition system
took only one data reading per pressure value. Therefore the
temperature dependency results (fig. d) for RU1 look much better
than the other data results.
2
Transport test:
No significant effect observed.
3
Low pressure test:
No significant effect observed.
4
Power supply test:
No significant effect observed.
5
High humidity test:
No significant effect observed.
6
Adjustment:
Because the pressure presented by the instrument is calculated from
the measured gas density, any calibration result may only be applied
if it is based on a gas with the same gas constant. For that reason
any calibration with nitrogen will be useless if a barometer will be
used to measure atmospheric pressure. To overcome this problem the
Ruska 6200 is provided with a switch, with the help of it the user
can choose between air (for operational measurements of atmospheric
pressure) or nitrogen (for calibration purposes). There are two
adjustment procedures: 1) to adjust for offset only (zero adjust)
and 2) based on a set of reference pressure values. The zero
adjustment is performed by pressing the zero button while connecting
to vacuum. The other procedure is based on three generated pressure
setting points offered to the system (≈0%, 50% and 100% full scale),
and to be entered in calibration mode by ’+’ and ’-’ keys on the
front panel. Based on these results the barometer calculates the new
calibration parameters and stores them into memory.
7
Comment:
RU1 showed very unstable (i.e. noisy) reading, may be caused by any
electronical malfunction.
A significant temperature related offset is found, identical for
both instruments. The use of a more appropriate set of parameters
for temperature compensation calculations may reduce this effect
largely.
71
fig. a
fig. b
fig. c
fig. d
RU1
72
RU1
fig. b
fig. c
fig. d
RU2
fig. a
RU2
73
6.10
Setra
Type of instrument:
S31, S32: Model 370 digital pressure
gage; S41, S42: Model 470 digital
pressure transducer
Number of instruments:
4 (S31, S32; S41, S42)
Participating member(s):
United Kingdom (S31, S32), U.S.A. (S41,
S42)
Manufacturer:
Setra Systems, Inc.; Acton,
Massachusetts, U.S.A.
General technical data
1
Principle of operation:
Both of the models are based on Setra’s Setraceram pressure sensor.
Because its elastic properties and in order to obtain good stability
over time and temperature alumina ceramic material was chosen for
this sensor. The sensor is a symmetrical variable capacitance
capsule which deforms proportionally to applied pressure. The
reference space inside the capsule is sealed under a high vacuum.
Gold plates electrodes on the inside surfaces of the ceramic capsule
comprise a variable capacitor. As pressure is applied on the capsule, the distance and therefore the capacitance between the electrodes changes. With an LC oscillator circuit a frequency output is
generated corresponding to the applied pressure. Also the temperature is measured for compensation of any residual thermal shifts. The
raw frequency signals are processed digitally by using calibration
parameters into polynomial expressions, including corrections for
thermal errors, providing the frequency to pressure conversion
function.
2
Power requirements:
Model 370: 110/220 VAC (-10%…+30%),
option: 12 VDC internal rechargeable
battery pack (8 hours); Model 470: 5 VDC
±1%
3
Operating pressure range:
S31, S32: 800 … 1100 hPa; S41, S42: 600 …
1100 hPa
4
Operating temperature range:
-1
5
Storage temperature range:
-18
6
Dimensions:
Model 370: 217×191×183 mm; model 470:
104×89×172 mm
7
Output:
a) type of reading:
… 43 C (both models)
… 66 C (both models)
Display (model 370 only) and serial
digital output
b) type of output:
RS 232C (ASCII, bidirectional), no
handshaking; baudrate selectable: 300…9600 baud.
c) example of output string:
- pressure information:
74
955.648
mbar A OK<CR><LF>
"955.648" (in hPa)
- extra information:
8
"mbar" for unit, "A" for absolute, "OK"
for ’system is stable’.
Other features:
All internal parameters can be altered remotely using the RS232
interface. Not only pressure readings may be reported but also
status messages. The barometers can be commanded to send only one
reading or to repetitive report readings within to be defined
intervals. The units to be chosen are: psi, hPa (mbar), mm Hg, inch
Hg, mm H20, inch H20, feet, meters. Altitude may be reported,
calculated from the ’Standard Atmosphere Curve’ and based on elevation parameter or zero setting entered in RAM previously. In stead of
absolute pressure, the system may present ’tare’ pressure readings
(the current pressure subtracted by a fixed previously measured
pressure). The system is able to send alarm messages, when the
pressure exceeds high or low setpoints. It also continuously captures maximum and minimum encountered values. Model 370 is provided
with a display to present data in 6 digits as well as status information. This model has also a keyboard with which all commands can
be carried out. An audible alarm is also provided with model 370 to
attract attention or for data communication control purposes
Intercomparison results:
1
The initial calibration plots, the extended calibration plots, the long
term stability plot and the temperature dependency plot are shown in
the figs. a, b, c and d respectively and for each barometer apart. (An
explanation of these plots is given on page 29)
Comment: Note the spontaneous negative offset for S31 and S32 in
april/may 1990. Notice the increase of accuracy of S42 after the
measurement of 17APR90.
2
Transport test:
No significant effect observed.
3
Low pressure test:
No significant effect observed.
4
Power supply test:
No significant effect observed.
5
High humidity test:
No significant effect observed.
6
Adjustment:
The zero and span of the barometers may be readjusted. All calibrations has to be carried out in a stable temperature environment. The
calibration procedures will require the same pressures, in the same
engineering units, as were used for calibration at the factory.
After applying precisely the requested pressures the barometer will
calculate the new parameters and put it into memory. In model 470 a
jumper has to be placed first to be able to reprogram the nonvolatile memory with new calibration parameters(reason: calibration
protection).
7
Comment:
After power drops S41 and S42 did not return in a previously set
baudrate, nor in a default or factory set baudrate. These baudrate
values were randomly chosen by the instruments at start-up and
sometimes outside the range of selectable baudrates. Model 470
requires a very stable power supply: 5 VDC ±1% or ±0.05 VDC.
75
fig. a
fig. b
fig. c
fig. d
S31
76
S31
fig. b
fig. c
fig. d
S32
fig. a
S32
77
fig. a
fig. b
fig. c
fig. d
S41
78
S41
fig. b
fig. c
fig. d
S42
fig. a
S42
79
6.11
Solartron
Type of instrument:
3088-1W air pressure transducer
Number of instruments:
2 (SO1, SO2)
Participating member(s):
United Kingdom
Manufacturer:
Schlumberger Industries, Farnborough,
Hampshire, England.
General technical data
1
Principle of operation:
The transducer operates on the vibrating cylinder principle. The
natural frequency of a cylinder, with one closed end, varies with
the internally applied pressure (the outside being exposed to
vacuum). The cylinder is maintained in resonance electromagnetically
and will always vibrate at the natural frequency. A 0…5 volt TTL
signal, at the frequency of vibration, is supplied so that the user
can measure the frequency of vibration. Pressure can be calculated
using a "surface fit" algorithm whose input parameters are time
period of cylinder vibration and the voltage across a temperature
sensing diode which varies in the range 0.8 to 0.4 volts for
temperatures in the range -55 C to +125 C.
2
Power requirements:
+15 ±1 VDC, 0 VDC and -15 ±1 VDC; the
transducer can also be easily configured
to operate from an 0 VDC, 24 VDC supply.
3
Operating pressure range:
35 … 1300 hPa (storage up to 4000 hPa)
4
Operating temperature range:
-55
… +112 C
5
Storage temperature range:
-60
… +125 C
6
Dimensions:
84×56×50 mm
7
Output:
a) type of reading:
transducer: frequency (pressure
information) and VDC (temperature information).
b) type of output:
[The participating Member provided a
system, consisting of a Campbell Scientific CR10 measurement
module connected with the SO1 and SO2 and a power supply module.
The CR10 counted the frequency outputs as well as the voltages
across the temperature sensitive diodes. Using these data and
calibration parameters the CR10 repeatedly calculated the
pressure and sent these directly to the SC32A interface (RS232
compatible). The CR10 was provided with a small keyboard and a
simple display to show the measured pressure values manually]
c) example of output string:
01+0001.
02+955.99 03+955.62<CR><LF>
[from the Campbell CR10]
80
- pressure information:
"+955.99" for SO1, "+955.62" for SO2
(both in hPa)
- extra information:
-
8
Other features:
-
Intercomparison results:
1
The initial calibration plots, the extended calibration plots, the long
term stability plot and the temperature dependency plot are shown in
the figs. a, b, c and d respectively and for each barometer apart. (An
explanation of these plots is given on page 29)
Comment:
-
2
Transport test:
No significant effect observed.
3
Low pressure test:
No significant effect observed.
4
Power supply test:
No significant effect observed.
5
High humidity test:
No significant effect observed.
6
Adjustment:
The transducer cannot be adjusted itself. In principle one should
calibrate the output of this transducer as function of pressure and
temperature to obtain a "surface fit", with parameters to be entered
in an automatic data acquisition/digital processing facility (like
the CR10).
7
Comment:
Note that this transducer act as an pressure/temperature sensing
unit. The pressure itself has to be calculated externally (e.g. by a
microprocessor unit) taking sensor specific calibration parameters
into account. Notice that in other systems, like Druck and Ruska,
this transducer is applied.
A significant temperature related offset is found. the use of a more
appropriate set of parameters for temperature compensation
calculations may reduce this effect largely.
81
fig. a
fig. b
fig. c
fig. d
SO1
82
SO1
fig. b
fig. c
fig. d
SO2
fig. a
SO2
83
6.12
Vaisala
Type of instrument:
Digital Barometer PA11
Number of instruments:
5 (PA0, PA1, PA2, PA3, PA4)
Participating member(s):
Finland (PA1, PA2), Denmark (PA3, PA4),
the Netherlands (PA0)
Manufacturer:
Vaisala Oy, Helsinki, Finland
General technical data
1
Principle of operation:
Pressure is measured by three independent aneroid capsules with the
capacitive elements installed in the vacuum inside each aneroid.
Capacitor plates are precision welded to ensure maximum possible
stability and protection. Three transducer units are used for
reliability. each transducer unit is a pressure-frequency converter
controlled by microprocessor. Compensation for the effects of
ambient temperature variations is done by continuous measurement of
capsule temperatures. This arrangement automatically compensates for
temperature drift of the electronics.
2
Power requirements:
10…28 VDC; provided AC adapter: 220 VAC
±10%; the barometer is provided with a
rechargeable battery (3 hours).
3
Operating pressure range:
500 … 1060 hPa (P>1060 hPa may damage
sensors)
4
Operating temperature range:
+5
5
Storage temperature range:
-25
6
Dimensions:
72×144×250 mm
7
Output:
… +55 C
… +70 C
a) type of reading:
output
Display (5 digits) and serial digital
b) type of output:
RS232 (ASCII), 300 baud and 7 bits, even
parity only; no handshaking.
c) example of output string:
AAXY 9557 -565 Q0q0<CR><LF>
- pressure information:
"9557" in units of 0.1 hPa, thousands of
hPa are omitted (e.g. "0135" ˆ
= 1013.5 hPa)
- extra information:
"AAXY" is a header, "-565" indicates the
trend (current one minute averaged pressure minus pressure three
hours ago), "Q0" indicates the minimum difference between two
transducers en "q0" indicates the maximum difference between two
transducers.
8
84
Other features:
Display may be switched manually from pressure to trend. In
principle all transducers are to be used to calculate the pressure.
However, it is possible to select only one or two of the set of
three. For instance in cases a transducer is out of range or during
calibration/adjustment actions. The instrument can be switched
manually into two modes: 1) data transmission automatically every
minute or 2) after receipt of a "P" (software trigger). The pressure
units are: hPa and psi. The manufacturer indicates that the
instrument should be protected against mechanical shock and
vibration and advices to use absorbent packaging during
transportation. The instrument is provided with an audible alarm,
which sounds if the battery voltage is getting low, or if one of the
transducers deviates from the others by more than 0.6 hPa.
Intercomparison results:
1
The initial calibration plots, the extended calibration plots, the long
term stability plot and the temperature dependency plot are shown in
the figs. a, b, c and d respectively and for each barometer apart. (An
explanation of these plots is given on page 29)
Comment:
Notice the temperature dependency effects for all instruments.
2
Transport test:
No significant effect observed.
3
Low pressure test:
No significant effect observed.
4
Power supply test:
No significant effect observed.
5
High humidity test:
No significant effect observed.
(Also see pt. 7, Comment)
6
Adjustment:
The user can set and clear an index correction (offset correction)
manually in steps of ±0.1 hPa. By pressing a button the pressure
reading increases or decreases. The button has to be released on the
moment the display reading is correct. Then the correction will be
stored in the EEPROM of each transducer. There is no protection to
prevent disadjustment during operational use by unqualified persons,
which can be done easily; the only way to control this is to reset
the correction and to check this value by subtracting the new
reading from the old one and then to set the calibration factor
again.
7
Comment:
From the temperature dependency test it followed that the
instruments may have unexpected results if placed in unconditioned
environments (especially VA2 and VA3 showed significant effects). As
shown by the figures there is not a specific trend or a relationship
with the temperature itself. These results are in contrast with the
standard calibration results, as summarized by the long term
stability figure. Investigations on possible typical effects caused
by the climate chamber did not demonstrate any clarification. The
serial interface of VA0 suffered from breakdowns and gave reason to
malfunction of the barometer itself; Vaisala has indicated that this
problem is solved for newer models (from 1988 and later), like VA1
to VA4. The VA3 could not be calibrated at 40 C because of complete
breakdown at that temperature (no display reading); after reducing
the temperature to 20 C the instrument started working as normal.
85
fig. a
fig. b
fig. c
fig. d
VA0
86
VA0
fig. b
fig. c
fig. d
VA1
fig. a
VA1
87
fig. a
fig. b
fig. c
fig. d
VA2
88
VA2
fig. b
fig. c
fig. d
VA3
fig. a
VA3
89
fig. a
fig. b
fig. c
fig. d
VA4
90
VA4
7. Conclusions and Recommendations
The findings of the Intercomparison can be summarized by the following
general statements. Of course these statements are based on the results of
the followed procedures and with respect to the submitted barometers only.
7.1
Conclusions
1
The operational reliability of most of the instruments submitted
in the intercomparison was very high.
2
The repeatability taking hysteresis and drift into account is very
good for many submitted instruments. The majority of these
barometers showed a repeatability of ± 0.1 hPa (or better).
3
Compared to the KNMI reference some submitted barometers showed
offsets larger than 0.3 hPa at the beginning of the
intercomparison. Presumably caused by improper calibration and
adjustment.
4
A large number (40%) of the submitted instruments showed a long
term stability of 0.1 hPa per year or better. With respect to the
required target accuracy of ±0.1 hPa and to the achievable
observing accuracy of ±0.3 hPa, stated in paragraph 1.2, the
calibration interval of many of these instruments may be one year
or more.
5
A large number of barometers suffer from temperature dependency
(larger than 0.3 hPa in the temperature range from 0° to 40°C).
6
Many barometers showed a pressure sensitive error, relatively
large compared to its overall accuracy. A number of these
instruments cannot be adjusted for this error.
7
Various instruments of the same manufacturer and type showed
remarkable differences in the overall performance.
8
Some barometers are un-adjustable and some are offset adjustable
only. A number of barometers can be re-adjusted by reprogramming a
set of parameters, to be used by its microprocessor, into an
(E)EPROM.
9
No instruments showed significant effects caused by the transport
test and the power-supply test. Only the barometers LE1, LE2, ME1,
ME2 and ME3 showed a significant effect due to low pressure
conditions.
10
A few barometers did not work correctly at high temperatures
(40°C, 95%RH).
91
11
No general standard for digital output is used (IEEE 488, RS232
with handshake, RS232 without handshake or frequency output). Also
quite different formats are used.
For one type of barometer, indicated as micro processor digital
barometer, it was necessary to start any output of data manually.
Therefore it is stated that not all automatic digital barometers
can be used in automatic weather stations.
12
Many barometers are suitable to work in a wide range of DC
voltages as power supply.
7.2
92
Recommendations
1
The WMO should make attention to its Members to recommend its
manufacturers to calibrate and adjust barometers properly before
shipment. It is necessary to calibrate any barometer upon receipt
to control or re-adjust the calibration setting. Such calibration
as well as the control of appropriate working has to be repeated
regularly.
2
If choosing for a barometer one should take great care of the
possibilities how to calibrate and re-adjust such a barometer. It
is recommended to investigate first the possibilities of its own
calibration facility before any decision is made.
3
There are some barometers which can be used as travelling
standard. These instruments showed excellent long term stability
and repeatability and did not suffer from transport and low
pressure effects.
4
Since many barometers suffer from insufficient temperature
compensation one should take great care of it. The manufacturers
or calibration laboratories responsible for adjustment of
barometers should also take care of proper temperature
compensation or improve the calibration as a function of
temperature.
5
In order to use barometers as automatic measuring devices, these
instruments should make use of communication interfaces with
international standardized protocols and formats. It is strongly
recommended to confirm very strictly to the international
agreements on digital communication. The WMO should make attention
to its Members to recommend its manufacturers to do so.
6
It is recommended, for barometers to be used in automatic
measuring systems, to be suitable for power supplies in a wide
range of DC voltages (e.g. 5 to 28 V DC).
7
Any decision when choosing for an new appropriate automatic
digital barometer, must not only be based on the stated instrument
specifications but also on the environmental circumstances and
maintenance issues.
8. Acknowledgements
The author would like to express his gratitude to the following people:
-
The staff personal of the KNMI calibration laboratory, In particular to
Mr. J. Middelkoop, who did all manual readings and supported the
instalment of the devices on all sites.
-
Mrs. I. Ritsma and mrs. E. van Duivenbooden, for secretarial support.
-
The IOC for the invaluable expert guidance given both in the planning
of the Intercomparison and the preparation of this report.
9. References
[1]
Van der Meulen, J.P., A Comparative Study of a Set of Digital
Automatic Barometers, WMO (Geneva, 1989): Instruments and Observing
Methods Report No. 35,
[2]
Bevington, Ph.R., Data Reduction and Error Analysis for the Physical
Sciences, McGraw-Hill (New York, 1969): Chapter 8.
93
Appendix A. The International Organizing Committee.
J.P. van der Meulen (Project leader)
the Netherlands
J.L.J. Boot (Chairman)
the Netherlands
A.G.M. Driedonks 1)
the Netherlands
A. van Gysegem
Belgium
P.J. Six 1)
the netherlands
WMO Secretariat
S. Klemm 1)
Senior Scientific Officer
K. Schulze 2)
Senior Scientific Officer
1: First session only (October 11 to 13, 1988).
2: Second session only (October 8 to 10, 1991).
94
Appendix B. Terminology.
To prevent confusion which may arise in usage of terms like accuracy,
correction, calibration, etc., the four international organizations
affiliated with metrology (BIPM, IEC, ISO and OIML) have published a
"International vocabulary of basic and general terms in metrology",
abbreviated as VIM. A number of these definitions, relevant for this report
are given here:
Accuracy (of a measuring instrument): The ability of a measuring instrument
to give indications approaching the true value of a measurand.
Accuracy of measurement: The closeness of the agreement between the result
of a measurement and the (conventional) true value of the measurand.
Note: The use of the term precision for accuracy should be avoided.
Calibration: The set of operations which establish, under specified
conditions, the relationship between values indicated by a measuring
instrument or measuring system, or values represented by a material
measure, and the corresponding known values of a measurand.
Note: Calibration does not imply any adjustment of the measuring
instrument.
Correction: The value which, added algebraically to the uncorrected result
of a measurement, compensates for an assumed systematic error.
Notes: 1. The correction is equal to the assumed systematic error, but
of opposite sign.
2. Since the systematic error cannot be known exactly, the
correction is subject to uncertainty.
Digital (measuring) instrument: Measurement instrument in which the
quantity to be measured is accepted as, or is converted into, coded
discrete signals and provides an output/or display in digital form.
Discrimination: The ability of a measuring instrument to respond to small
changes in the value of the stimulus.
Drift: The slow variation with time of a metrological characteristic of a
measuring instrument.
Error (of indication) of a measuring instrument: The indication of a
measuring instrument minus the (conventional) true value of the
measurand.
Hysteresis: The property of a measuring instrument whereby its response to
a given stimulus depends on the sequence of preceding stimuli.
Reference standard: A standard, generally of the highest metrological
quality available at a location, from which the measurements made at
that location are derived.
Repeatability of measurements: The closeness of the agreement between the
results of successive measurements of the same measurand carried out
subject to all of the following conditions:
- the same method of measurement,
- the same observer,
- the same measuring instrument,
- the same location,
- the same conditions of use,
- repetition over a short period of time.
95
Reproducibility of measurements: The closeness of the agreement between the
results of measurements of the same measurand, where the individual
measurements are carried out changing conditions such as:
- method of measurement,
- observer,
- measuring instrument,
- location
- conditions of use,
- time.
Repeatability (of a measuring instrument): The ability of a measuring
instrument to give, under defined conditions of use, closely similar
responses for repeated applications of the same stimulus.
Repeatability error (of a measuring instrument): The random component of
the error of a measuring instrument.
Resolution (of an indicating device): A quantitative expression of the
ability of an indicating device to distinguish meaningfully between
closely adjacent values of the quantity indicated.
Sensor: The part of a measuring instrument which responds directly to the
measured quantity,
Specified measuring range: The set of values of a measurand for which the
error of a measuring instrument is intended to lie within specified
limits.
Transfer standard: A standard used as an intermediary to compare standards,
material measures or measuring instruments.
Stability: The ability of a measuring instrument to maintain constant its
metrological characteristics.
Travelling standard: A standard, sometimes of special construction,
intended for transport between different locations.
Uncertainty of measurement: An estimate characterizing the range of values
within which the true value of a measurand lies.
Working standard: A standard which, usually calibrated against a reference
standard, is used routinely to calibrate or check material measures or
measuring instruments.
The International Laboratory Accreditation Conference (ILAC) is more
specific in defining terms with respect to testing of instruments:
To express quality of test results, there are four basic terms;
accuracy, precision, repeatability and reproducibility:
Accuracy expresses the degree of departure or coincidence between the
test result and a "true value".
The term precision is a general term used to express "the closeness of
agreement between repeated test results".
Repeatability is the measure of variations between test results of
successive tests of the same sample carried out under the same
conditions; i.e. the same test method, the same operator, the same
testing equipment, the same laboratory and a short interval of time.
Reproducibility is the term used to express the measure of variations
between test results obtained with the same test method on identical
test samples under different conditions; i.e. different operators,
different testing equipment, different laboratories and/or different
time.
96
Appendix C. Fitting mathematics.
Used formulas to calculate the fitting curve and standard error (for more
details about curve fitting see for instance ref. [2], page 93).
Definition of the data points to be fitted:
Ci
= C(Pi)
≡ Pref,i - Pi, with Pi ∈ [950, 1050] hPa; i = 1…44
To enable a brief impression of the characteristics of the fit for the
separate terms a new parameter p was defined, to be interpreted as reduced
pressure parameter. (notice the difference between lower case p and capital
P). This parameter is defined as:
p
≡ (P - 1000 hPa) / (50 hPa),
which implies that:
Pi ∈ [950, 1050] hPa ⇔ pi ∈ [-1, +1]
and
Pi = 1000 hPa
⇔ pi = 0
By fitting the data C(pi) to the second order polynome
Y(p) = a + bp + cp2 ,
the order of magnitude of a, b and c are comparable since pi and pi2 ≤
1, and because p is dimensionless. Notice that the dimension of a, b and c
is in terms of pressure units, i.e.
{a} = {b} = {c} = hPa.
Moreover, since we have stated that analysis of the long term stability
will be related to the trend of the correction C at the fixed pressure
level P = 1000 hPa, the trend of parameter a can be used directly as input
for that analysis since at that pressure,
C(P = 1000 hPa) = C(p = 0) = a.
The parameters b and c are the pressure related parameters and yield
information used to calculate any pressure dependency part of the
correction. Parameter c, representative for second and higher order terms
in C(p) is a good measure for the non-linearity of C(p).
These parameters a, b and c are calculated according to the method to draw
a least-squares fit to a polynomial (with all weighting factors ≡ 1), i.e.
97
<C>
<p>
1
a =
∆
2
∆
3
<Cp>
<p >
<p >
<Cp2>
<p3>
<p4>
1
<C>
<p2>
<p>
<Cp>
<p3>
1
b =
<p2>
2
<p >
2
<Cp >
1
1
,
,
c =
∆ =
4
∆
2
<C>
<p>
<p >
<Cp>
<p2>
<p3>
<Cp2>
1
<p>
<p2>
<p>
<p2>
<p3>
2
<p >
<p>
<p >
3
<p >
,
.
4
<p >
The brackets "<" and ">" stand for average, i.e.:
<x> ≡
1
N
N
xi ;
N = 44
i=1
The standard error s of Y(p) is defined by the square root of the
experimental variance v of the data points Ci to Y(p), i.e. s = v, where
v
98
≡
N
N-3
<C - Y(p)2> ;
N = 44
Appendix D. Questionnaire.
QUESTIONNAIRE
FOR PARTICIPATION IN THE WMO AUTOMATIC DIGITAL BAROMETER
INTERCOMPARISON
(DE BILT, MARCH 1989 to APRIL 1990)
The intercomparison will be hosted by the Netherlands at KNMI
commencing March 1989. In order to prepare the site and data logging
facilities participants are asked to provide the following
information.
(please copy and complete separate forms if more than one type of
instrument is being entered).
a.
PARTICIPANT:
b.
INSTRUMENT/SENSOR
1. Manufacturer, Model/Type and number of instruments of this type
to be entered.
2. Principle of operation (in short):
3. Power requirements (with tolerance of variation) and power
consumption:
(if available, 220 VAC/50Hz is recommended)
4. Instrument/sensor output and communication protocol:
(interfacing facilities, e.g. IEEE-488, RS 422, RS 232,
centronics. Please specify in detail).
5. Technical data:
Operating pressure range (in hPa):
Operating temperature range (in
°C):
Storage pressure range (in hPa):
Storage temperature range (in
Storage humidity range (in
°C):
%RH):
Nominal repeatability/hysteresis (in hPa):
Long-term stability (in hPa/year):
99
6. Type of instrument/sensor output connector:
(please enclose one or more spare connectors).
7. Can the instrument/sensor be connected to an air inlet?
8. Is it possible to seal the instrument hermetically?
c.
CONTACT PERSON
Please nominate a contact person for all technical queries relating
to the instruments.
Name:
Full address:
Telephone number:
Telex:
Telefax:
Please return this questionnaire by 1 January 1989 to:
dr. J.P. van der Meulen
Projectleader WMO Intercomparison of Automatic Digital Barometers
KNMI
P.O. Box 201
3730 AE DE BILT
The Netherlands
telephone: +31 - 30 - 206432
telefax : +31 - 30 - 210407
telex
: 47096 knmi nl
100