‫חידושים במדידת לחות‪,‬‬
‫טמפרטורה ונקודת הטל‪.‬‬
Humidity Theory
? ‫מה זה לחות יחסית‬
Humidity= ‫לחות‬
 Water vapor content in a gas (air)
 If you can see the water it is not in
the gas phase anymore!
‫כמות של אדי מים בגז‬
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Dalton’s Law
 “The total pressure of a gas is the sum of
different gases’ partial pressures”
Pt = P1 + P2 + ...
E.g. air pressure is the sum of the partial
pressures of nitrogen, oxygen, water
vapor, carbon dioxide etc...
 In simple terms, ambient air consists of
water vapor and other gases -> water
vapor has a partial pressure Pw which is a
part of the total pressure
Pt = Pw + Pdry
 Pdry is the sum of the partial pressures of
all other gases
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John Dalton
6.9.1766 - 27.7.1844
Water Vapor Saturation Pressure
Maximum amount of water vapor that can be present in a gas is
water vapor saturation pressure Pws
Pws depends on the temperature of the gas: warmer air can
contain MUCH more water vapor
‫אוויר חם יכול להכליל הרבה יותר אדי מים מאוויר קר‬
Water vapour’s
saturation
pressure as a
function of
temperature
Amount at 20°C
Amount at 100°C
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Relative Humidity
%RH = 100 x Pw / Pws (t)
Relative humidity is the ratio of water vapor pressure present
in a gas (Pw) to the maximum pressure of water vapor that
could be present in the gas in that temperature [Pws(t)]
‫יחס אדי מים באוויר מול כמות מקסימלית‬
‫של אדי מים שהאוויר יכול להכליל‬
:
Ta = 60 °C
Pws = 200 hPa
Pw = 100 hPa
RH = 100 x 100/200 = 50 %RH
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Ta = 100 °C
Pws = 1013 hPa
Pw = 100 hPa
RH = 100 x 100/1013 = 10 %RH
Relative humidity is strongly proportional to temperature and its
Dewpoint
‫נקודת הטל‬
 Dewpoint is the temperature to which a gas
must be cooled for the water vapor to
condense into water
 At dewpoint, the relative humidity is 100 %RH
 The gas is saturated: contains its full capacity
of water vapor
. ‫ הטמפרטורה שצריך לקרר הגז כדי שהאדי מים הִ תְ עַ ּבו‬
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Dewpoint (continued)
Changing the Pressure
changes the dew point as the
vapor pressure also changes
The vapor pressure is directly
proportional to total pressure
Ta = 60 °C
Td = 47 °C
Pws = 200 hPa
Pw = 100 hPa
Ptot =1 bar
RH = 100 x 100/200 = 50 %RH
Page 8 / 10-2011 / LES / ©Vaisala
Ta = 60 °C
Td = 60 °C
Pws = 200 hPa
Pw = 200 hPa
Ptot =2 bar
RH = 100 x 200/200 = 100 %RH
Dewpoint (continued)
Changing the temperature
doesn´t change the dewpoint,
it depends only on the vapor
pressure
Ta = 60 °C
Td = 47 °C
Pws = 200 hPa
Pw = 100 hPa
RH = 100 x 100/200 = 50 %RH
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Ta = 100 °C
Td = 47 °C
Pws = 1013 hPa
Pw = 100 hPa
RH = 100 x 100/1013 = 10 %RH
When RH and when Td?
 Td is the preferred parameter in low humidities
 (~below 10%RH, Tf -10°C)
Humidity low
i.e. Td << T
Humidity high
i.e. Td close to T
Td
RH
Also in some high humidity cases when the interest is in
avoiding condensation
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Different Measures for Humidity - RH
Uses:
 How much water is there compared to the
maximum possible AT THIS TEMPERATURE?
 Usually the best parameter to use when comfort
of humans or in "life sciences"
 Also traditional measure in meteorology
Relative Humidity [%RH]
Properties:
 Range 0…100%
 VERY strong temperature dependence
 Also directly proportional to pressure if a gas
sample is compressed
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Key Takeaways:
Relative humidity depends strongly on temperature
If you know temperature and one humidity parameter the
others can be calculated
Dewpoint does not depend on temperature
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Capacitive HUMIDITY Sensors
Structure of the Capacitive Humidity
Sensor
Only the area where the
electrodes overlap matter
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Structure of the Capacitive Humidity
Sensor (continued)
Upper electrode
 Conductive material, corrosion resistant.
 Protects the active material of the sensor from dust, dirt and conductive
particles
 Lets through water vapour
 Functions as one of the two electrodes in a capacitor
Active material
 A polymer film absorbs water vapour: amount is a function of ambient
relative humidity
Lower electrode
 Made of conductive material,corrosion resistant
 Functions as one of the two electrodes in a capacitor
Glass substrate
 Base that supports the sensor structure
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Operation of the Capacitive Humidity
Sensor
 Water vapour penetrates the upper electrode and reaches the active
polymer film.
 How much water vapour is absorbed into the film depends on the
ambient relative humidity.
 Because the sensor is small and the polymer film thin, the sensor
responds quickly to changes in ambient humidity.
 The water vapour absorbed in the film changes the dielectric
properties of the sensor: the capacitance of the sensor changes.
 The change in the sensor’s capacitance is measured with
measurement electronics.
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The solution
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Where to Use Vaisala
HUMICAP® Sensors
 Most normal relative humidity
measurements
 In temperatures of -70 ...+180 °C
 In humidities 0...100 %RH (with
some exceptions in high humidities)
 In processes without any high
concentrations of corrosive gases
and/or solvents
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Difficult applications
 In combined high temperature/high
humidity (e.g. +100 °C/95 %RH); more
frequent calibration required.
 When the process contains high
concentrations of corrosive gases and/or
solvents.
 In low humidity/high temperature (e.g.
+160 °C/ +20 °C dewpoint temperature); in
low humidities the measurement accuracy
of a RH transmitter may not be good
enough
 DRYCAP ® instruments are more suitable
here. Consider Drycap instruments if RH is
always <10%RH
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Response Time
INTERCAP®
T90 with filters/slow moving air:
 open grid
8s
 steel netting
20 s
 sintered filter
40 s
Typical time constant of bare chip at 20°C: 2s
HUMICAP®180R
T90 with filters/slow moving air:
 open grid
17 s
 steel netting
50 s
 sintered filter
60 s
Typical time constant of bare chip at 20°C: 10s
The response time varies exponentially with the temperature, at -40°C 100x longer than at 20°C (H180):
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Stability – the Key to
Good Measurements
The graphs below show the excellent
stability of the Humicap180R sensor
during a 4 years test at the Vaisala
outdoor test site
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Application Tests – Wood Dryer
 Samples:
HUMICAP®180R with and without chemical purge
 Conditions:
-30 ... 100°C, 10 ... 100%RH, various chemicals
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A Source for Drift - Chemicals
 Some chemicals (mainly hydrocarbons) can slowly penetrate the sensor and cause
changes in its behaviour. As the time scales involved are many times long this is
perceived as drift by the customer instead of cross sensitivity
 Chemicals usually cause a decrease of sensitivity (gain) of the sensors. If our
instruments are found to show too low values at high humidity chemicals are usually to
blame if the errors are large.
dC0%
dC0%, d75% } => Drift in RH-reading
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Capacitance
dC75%
Chemical Tests – Drift, Saturated
Isopropanol Vapor
Isopropanol vapor exposure, sensor drift at 0% and 75%RH:
 Humicap180R can tolerate even large concentrations in many
cases
 Some drift is
still seen after
long exposures
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Chemical Purge – Principle
 HUMICAP®180R is briefly heated with an attached PT-100
element. Diffused chemicals are removed from the sensor’s
active polymer.
Polymer sensor
PT-100
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Chemical Purge – Example,
Isopropanol (saturated)
Chemical purge after 35 days exposure to Isopropanol (saturated)
A
chemical
purge
corrects
the error
caused by
diffused
chemicals
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Vaisala’s Patented Warmed Probe
Technology
Vaisala HUMICAP® sensor
Heating element
inside the probe
Ambient: RHa, Ta , Tda
Conditions inside the
probe: RHs , Ts , Tda
Ambient:
Ta = 14 °C
Humidity sensor: Ts = 16 °C
RHa = 97 %RH
RHs = 83 %RH
Tda = 13 °C
Tda = 13 °C (calculated)
NOTE! The heating does not effect in dewpoint!
The ambient temperature is measured with separate T-probe!
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Comparison of Warmed and Non-Warmed
Sensor Heads in High Humidity Weather
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Key Takeaways
 HUMICAP®180R is the best sensor on the market with
excellent stability and good chemical tolerance
 Chemical purge can improve stability in applications where
chemicals are constantly present.
 Heated probe technology improves perfomance in
condensing conditions
Page 30 / 10-2011 / LES / ©Vaisala
5 Things to Remember
Relative humidity depends strongly on temperature
If you know temperature and one humidity parameter the
others can be calculated
HUMICAP®180R is the best sensor on the market with
excellent stability and good chemical tolerance
Heated probe technology improves perfomance in
condensing conditions
Chemical purge can improve stability in applications
where chemicals are constantly present.
Page 31 / 10-2011 / LES / ©Vaisala
Vaisala Technologies for Humidity/
Dewpoint Measurement
 Dewpoint is mostly used as a parameter in dry applications, but
can be used all over the humidity range
 Vaisala instruments:
 DRYCAP® technology for low humidity (below 10%RH)
 HUMICAP® technology for high humidity (over 10 %RH)
 Both DRYCAP® and HUMICAP® instruments may have RH and
Td/f outputs available. The choice of technology is not about
parameter but humidity range of the application
Humidity low
i.e. Td << T
Humidity high
i.e. Td close to T
DRYCAP
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HUMICAP
Vaisala DRYCAP® Sensor
 Polymer sensor - capacitance
signal
 Measures the ambient water
vapor
 Relative to the number of
absorbed water molecules
(RH= Pw/Pws )
 Temperature sensor resistance signal (Pt-100)
 Measures the sensor
temperature
 Td is the calculated result of
measured RH and T
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Polymer sensor
PT-100
Vaisala Autocalibration
 Vaisala DRYCAP® technology utilize autocalibration
procedure for accurate dewpoint measurement
 Autocalibration warms up the sensor automatically and
makes the offset (= zero point) correction:


1) guarantees the very good accuracy in low dewpoints
2) offers excellent long term stability
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Auto-calibration in detail
3. RH and T values logged during
cooling phase
4. When drawing a straight line
through the collected RH&T
points the offset correction value
is got as a result
5. Auto-calibration ends:
Sensor is cooled and normal
measurement mode gets active
again using the correction value
6. After set interval (once/hour)
auto-calibration repeated
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Sensor
temperature
Sensor
humidity
1. 2.
3.
humidity
increases
5.
6.
time
RH/T point before
warming
1.
RH/T points during
the cooling phase
Humidity
1. Auto-calibration start:
Sensor warmed
RH value decreases when T
increases
2. Sensor starts to cool down
after short warming.
RH reading starts to increase.
3.
RH/T point
right after
the warming
2.
= Humidity values
after offset
correction
low
humidity
0
T increases
4.
offset correction value
(RH is this much off, e.g. 0.01 %RH)
Temperature
low T
DRYCAP® instruments equipped with
intelligent functions
 Auto-calibration
 typically once an hour and when the instrument switched on
 offset correction (zero point correction)
 Sensor purge
 once a day and when the instrument switched on
 gain correction (wet point correction)
 Sensor head warming
 Long term stability
 2 years recommended calibration
interval
 in case of humidity > 70 %RH
 to avoid sensor wetting
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DRYCAP® sensor long term stability 55 months
Long term stability of VAISALA DRYCAP®180M
Dewpoint output reading of three randomly selected units
against reference dewpoint at -10, -40 and -60 °C Td
test results at +20 °C room temperature, no adjustments done for test units during the test
Dewpoint value
Reference Thunder humidity generator
0
-10
unit #1
-20
unit #2
2 years
4 years
unit #3
-30
Unit # 1 - DMT242
Stored in N2 gas
-40
Unit # 2 - DMT242
Stored on room table
-50
Unit # 3 - DMT242
Stored on room table
-60
accuracy
specification
±2°C Td / 2 yrs
-70
0
5
10
15
20
25
30
35
time units tested / months
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40
45
50
55
Drycap recovery from liquid water exposure
Dew point reading, -40 Tfp --> immersed in water 1 hour -- > -40 Tfp
-10
-15
immersed
-20
['C]
-25
-30
-35
-40
-45
0
0.5
1
1.5
2
time [h]
Page 38 / 10-2011 / LES / ©Vaisala
2.5
3
3.5
4
DRYCAP® vs. AlOx sensor
DRYCAP® sensor
AlOx sensor
Excellent long term stability
Tendency to drift
Fast Response time
- Necessary especially in dewpoint dependent switching
First reaction quick, but slow to stabilize
Durable against high humidity and liquid water
- no need to protect sensor from ambient humidity or water
spikes
Loss of calibration when exposed to high humidity
Selection of transmitters and sensors available
- Optimum sensors for all dryers
One old sensor technology available
Easy field checks
- Compatibility with the hand-held DM70 for easy field checks
Typically no field check possibility
Low cost of ownership
- Long calibration interval
High lifetime cost
- service costs high vs. purchase price
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Adventages
 Excellent long term stability: Accuracy
specification includes 2 year drift.
 Fast Response time: Essential especially in
dewpoint dependent switching.
 Durable against high humidity and liquid water:
no need to protect sensor from ambient
humidity or water spikes.
 Due to the sensor purge function, durable
against typical chemical contaminants & air
pollutants often present at industrial sites.
 Selection of transmitters and sensors available:
Optimum sensors for all dryers/ systems.
 Easy verification on-site: Compatibility with the
hand-held DM70 for easy field checks
 Low cost of ownership: Long calibration interval
Page 40 / 10-2011 / LES / ©Vaisala
Sampling cells
DMT242SC
•
G3/8”, G1/4” ISO (inlet,
outlet)
•
G1/2” for probe
DMT242SC2
•
Like above, but with 1/4”
swagelok fittings
DSC74C
•
For pressurized air, quick
connector and leak screw
•
Pressurized measurement
DSC74C
•
•
For measurement in
atmospheric pressure
Fixed leak screw 3 ..1 0
bar
DMCOIL
Page 41 / 10-2011 / LES / ©Vaisala
•
For venting the sample
out in low dew point
DMT348 - For Pressurized Pipelines
• Durability up to 40 bar (580 psi)
• Also long 400 mm probe available
• 2 m, 5, 10 and 15 m probe cable
• Quick and easy installation
in pressurized pipelines
• A selection of installation accessories:
• Sampling cells
• one with thread connection
• one with Swagelok connectors
• Ball valve
• Duct installation flange
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filter options:
DMT342 - With Small Size Flanged Probe
for use with sampling cell
• Durability with specified accuracy up to 50 bar (725 psia)
• Mechanical durability up to 250 bar (3625 psi)
• 2 m, 5 m and 10 m probe cable
• Excellent for high pressure processes
that require small size probe
• Direct installation to process
or using sampling cell (HMP302SC)
filter options:
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Hand-Held Devices as Spot Check and
Field Reference Instruments
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4
12/1
[Name]
Calibration
Page 45 / 10-2011 / LES / ©Vaisala
HM70 as Field Reference Instrument
 MI70 indicator has two probe ports:
 One for reference probe (e.g. HMP75)
 Other can be simultaneously used for the
fixed transmitter to be checked
– depending on the transmitter, the
adjustment can be done with the
MI70 interface, transmitters push
buttons or trimmers
– transmitter settings (e.g. pressure)
Page 46 / 10-2011 / LES / ©Vaisala
4
12/1
[Name]
Transmitter not working?
 Clipping to min / max
 Frozen value
 Fluctuating too rapidly
 Impossible values (sub-zero with refrigerant etc)
Td
Td1 --43,5°C
43,5°C
MI70
Td2- 43,3°C
DMT152
dewpoint
transmitt
er
 Easy checking with DM70
 Display and datalogger
 Data link verification
 Reference measurement
 Powers the ref unit
Page 47 / 10-2011 / LES / ©Vaisala
Dryer
control
hand-held meter
Referenc
e
DMT152
dewpoint
transmitt
er
SEE YOU IN THE NEXT
APPLICATION .