Fluke Calibration
W b Seminar
Web
S i
Series
S i
Principles
p
and p
practical tips
p
about electrical, flow, pressure,
RF and temperature calibration
How to Calibrate an RTD
or Platinum Resistance
Thermometer
©2010 Fluke Corporation.
Common Calibration Techniques
Introduction
• Thermometers are transducers
• Exhibit a change in a characteristic which is
proportional to the change in temperature
• Fundamentally,
F d
t ll calibration
lib ti iis characterization
h
t i ti off thi
this
relationship
PRTs - Characteristics
• Medium to high accuracy
• High purity or doped high purity
platinum sensor
• Wire
Wi wound
d or thi
thin fil
film
construction
a yd
different
e e types
ypes a
and
d sstyles
y es
• Many
of assembly
• Very common in industry
Photo courtesy of RTD Co.
PRTs - Characteristics
• Temperature range (-260 to 1000
°C)
• Stable over time and temperature
• Well
W ll d
defined
fi d mathematically
th
ti ll
• Relatively linear with low sensitivity
• Relatively easy to measure &
calibrate
• Available in many configurations
Photo courtesy of
Burns Engineering
Instruments-Standards-Apparatus
• Reference Probe
• Readout for the reference
• Readout for the UUT (unit
under test)
• Temperature source
Reference Probes - Types
• SPRTs
–
–
–
–
–
Standardized
-200 to 1000 °C range
0.25, 2.5, 25.5 Ohm versions
Highly stable & accurate
Typical uncertainties from
0.001 to 0.010 °C
– Expensive & fragile
• PRTs
–
–
–
–
–
Not standardized
-200 to 1000 °C range
100 ohm typical
Quite stable and accurate
Typical uncertainties from
0.010 to 0.025 °C
– Less expensive & less fragile
than SPRTs
Readouts - General Requirements
• DMMs provide moderate results
• Readouts designed for temperature measurement
provide better results
• Readouts designed for temperature calibration
provide best results
• Switch or multiplexer increases efficiency of
measurement system
Readouts - PRTs
•E
Ensure th
thatt the
th resistance
i t
range is
i appropriate
i t ffor
temperature range of interest
– 25  SPRTs and PRTs from  4 to 85  ((-200 °C to 660
°C)
– 100  PRTs from  18 to 340  (-200 °C to 660 °C)
Readouts – excitation current
•E
Ensure that
th t the
th readout
d t is
i using
i th
the proper source
current - too much source current will cause excessive
self-heating
g and calibration errors
– Range changes in DMMs cause inconsistent self-heating
• SPRTs -1 mA (25 ), 5 mA (2.5 ), 14 mA (0.25 )
• PRTs - 1 mA is recommended
Temp Sources - Requirements
• Stability & uniformity consistent with desired uncertainty
(10:1 recommended by NVLAP)
• Temperature range appropriate for range desired
• Temperature sources designed for temperature
calibration provide best results
Temperature Sources - Types
• Dry Wells
–
–
–
–
–
–
Moderate accuracy
Fixed hole diameter
Fi d iimmersion
Fixed
i d
depth
h
Dry and clean
Portable
Faster temperature
changes
– Internal reference probe
• Calibration Baths
– High accuracy
– Flexible with immersion depth
and probe diameter
– Can be messy
– Not usually portable
– Slower temperature changes
– Requires external reference
probe
Temperature Sources - Types
• Dry-blocks for higher temperatures
(above ~500
°C)
• LN2 comparison device or variable cryostat for lower
temperatures (below -100 °C)
Procedures - PRTs
• Characterization
Ch
t i ti
– UUT resistance is
measured at several
temperature points over a
range and the data is fitted
to a mathematical formula
(mathematical model).
model)
• Tolerance
T l
T
Testing
ti
– UUT resistance is
measured at several
temperature points and
the data is compared to
defined values at those
temperatures No fitting is
temperatures.
performed.
Procedures - Characterization
•M
Mostt common approach
h to
t PRT calibration
lib ti
• The resistance vs. temperature relationship is redetermined with each calibration
• Usually, calibration coefficients and an interpolation
table are provided as a product of the calibration
Procedures - Characterization
•C
Connect to readout
• Insert the reference and UUT(s)
into the temperature source in
close proximity
• Measure the reference probe
and determine the temperature
• Measure and record the UUT
resistance
• Fit the data
Procedures - Characterization
P b placement
Probe
l
t
• Circular pattern with reference in center
• Sufficient immersion
(20*) x (probe diameter) + (sensor length)
i.e., 20 x 3/16” + 1” = 3.75” + 1” = 4.75”
* Note: 15X and 20X are often used with some small
uncertainty due to immersion error. When practical,
immersion of 30X essentially eliminates all error from
this component.
Procedures - Characterization
C
Connection
ti tto readout
d t
• Proper 2, 3, or 4 wire configuration
• Insure that connections are tight
Procedures - Characterization
R f
Reference
probe
b measurementt
• Measure temperature directly
• Measure resistance and convert to temperature (linear
interpolation)
Procedures - Characterization
Single DMM Method
Procedures - Characterization
D l DMM M
Dual
Method
th d
Procedures - Characterization
Thermometer Readout Method
Procedures - Characterization
D t Fitting
Data
Fitti
• PRTs
– ITS-90
– Callendar Van Dusen
– polynomials
ITS-90 Equations
Resistance Ratio
W(T90 ) 
R(T90 )
R TPW
ITS-90 reference function (above zero)
 (T  754.15) 
Wr (T90 )  C 0   Ci   90

481


i 1
9
i
Deviation from ITS-90 reference function
 W(T90 )  W(T90 )  Wr (T90 )
http://www.bipm.org/en/publications/its-90.html
ITS-90 Characterization
Deviations from the reference function are characterized
using calibration coefficients (a,b,c) and the deviation
functions below:
• Above zero (0 °C to 660.323 °C)
 W(T90 )  a  (W(T90 )  1)  b  (W(T90 )  1) 2  c  (W(T90 )  1) 3
• Below zero (range 5: –38
38.8344
8344 °C
C to 29
29.7646
7646 °C)
C)
 W(T90 )  a4  (W(T90 )  1)  b4  (W(T90 )  1)  ln(W(T90 ))
 W(T90 )  a5  (W(T90 )  1)  b5  (W(T90 )  1) 2
Example Data Set
4 points + Rtpw
Temperature
Measured 
0.010°C
99.96653
156.599°C
160.89476
231.928°C
189.16982
300.000°C
°C
214.15407
419.527°C
256.72668
Calculate Coefficients using ITS90 Deviation Functions
S t off equations
Set
ti
W
T1
WT2
WT3
W
T 4
a ( W
 a ( W
 a ( W
 a ( W

T1
 1)  b (W
T 2
 1)  b (W T 2  1) 2
T 3
 1)  b (W T 3  1) 2
T 4
 1)  b (W
T1
T 4
 1) 2
 1) 2




Solving for a and b using example data:
a = -5.3581671E-04
b = 2.0307049E-05
Procedures - Characterization
PRTs: Polynomials
typical expressions take the form:
t  a  b R c R2  d R3  e R4
Electrical Properties of Platinum
Thermometers
• Callendar-Van Dusen equation describes the R vs t
relationship of platinum (C=0 when t<0)

R(t)  R0C 1 At  Bt2  C1100t 3


t 
 t 
R(t)  R0C 1t  1 
100 100

Precision PRT (α=392) Industrial PRT (α=385)
A
3.985 X 10-3 °C-1
3.908 X 10-3 °C-1
B
-5.870
5 870 X 10-77 °C
C-22
-5.775
5 775 X 10-77 °C
C-22
C
-4.000 X 10-12 °C-4
-4.183 X 10-12 °C-4
Procedures - Tolerance Testing
• Typical approach for
medium to low accuracy
and
d iindustrial
d t i l applications
li ti
• Resistance at temperature
T is compared to defined
(table) values
• Usually
Usually, DIN,
DIN IEC
IEC-751
751, or
ASTM 1137 defined
equations are used
Procedures - Tolerance Testing
• ASTM 1137 class A
= ±[0.13 + 0.0017|t|]°C
@ 100 °C = [0.13 + 0.0017|100|] = ±0.30 °C
• ASTM 1137 class B
= ±[0.25
[0 2 + 0
0.0042|t|]°C
0042| |]°C
@ 100 °C = [0.25 + 0.0042|100|] = ±0.67 °C
Example System for Industrial
Tolerance Testing
• Basic
B i system
t
– Fluke-744 Documenting Process
Calibrator
– Hart Field Dry-Well
y
• 9103 for -25°C to 140°C
• 9141 for 50°C to 650°C
• Approximate system uncertainty
±0 6°C with
±0.6°C
ith 9141 or ±0.4°C
±0 4°C with
ith
9103 (rss method)
– 744 PRT measurement accuracy
±0.3°C
– 9141 source accuracy ±0.5°C @
400°C
– 9103 source accuracy ±0.25°C
Example System for Precision
PRTs
• Basic System
– 1529 CHUB-E4 Readout
– 5626 Secondary SPRT
– Deep Immersion Compact
Bath
– 9938 MET/TEMP II
Software
• Approximate uncertainty
±0 03°C
±0.03°C
– 1529 CHUB ±0.012°C @
200°C
– 5626 SPRT ±0.009°C @
420°C
°
– 6331 combined stability and
uniformity ±0.03°C @
300°C
Example System for SPRTs
• Basic equipment
–
–
–
–
–
1590 SuperThermometer
5681 Quartz SPRT
9114 Freeze-Point Furnace
590X Fixed Point Cells
9938 MET/TEMP II
Software
• Approximate uncertainty
±3mK to ±5mK
– 1590 ±1.5mK
– 5681 SPRT ±1mK
– 5906 Zinc Point ±1mK
Where can I find out more?
• Application Note
• Fluke Hart Scientific Division Catalog
• http://www.bipm.org/en/publications/its-90.html
Th k you.
Thank
For information about other web seminars in this series,
i l di previously
including
i
l recorded
d d web
b seminars,
i
visit:
i it
www.fluke.com/calwebsem
Fluke also offers in-depth training courses in calibration
and metrology.
metrology For class descriptions,
descriptions schedules
schedules, and
registration, visit:
www.fluke.com/caltraining
Be the first to know. Sign up for Fluke Calibration
e-news bulletins, and the quarterly Total Solutions in
Calibration newsletter:
www.fluke.com/signmeup
©2010 Fluke Corporation.