Fluke 5622 Application Note
How to calibrate an RTD
or Platinum Resistance
Thermometer (PRT)
Topics Covered
●
Calibration by characterization
ITS-90
Callendar-Van Dusen
Polynomial
●
Tolerance testing
ASTM 1137
IEC 60751
Application Note
Introduction
There are two types of calibrations applicable to PRTs—characterization and tolerance
testing. The type of calibration to perform is determined by the way in which the UUT is
to be used and the accuracy required by the user. Characterization is the type of calibration in which the unit under test (UUT) resistance is determined at several temperature
points and the data are fitted to a mathematical expression. Tolerance testing on the
other hand is a calibration in which the UUT resistance is compared to defined values at
specific temperatures. No data fitting is performed. In the laboratory, we are required to
perform both types of calibration depending upon our customer’s needs.
Calibration Procedures
Characterization
Characterization is the method that is most often used for medium to high accuracy PRT
calibration. With this method, a new resistance vs. temperature relationship is determined anew with each calibration. Generally, with this type of calibration, new calibration coefficients and a calibration table are provided as a product of the calibration.
There are five basic steps to perform as listed below:
1. Place the reference probe and the UUTs in the temperature source in close proximity to one another.
2. Connect the leads to the readout(s) ensuring proper 2-, 3-, or 4-wire connection.
3. Measure the reference probe and determine the temperature.
4. Measure and record the resistance of the UUT(s).
5. Fit the data.
Some readouts simplify the technique by combining or eliminating some of the steps.
In the following discussion, we will consider an application involving PRT characterization by comparison to an SPRT.
Step 1: Probe Placement
All temperature sources have instabilities and gradients. These translate into calibration errors and/or uncertainties. To minimize the effects, the probes should be placed
as close together as practical. In baths the probes to be calibrated should be placed
in a radial pattern with the reference probe in the center (focus) of the circle. This
ensures an equal distance from the reference probe to each of the UUTs. In dry-well
temperature sources, the reference probe and probes to be calibrated should all be
placed the same distance from the center for best results, but the reference may be
placed in the center if needed.
Also, the sensing elements should be on the same horizontal plane. Even though
sensing elements are different lengths, having the bottoms of the probes at the same
level is sufficient. Sufficient immersion must be achieved so that stem losses do not
occur. Generally, sufficient immersion is achieved when the probes are immersed
to a depth equal to 20 times the probe diameter plus the length of the sensing element. For example, consider a 3/16 inch diameter probe with a 1 inch long sensing
element. Using the rule of thumb, 20 x 3/16 in + 1 in = 3 3/4 in + 1 in = 4 3/4 in.
In this example, minimum immersion is achieved at 4 3/4 inches. This rule of thumb
is generally correct with thin wall probe construction and in situations of good heat
transfer. If the probe has thick wall construction and/or poor heat transfer is present
(such as in the case of a dry-well with incorrectly sized holes), more immersion is
required.
F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y
Step 2: Connection to Readout
This step is straightforward. Connections must be
tight and in proper 2-, 3-, or 4-wire configuration. If
using 4-wire configuration, ensure that the current
and voltage connections are correct. See Figure 1.
temperature data is available in real time. Some
modern readouts also display the data in graphical
format, allowing the operator to determine stability at a glance. Both of these features speed up the
process and eliminate possible operator error due to
incorrect table interpolation.
The second method is used when the readout
does not provide for proper temperature calculation.
(Some readouts, particularly DMMs, have some of
the more popular temperature conversions built in.
These typically do not allow use of unique calibration coefficients and cannot be used for accurate
temperature calibration.) In this case, the resistance
is measured and the temperature is determined
from either a calibration table or from a computer or
calculator program.
Since the temperature must be calculated after
the resistance is measured, the process is slower
and does not provide immediate, real time temperature data. See Tables 1 and 2 below.
Table 1. Interpolation from an RTD calibration table
(resistance vs. temperature).
t( °C) R(t) (W) dR/dt(t) W/°C
400 249.8820 0.3514
401 250.2335 0.3513
402 250.5848 0.3512
403 250.9360 0.3511
450 267.3108 0.3456
451 267.6564 0.3455
452 268.0019 0.3454
453 268.3472 0.3452
Figure 1. Thermometer readout connection schematics
Step 3: Measurement of Reference Probe
and Temperature Determination
There are two ways to measure the reference probe
and determine the temperature. Both techniques
have the same potential accuracy. That is, if done
correctly, neither technique is inherently more
accurate than the other.
The first and best method is used with sophisticated readouts designed for temperature work.
The resistance is measured and the temperature
calculated from calibration coefficients which were
entered into the readout previously. Once these
calibration coefficients have been entered, the temperature calculations are accomplished internally
and the readout displays in temperature units. The
1. Measure the reference
probe resistance
2. Locate where it falls on
the table
3. Subtract lower table value
from measured value
4. Divide by dR/dT(t) (slope
of curve)
5. Add fractional
temperature to table value
249.9071 W
between 249.8820 W and
250.2335 W
249.9071 W – 249.8820 W =
0.0251 W
0.0251 / 0.3514 = 0.0714 °C
0.0714 ’C + 400 =
400.0714 °C
Tech Tip
Manual calculation is more
prone to human error and
is more time consuming than using a readout
designed for temperature
work.
2 Fluke Corporation, Hart Scientific Division How to calibrate an RTD or Platinum Resistance Thermometer (PRT)
Table 2. Interpolation from an RTD calibration
(resistance ratio (W) table).
t( °C) W(t) dt/dW(t)
300 2.1429223 275.2199
301 2.1465557 275.3075
302 2.1501880 275.3951
303 2.1538192 275.4827
350 2.3231801 279.6655
351 2.3267558 279.7559
352 2.3303304 279.8464
353 2.3339037 279.9369
1. Measure reference probe
resistance
2. Calculate W (Rt/R
= 25.54964)
3. Locate where it falls on
the table
4. Subtract lower table value
from measured value
5. Multiply by dt/dW(t)
(inverse slope of curve)
6. 6) Add fractional
temperature to table value
) (R
tpw
54.75258 W
54.75258 W / 25.54964 W =
tpw
2.1429883
between 2.1429223 and
2.1465557
2.1429883 – 2.1429223 =
0.000066
0.000066 • 275.2199 =
0.0182 °C
0.01821 °C + 300 °C =
300.0182 °C
Number of readouts - will the reference probe
•
and UUTs be measured with the same readout or
different readouts?
Type of readout - a readout designed for temper-
•
ature calibration often has features which allow
flexibility in the measurement scheme.
UUT characteristics - self-heating time, source
•
current requirements, stability, and overall quality influence the measurement process.
It is not possible for us to anticipate all of the
variables and discuss the optimum solutions here.
However, in the following examples, we will consider some typical calibration scenarios and suggested measurement schemes.
Example 1: 2 DMM readouts, 1 reference
probe and 5 UUTs
Step 4: Measurement of Units Under Test
(UUTs)
Since the UUTs are resistance thermometers similar to the reference probe, they are measured in
a similar manner. If several UUTs are undergoing
calibration, ensure that when they are connected
or switched in, sufficient time is allowed for selfheating to occur before the data is recorded. Also,
ensure that the readout is set to the correct range
to provide the proper source current and to prevent
range changes between the measurements at different temperatures. Typically, the measurements
are conducted starting at the highest temperature
of calibration and working down. Additionally, it
increases the precision of the calibration to use
a mean (average) value calculated from multiple
measurements at the same temperature. Often, the
readout is designed with statistical features to facilitate this practice. It is also a good practice to close
the process with an additional measurement of the
reference probe. The sequence in which the probes
(reference and UUT) are measured is referred to as a
measurement scheme. There are many variables to
consider when designing a measurement scheme.
Some points to consider are:
Accuracy - the higher the accuracy desired, the
•
more all of the following must be considered.
Temperature source stability - the more stable
•
the source, the more time exists to conduct
the measurements before temperature changes
cause unwanted error.
Number of UUTs - the higher the number, the
•
longer it takes to cycle through all UUTs.
The reference probe is connected to one readout
and the first UUT is connected to the second readout. This places the probes to be measured under
current at all times, thus, eliminating self-heating
errors caused by changing current conditions. The
UUTs will be connected and measured individually.
The scheme is as follows:
REF(1)-UUT (1) - REF(2)-UUT (2) - REF(3)-UUT (3) -
REF(4)-UUT (4) - REF(5)-UUT (5)
This provides 5 readings each of the reference
and the UUT. Take the average of the readings and
use it for the data fit. If the reference probe readings are in resistance, the temperature will have to
be computed. After completion, repeat the process
for the additional UUTs.
Fluke Corporation, Hart Scientific Division How to calibrate an RTD or Platinum Resistance Thermometer (PRT) 3
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