How to calibrate an RTD ?

By | February 29, 2016

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 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.

 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.

 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


 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 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.


 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.

• 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 temperature 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. 

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.


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