Platinum Resistance Thermometers (PRT)

Characteristics

A platinum resistance thermometer (PRT) is a thermometer constructed from a high purity platinum element (wire-wound coil or thin film) placed in a tube of metal or glass and sealed with an inert atmosphere and/or mineral insulator. Two, three, or four leads are connected to the element and are used to provide for the measurement of the electrical resistance of the element. Some of these characteristics are:

Wide temperature Range (–260°C to 1000°C)
Electrical resistance is typically between 0Ω and 400Ω and depends on temperature
Excitation current is typically 1mA
Stable over time
Stable over temperature
Shallow slope (i.e. 0.4Ω/°C for a 100Ω PRT)
Relatively easy to measure
Relatively easy to calibrate
Commercially available in many configurations

The list above shows that PRTs are suitable for use over a wide temperature range. This is true, but actual design and construction will differ in instruments intended for different ranges. No single instrument will be suitable for use over the entire range shown above. In calibration, the electrical resistance is measured at several temperature points and fitted to a mathematical expression. The number of calibration points depends on the range and accuracy desired but, because the temperature response of platinum is relatively linear and very well known, fewer calibration points are required for a given range compared to other sensor types. Also, because of the shallow slope, the readout used for the resistance measurement need not have a large range. PRTs, like any probe, have immersion requirements which vary from configuration to configuration. Often, the required immersion is not stated or specified. Since PRTs are used in so many different applications, we are presented with a large variety of shapes, sizes, and types. Although the basic calibration requirements are the same, these various configurations pose different problems in the laboratory. We must solve these problems satisfactorily to provide a proper calibration. Therefore, we must understand the requirements to an extent that allows us to adapt our process, if necessary, to accommodate a new or unusual configuration.

Instruments, Standards, and Apparatus

Calibration is performed by measurement of the resistance of the unit under test (UUT) while it is exposed to a temperature. Fundamentally, four instruments are required as follows:

Reference probe
Readout for the reference
Readout for the UUT
Temperature source

Reference Probe

Since the temperature is determined by this instrument for our calibration, its accuracy and stability are of paramount importance.

Standard Platinum Resistance Thermometers (SPRTs)

SPRTs (Standard Platinum Resistance Thermometers) are the most accurate and stable instruments available for this purpose. They are generally available in 0.25, 2.5, 25, and 100Ω versions with either borosilicate glass (Pyrex), fused silica glass (quartz), stainless steel, or INCONEL sheath materials. The different resistance values and different sheath materials are intended for different temperature ranges. A typical quartz sheathed 25Ω SPRT will have a temperature range of –200°C to 660°C and with a high quality calibration will have calibration uncertainties from 0.001°C to 0.010°C. Additionally, since these instruments are actually part of the definition of the ITS-90, they are standardized. That is, there are minimum requirements for the purity of the platinum wire and the type of construction used. This results in less confusion as to the suitability of the instrument for a particular application and almost guaranteed good performance if calibrated and used correctly. These instruments are highly stable and accurate, but they are expensive and extremely delicate. They should be reserved for high accuracy applications only.

Platinum Resistance Thermometers (PRTs)

When accuracy requirements are less severe, PRTs can be used successfully. As mentioned, PRTs are available in many configurations, however PRTs which are suitable for use as calibration standards are generally available as 100Ω stainless steel sheathed probes. Historically, they have been limited to a temperature range of –200°C to 420°C but a new type has been introduced which has extended the upper limit to 1000°C. Calibration uncertainties range from 0.010°C to 0.025°C. These instruments are not as accurate as SPRTs but they are generally more rugged and easier to work with. Additionally, unlike SPRTs, the design of PRTs is left to the discretion and ingenuity of the manufacturer. Not all designs perform to the level required for use as a reference. Be careful in the selection of a PRT to ensure that the type selected is appropriate for use as a calibration reference over the range of interest and with the required accuracy.

SPRT	PRT
Capable of very high accuracy	Capable of moderate to high accuracy
Capable of large temperature range	Capable of large temperature range
Extremely stable	Very stable
Standardized	Not standardized
Relatively expensive to purchase	Relatively inexpensive to purchase
Relatively expensive to calibrate	Relatively inexpensive to calibrate
Extremely delicate	Less delicate

Thermometer Readout

When calibrating PRTs against a reference PRT or SPRT, the technical requirements for the readout are the same for the UUTs and the reference. If a switching system is available, one readout can usually be used for both. If the readout is designed for temperature calibration (not just temperature measurement) and has variable settings (current, timing, etc.), then certainly it can be used for both. If the readout is not designed for temperature calibration and/or a switching system is not available, then two or more readouts will probably be required. Before selecting a readout, review the information presented in the readouts section with regard to current settings, timing, multiplexing, etc. Best results will be obtained with readouts designed specifically for thermometer calibration. There are two important points to consider with regard to PRT and SPRT readouts which bear repeating:

Ensure that the readout has a resistance range appropriate for the reference probe and UUTs for which it is intended. Over the range of –200 to 660°C, a 25Ω SPRT will vary in resistance from approximately 4.6Ω to 84.4Ω, a 100Ω PRT from approximately 18Ω to 338Ω. This will usually require 2 or 3 range changes for typical DMMs (10Ω, 100Ω, and 1kΩ ranges). Many modern thermometer readouts are designed to cover this span on a single range. Changing ranges can cause discontinuities in the math fit (the equations are intended to fit platinum, not DMM range offsets or gain errors).
Ensure that the readout is using the proper source current. Too much source current will result in excessive self-heating and incorrect calibration. In some cases, particularly with older DMMs, the source current is so high that damage to the sensor is likely. Additionally, some DMMs use unconventional values of source current such as decades of 2 or 3 rather than 1 (2 mA or 3 mA, not 1 mA). Most certainly, these values of current are not reproduced during calibration of the reference or use of the UUT. Moreover, if the readout is a DMM which requires range changes as mentioned above, the source current will change with the range, meaning different current values for measurements at different temperatures. This will result in inconsistent self-heating and additional calibration errors.

Temperature Source

The most common temperature sources for PRT calibration are dry-wells (dry block calibrators) and calibration baths. Dry-wells are used in applications where probe consistency (diameter and length) is present and modest accuracy is desired. When probes of different shapes and sizes must be accommodated, or higher accuracy is required, calibration baths should be utilized. For the lowest temperatures (below –100°C) use an LN2 comparison device and for the highest temperatures (above 500°C) use a calibration furnace.

The two most important considerations are uniformity and stability. Immersion depth is also an issue. Insufficient immersion depth will result in calibration errors. Additionally, if the reference probe is a glass sheath SPRT, then some form of protection should be used at higher temperatures to prevent devitrification of the glass sheath and contamination of the platinum sensor. This is a concern particularly with dry-wells and furnaces at temperatures above 400°C and with calibration baths that use liquid salt as the bath fluid.

Calibration of short UUTs presents many problems with regard to the temperature source. The probe must be immersed sufficiently without subjecting the transition junction (where the leads join the probe) to extreme temperatures. Often dry-well temperature sources are a better solution in these situations. Some calibration baths have fluid level adapters which actually raise the fluid up to the top of the bath lid. These adapters can also be used successfully in the calibration of short probes. Whatever type of temperature source is used, the most important consideration is the application itself. Even an excellent instrument may not perform adequately in a specific application if it is not matched to that application. Carefully evaluate the requirements before selecting the temperature source to ensure a good fit.