What is a PRT?

A Platinum Resistance Thermometer (PRT) is a type of temperature sensor that relies on the predictable change in electrical resistance of platinum as a function of temperature. Platinum is chosen for its stability, repeatability, and wide temperature range. PRTs are known for their accuracy and are commonly used in laboratory and industrial environments where precise temperature measurement is essential.  The stability of SPRT’s and PRT’s do not make them more accurate out of the box, but it does allow for individual characterization of probes using ITS-90 coefficients.  The customized coefficients allow for an accuracy unmatched by of-the-shelf RTD’s.

What is an RTD?

A Resistance Temperature Detector (RTD) is a broader category of temperature sensors that also function based on the principle that the resistance of a material changes with temperature. While PRTs are a specific type of RTD that use platinum as the sensing element, RTDs can be made from other materials such as nickel or copper. RTDs are widely used across various industries due to their reliability and ease of use.  Coefficients for standard RTD’s are set by the material of the wire and are not changed from year to year during the calibration like PRT’s.

Let's Talk About Key Differences:

While PRTs are technically a subset of RTDs, they differ significantly in their application, performance, and construction:

1. Material and Stability: PRTs use platinum, which offers superior stability and a wider temperature range compared to other materials like nickel or copper, commonly found in general RTDs.

1. Accuracy: PRTs are often more accurate than other types of RTDs. This makes them ideal for applications requiring high-precision temperature measurements, such as in laboratories or quality control processes, or within commercial calibration houses.

1. Temperature Range: PRTs typically operate over a broader temperature range, from -200°C to 850°C, whereas other RTDs may have more limited ranges.

1. Cost: Due to the use of platinum and precise construction of the probe, PRTs are generally more expensive than other RTDs, but the investment is justified by their accuracy and stability in critical applications.  RTD’s present a much more cost effective solution for applications that may not require precision under 100 mK, such as oven or process controllers.

Calibration is the process of comparing a sensor’s output against a known standard to ensure accuracy. However, the calibration process for a PRT should not be approached the same way as for a general RTD.

1. Calibration of PRT’s

Calibration of a PRT/SPRT is required to determine drift from current ITS-90 coefficients and assign new coefficients.  This is accomplished by comparison to ITS-90 fixed points through the range of the PRT, or occasionally by comparison to a Primary SPRT.  Precision resistance measurements of the probe are made after the PRT has stabilized at an ITS-90 fixed point.  These resistance values are then used to provide AS FOUND data using the previous coefficient and to provide AS LEFT data using the newly calculated coefficients. Utilization of ITS-90 fixed points is essential to minimize measurement uncertainty and mitigate measurement risk in your process.

1. Calibration of RTD’s

Calibration of an RTD is required to ensure the probe has not drifted since the last calibration.  Unlike PRT’s, new coefficient calculation is not required because an RTD maintains the same standard coefficients throughout its lifetime.  ITS-90 fixed points are not normally utilized for calibrations as comparisons to a PRT in baths or drywells will provide a comfortable TUR.  An ohmmeter is used to collect the resistance values at test points throughout the range of the RTD.  These resistance values are translated to temperature using calculations or tables using the coefficients corresponding to the material of the RTD wire.

3    Temperature Range Considerations: Since PRTs cover a wider temperature range, calibration must account for the sensor's performance across the entire range. This can be more complex than the calibration of a standard RTD, which might only require validation over a narrower range.  In purchasing a sensor appropriate for the range you require, you can avoid additional cost and maintenance by not calibrating a probe above and below the range you need.

4    Calibration Equipment: The equipment used to calibrate PRTs must be capable of matching the sensor's high accuracy.  Fixed point and resistance bridges provide the stability and accuracy required to set your PRT coefficients, maintain your TUR, and minimize your risk of measurement errors in your process.  Standard RTD calibration equipment includes baths, drywells, and ohmmeters.  This equipment, while not as accurate as fixed-point cells and resistance bridges, offers accuracy sufficient to calibrate your RTD.  While it varies, the accuracy of an RTD may be about ten times worse than that of a PRT.

5    Long-Term Stability: PRTs are chosen for applications where long-term stability is crucial. Regular calibrations and ITS-90 coefficient characterizations are critical to monitor and maintain the accuracy you need from your PRT.  Additionally, regular checks such as a Triple Point of Water reading can be instituted to monitor PRT drift between schedule calibrations to minimize process risk.

In Conclusion - Don't treat a PRT like a normal RTD:

Understanding the differences between PRTs and RTDs is essential for anyone involved in temperature measurements that matter. While both sensors operate on similar principles, their applications and calibration requirements can vary significantly. By recognizing these differences and ensuring that PRTs are calibrated with the appropriate methods and equipment, you can achieve the accuracy and reliability your process demands. For more detailed information on selecting and calibrating the right temperature sensor for your needs, just let us know. We’re here to help you make informed decisions that ensure the precision and success of your operations.

For PRTs & SPRTs, we focus on utilizing ITS-90 Fixed Point Cells to provide a higher precision temperature calibration service to our friends in the industry.  If you're looking for a professional, accredited laboratory to help with PRT & SPRT calibrations, take a look at us here:

ISO / IEC 17025 Calibration Service for PRTs & SPRTS