The Pt100 temperature sensors are very common sensors in the process industry. This article discusses many useful and practical things to know about the Pt100 sensors. There’s information on RTD and PRT sensors, different Pt100 mechanical structures, temperature-resistance relationship, temperature coefficients, accuracy classes and on many more
The abbreviation RTD comes from “Resistance Temperature Detector.” It is a temperature sensor in which the resistance depends on temperature; when temperature changes, the sensor’s resistance changes. So, by measuring the sensor’s resistance, an RTD sensor can be used to measure temperature. RTD sensors are most commonly made from platinum, copper, nickel alloys or various metal oxides
Platinum is the most common material for RTD sensors. Platinum has a reliable, repeatable and linear temperature resistance relationship. RTD sensors made of platinum are called PRT, “Platinum Resistance Thermometer.” The most common platinum PRT sensor used in the process industry is the Pt100 sensor. The number “100” in the name indicates that is has a resistance of 100 ohms in 0 °C (32 °F) temperature
Measuring RTD/PRT sensor
There are different ways to measure resistance. You can use a 2, 3 or 4 wire connection. The 2-wire connection is only suitable for very low accuracy measurement because any wire resistance or connection resistance will introduce error to the measurement. Any normal process measurement should be done using 3 or 4 wire measurement. For example, the IEC 60751 standard specifies that any sensor better than accuracy class B must be measured with a 3 or 4 wire measurement.
The most common RTD sensor in process industry is the Pt100 sensor, which has a resistance of 100 ohms at 0 °C (32 °F). The resistance at higher temperatures depends on the version of the Pt100 sensor, as there are a few different versions of the Pt100 sensor, which have slightly different temperature coefficients. Globally, the most common is the “385” version. If the coefficient is not mentioned, it is typically the 385. The temperature coefficient of the Pt100 sensor is indicated as the difference of the resistance at 100 °C and 0 °C, divided by the resistance at 0 °C multiplied with 100 °C.
a = temperature coefficient
R100 = resistance at 100 °C
R0 = resistance at 0 °C
Pt100 (385) temperature resistance relationship
In the graphics below, you can see how a Pt100 (385) sensor’s resistance depends on temperature:
When looking at these, you can see that the resistance temperature relationship of a Pt100 sensor is not perfectly linear, but the relationship is somewhat “curved.”
Pt100 accuracy (tolerance) classes
Pt100 sensors are available in different accuracy classes. The most common accuracy classes are AA, A, B and C which are defined in the IEC 60751 standard. As Pt100 sensors cannot be adjusted to compensate for errors, you should buy a sensor with a suitable accuracy for the application. Accuracies of the different accuracy classes (per IEC 60751:2008):