RTDs (resistance temperature detectors) are one of the most common temperature sensor types used in industrial applications. Thermocouples and thermistors are popular temperature sensors as well, but RTD sensors are more accurate over a wide temperature range and more stable over time, making them an excellent choice for many applications.
An RTD sensor is essentially a resistor whose resistance value increases with temperature. Due to the predictable change in resistance of certain materials as temperature changes, it is possible to acquire highly accurate and consistent temperature measurements. Most RTD sensors have a response time between 0.5 to 5 seconds or more. RTD sensors can be constructed with pure platinum, nickel or copper. RTDs made with platinum are also known as PRTs (platinum resistance thermometer) and are the most frequently used given their higher temperature capabilities, stability and repeatability.
Specifications for RTD sensors include a base resistance value and a temperature coefficient of resistance (TCR) value. Typical base resistance values can range from 10 to several thousands of Ohms (Ω) depending on material and type. The base resistance value indicates the nominal resistance of the sensor at 0°C (nickel and platinum) or 25°C (copper), with 100Ω being the most common.
The temperature coefficient of resistance does not affect a sensor’s accuracy, but is important to the measuring device that calculates changes in temperature based on the base resistance. PRTs have two standards of TCRs; the European standard (IEC 751) requires a TCR of 0.00385Ω/Ω/°C; and the American standard requires a TCR of 0.00392Ω/Ω/°C. Assuming a TCR of 0.00385Ω/Ω/°C — meaning that for every degree change in temperature, the resistance increases by 0.385Ω — a 100Ω PRT’s resistance will be 138.5Ω at 100°C. Thus, the measuring device used needs to be attuned to the TCR of an RTD sensor in order to accurately report changes in temperature, but the difference in TCR value has no impact on the sensor itself.
Between RTDs, thermocouples and thermistors, RTD sensors are the most accurate and stable over time, and are resistant to contamination under 660°C. They also boast high repeatability, which means that RTDs can accurately measure identical temperatures even when exposed to repeated heating and cooling cycles with minimal discrepancies. An RTD sensor will consistently measure 100°C after being put into an oven and subsequently a freezer multiple times. In contrast, a thermocouple is more likely to measure 100°C, then 98°C, then 103°C and so on when placed in the same situation. Since most applications do not require immediate responses (less than 0.5 to 5 seconds) to temperature changes, RTDs are an ideal solution for many industrial applications, which NTI includes in its product line of ENVIROMUX® Enterprise Environment Monitoring Systems and Accessories.