Should I Use an RTD or a Thermocouple?

The two most common ways of measuring industrial temperatures are with resistance temperature detectors (RTDs) and thermocouples. But when should engineers use a thermocouple and when should they use an RTD? The answer is usually determined by four factors: temperature, time, size, and overall accuracy requirements.

1. Temperature

What are the temperature requirements? If process temperatures range from -200 to 500°C, then an industrial RTD is an option. But for extremely high temperatures, a thermocouple may be the only choice.

2. Time

What are the time-response requirements? If the process requires a very fast response to temperature changes – fractions of a second as opposed to seconds (i.e. 2.5 to 10 sec) – then a thermocouple is the best choice. Keep in mind that time response is measured by immersing the sensor in water moving at 3 ft/sec with a 63.2% step change.

3. Size

What are the size requirements? A standard RTD sheath is 3.175 to 6.35 mm dia, while sheath diameters for thermocouples can be less than 1.57 mm.

4. Accuracy

What are the overall requirements for accuracy? If the process only requires a tolerance of 2°C or greater, then a thermocouple is appropriate. If the process needs less than 2°C tolerance, then an RTD is the only choice. Keep in mind, unlike RTDs that can maintain stability for many years, thermocouples can drift within the first few hours of use.