The two most common ways of measuring industrial temperatures are with resistance temperature detectors (RTDs) and thermocouples. But when should control 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.

What are the temperature requirements? If process temperatures fall from –328 to 932 °F (–200 to 500 8C), then an industrial RTD is an option. But for extremely high temperatures, a thermocouple may be the only choice.

• 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.
• What are the size requirements? A standard RTD sheath is 0.125 to 0.25 in. dia., while sheath diameters for thermocouples can be less than 0.062 in.
• 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.

Although not a technical point, price may be another consideration. An average thermocouple costs approximately $35, while an average RTD costs $55. Cost of extension wire must also be considered. Thermocouples require the same type of extension wire material as the thermocouple, which can cost up to $1 per ft. Standard nickel-plated, teflon-coated RTD wire averages pennies per ft.

Once parameters are defined, the type of RTD or thermocouple is chosen. RTDs provide a resistance vs. temperature output and are passive devices, needing no more than 1.0 mA to run. The most common RTD is a 100 ohm, platinum sensor, with an alpha coefficient of 0.00385 ohms/ohm/C. It can be ordered as DIN A or DIN B which specifies the initial accuracy at 0 °C (ice point) and the interchangeability over the operating range. IEC 751 states that DIN A is 0.15 °C±0.002/t*, where t*=specified temperature. DIN B is 0.3 °C±0.005/t*.

RTDs can also be constructed from nickel, copper, or nickel/iron. Each metal has a different alpha coefficient and operating range. An RTDs alpha coefficient must be matched to its instrumentation or an error of several degrees can occur.

Thermocouples can be made with any combination of two dissimilar materials. ISA recognizes twelve thermocouples. Eight of the 12 have letter designations including Type J, Type K, Type T and Type E.

The most common determining factor for chosing thermocouple type is the temperature range of its intended application. Type J is suitable for a temperature range of 32 to 1,400 °F (0 to 759.99 °C). Type K is appropriate for a temperature range of 32 to 2,300 °F (0 to 1,259.99 °C). Type T handles a temperature range of –300 to 700 °F (–184.44 to 371 °C). Type E fits a temperature range of 32 to 1,600 °F (0 to 871.11 °C).

Standard limits of error and special limits of error must also be considered. These values relate to the purity of the wire used to manufacture the thermocouple. For very little additional cost, thermocouple specifiers can often improve accuracies greatly (100% or greater).

Specifying the correct thermocouple or RTD for an unconventional application may be a difficult task. Many manufacturers of RTDs and thermocouples offer applications engineering support to help customers select the right combination of temperature measurement equipment.