Overview of Thermocouple Types and Ranges

Thermocouples have been classified by the International Society of Automation (formerly Instrument Society of America) and the American National Standards Institute (ANSI), and are available for temperatures ranging from -200 deg. to 1700 deg.C (-330 deg. to 3100 deg.F). These standard tolerance thermocouples range in tolerance from ±0.5 percent to ±2 percent of true temperature. The table below presents commonly available thermocouple types and operating ranges.

Commonly Available Thermocouple Types and Operating Ranges

Thermocouples must be selected to meet the conditions of the application. Thermocouple and extension wires (used to transmit the voltage from the thermocouple to the monitoring point) are generally specified and ordered by their ANSI letter designations for wire types. Positive and negative legs are identified by the letter suffixes P and N, respectively. General size and type recommendations are based on length of service, temperature, type of atmosphere (gas or liquid constituents), and desired response times. Smaller wire gauges provide faster response but do not last as long under adverse conditions. Conversely, larger gauges provide longer service life but with longer response times. Thermowells and sheaths are recommended by thermocouple manufacturers for the extension of thermocouple life. Instruments used to convert thermocouple voltage to temperature scales are coded using the same letter designations. Failure to use matching thermocouples and instruments will result in erroneous readings.

Type J thermocouples use iron for the positive leg and copper-nickel (constantin) alloys for the negative leg. They may be used unprotected where there is an oxygen-deficient atmosphere, but a thermowell is recommended for cleanliness and generally longer life. Because the iron (positive leg) wire oxidizes rapidly at temperatures over 1000 deg.F, manufacturers recommend using larger gauge wires to extend the life of the thermocouple when temperatures approach the maximum operating temperature.

Type K thermocouples use chromium-nickel alloys for the positive leg and copper alloys for the negative leg. They are reliable and relatively accurate over a wide temperature range. It is a good practice to protect Type K thermocouples with a suitable ceramic tube, especially in reducing atmospheres. In oxidizing atmospheres, such as electric arc furnaces, tube protection may not be necessary as long as other conditions are suitable; however, manufacturers still recommend protection for cleanliness and prevention of mechanical damage. Type K thermocouples generally outlast Type J, because the iron wire in a Type J thermocouple oxidizes rapidly at higher temperatures.

Type N thermocouples use nickel alloys for both the positive and negative legs to achieve operation at higher temperatures, especially where sulfur compounds are present. They provide better resistance to oxidation, leading to longer service life overall.

Type T thermocouples use copper for the positive leg and copper-nickel alloys for the negative leg. They can be used in either oxidizing or reducing atmospheres, but, again, manufacturers recommend the use of thermowells. These are good stable thermocouples for lower temperatures.

Types S, R, and B thermocouples use noble metals for the leg wires and are able to perform at higher temperatures than the common Types J and K. They are, however, easily contaminated, and reducing atmospheres are particularly detrimental to their accuracy. Manufacturers of such thermocouples recommend gas-tight ceramic tubes, secondary porcelain protective tubes, and a silicon carbide or metal outer protective tube depending on service locations.

For more information about thermocouples, contact Duro-Sense Corporation by visiting https://duro-sense.com or calling 310-533-6877.