Choice of the Thermocouple Materials

Of the approximately 300 different types of temperature measuring thermocouples that have been identified and studied, only a few types, having the more favorable characteristics, are in general use. There are eight types of thermocouples that have been standardized, because they are the ones most commonly used industrially. In the United States each type is identified by a letter. This practice was originated by the Instrument Society of America (ISA) and adopted in 1964 as an American Standard to eliminate the use of proprietary names. The standards of the American National Standards Institute (ANSI-MC96.1, 1982) and the American Society for Testing and Materials (ASTM 230-87) utilize the reference tables from National Institute of Standards and Technology Monograph 125 as the basis for standardization. As noted in the ANSI and ASTM standards, the letter designations actually identify the tables and may be applied to any thermocouple that has a temperature-emf relationship agreeing within the tolerances specified in the standards with that of the table, regardless of the composition of the thermocouple. Substantial variations in composition for a given letter type do occur, particularly for types J, K, and E.

• Type B = platinum- 30% rhodium/platinum-6% rhodium - 0 to 1820°C ^*
• Type E = nickel-chromium alloy/a copper-nickel alloy -270 to 1000°C^*
• Type J = iron/another slightly different copper-nickel alloy -210 to 1200°C^*
• Type K = nickel - chromium alloy/nickel - aluminum alloy -270 to 1372°C
• Type N = nickel-chromium-silicon alloy nickel-silicon alloy -270 to 1300°C^*
• Type R = platinum- 13% rhodium/platinum -50 to 1768°C^*
• Type S = platinum- 10% rhodium/platinum -50 to 1768°C^*
• Type T = copper/a copper-nickel alloy -270 to 400°C^*

* temperature range as per NIST Table I: Thermocouple Types Definitions.

Certain combinations of alloys, such as Type J and K, have become popular as industry standards. Thermocouple type selection is driven by cost, availability, melting point, chemical properties, stability, and output. Different type thermocouples are best suited for different uses/applications. Thermocouple types are usually selected on the basis of the temperature range and accuracy needed. Other selection criteria include the chemical inertness of the thermocouple material and whether it is magnetic or not. 

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