Why the thermocouples are called as J, S, K, R etc

How are thermo-couples named as J, S, K, R etc

The naming is based on the type of material used to make the Temperature sensor. The range will vary according to the name

Letter designations are assigned by national or international standards.

  1. Noble metal T/C letter designations

The noble metal thermocouples received international letter designation, Type R or Type S in the late 1960’s.

The 1973 paper entitled “Platinum Thermocouple Reference Tables: Agreement on International Standards” by P. H. Wells of Johnson Matthey Metsls Lit., London announced the adoption of a new international standard, IPTS-68 (International Practical Temperature Scale) in which the “American letter notation” was adopted.

“Thus the platinum: 10 percent rhodium-platinum thermocouple is designated “Type S” and the platinum: 13 percent rhodium-platinum thermocouple is referred to as 'Type R”.

  1. Base metal and noble metal thermocouple letter designations in the USA

The ISA (Instrument Society of America) assigns letter designations to thermocouple types in its ISA MC96.1-1982 standard.

The preface to that standard cites some history:

“The development of this standard has resulted from the work of the American National Standards Committee on Temperature Measurement, MC96. The Committee was organized in 1946 under the sponsorship of ISA, the scope of the Committee being designated as follows: Requirements for temperature measurement thermocouples, including terminology, fabrication, wire sizes, installation, color codes of thermocouple and thermocouple extension wire, temperature-EMF tables and tolerances have been coordinated with the International Electrotechnical Commission (IEC).”

Analog (non-microprocessor) temperature controllers through the 1960’s that were designed for a specific type of thermocouple (input circuitry, non-linear scaling) typically had the thermocouple identified on the temperature scale somewhere, labeled by its metal composition, for instance, CA, for Chromel Alumel than the letter designations, K or Type K.