We use different types of thermocouples in industry to measure temperature at various application, One question always bothering me was -- Why the thermocouple types are named like B, K, T, S etc. What is the scientific/technical reason behind this naming?

Even though <b>ISA-MC 96.1 - 1982</b> standard very elaborately explains each and every aspect of thermocouples, it doesn't discuss anything on the naming (selection of type names).

 

The "Manual on the Use of Thermocouples in Temperature Measurement", ASTM Special Technical Publication 470A, dated 1974 states, "The commonly used thermocouple types are identified by letter designations originally assigned by the Instrument Society of America (ISA) and adopted as an American Standard in ANSI C96.1-1964." Page 19.

Mention of thermocouple letter designations is made in the forward to the MC96.1-1975 standard published in the 1980 "Standards and Practices for Instrumentation (6th Edition)".

"On the basis of general usage and NBS recognition, seven types of thermocouples, T, J, E, K, S, R, and B, have been coded. These letters, which indicate standard combinations, were chosen arbitrarily, but with an effort to eliminate any confusion which might arise from the use of such general letter usage as AC, DC, G, etc., and differences which already exist between manufacturers.

The use of the letter X to indiate thermocouple extension wire appeared obvious."

<snip>

"In ANSI-MC96.1-1975 thermocouple and thermocouple extension wire are designated by letters. This has been done primarily to eliminate the use of proprietary names." (page vi, ANSI MC96.1-1975)

A mid 1950's report, "Progress Report I, Thermocouple Research Report for the Period November 1, 1956 to October 31, 1957" by David L. McElroy published by Oak Ridge National Lab (ORNL-2467), refers to T/Cs as Chromel P-Alumel (I.S.A Type K) or Iron-Constantan (I.S.A. Type J) so letter designations were in use some years prior to the adoption of the standard.