NIST reference functions and tables of thermocouple electromotive force (emf) versus temperature have been adopted as standards by the American Society for Testing and Materials ( ASTM)  and the International Electrotechnical Commission (IEC) . All letter-designated thermocouple types are manufactured to match these standards to within specified tolerances. Furthermore, calibrations of individual thermocouples are often expressed in terms of the deviation in emf of the test thermocouple from the emf value given by the reference function.
This database reproduces a subset of the tables and reference functions of NIST Monograph 175, "Temperature Electromotive Force Reference Functions and Tables for the Letter-Designated Thermocouple Types Based on the ITS-90" , by G. W. Burns, M. G. Kaeser (formerly Scroger), G. F. Strouse, M. C. Croarkin, and W. F. Guthrie. All temperatures used by this database are given on the International Temperature Scale of 1990 (ITS-90) , and all emf values are given in units of the 1990 SI volt . Details of the ITS-90 are given in Appendix A .
Reference functions and tables are given for the eight letter-designated thermocouple types: noble-metal types B, R, and S; and base-metal types E, J, K, N, and T. The reference functions and tables for types B, E, J, K, and T are mathematical conversions of those given in NBS Monograph 125 . The reference function for type N is a conversion of functions given in NBS Monograph 161 . The conversions were based on the published differences between the temperatures on the ITS-90 and those on the IPTS-68 [4,8,9]. The reference functions given for types R and S thermocouples are taken directly from recently published research [10-12], and they were used to compute the reference tables presented for those two types. The nominal compositions, representative trade names, and temperature ranges of the letter-designated thermocouple types are given in Table I and Table II . The temperature ranges given in Table I are those of the reference functions.
The letter designations used in this database follow the recommendations of the Instrumentation, Systems, and Automation Society ( ISA) and the American Society for Testing and Materials ( ASTM). The practice of identifying each of the commonly used thermocouple types with a letter was originated by the ISA and adopted in 1964 as an American National Standard (C96.1) . As noted in the present ASTM American National Standard , the letter designations actually identify the reference tables and may be applied to any thermocouple that has a temperature-voltage relationship agreeing within the tolerances specified in the standards with the values given by 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.
A discussion of each thermocouple type is included in Monograph 175. For each thermocouple type, Monograph 175 gives a summary of the history of development, special precautions on usage, recommended temperature ranges of use, initial calibration tolerances, and nominal chemical compositions.