Conversion of Reported Temperatures from Historical Scales to the ITS-90

Abstract While the International Temperature Scale of 1990 (ITS-90) has been a standard for many years, there is a large amount of historical thermophysical data reported on other scales. The differences of these scales from ITS-90 are typically small, but not necessarily negligible for reference-quality work. We describe conversion to ITS-90 from 36 different scales. These include not only the relatively well-known International Temperature Scales from 1968, 1948, and 1927, but also scales that have been used for cryogenic work at temperatures below the ranges of the older international scales, including the many scales that were based on the vapor pressure of helium. Scales specific to certain laboratories are also considered. Python code has been made available for the conversions described.