ITS-90 was adopted in 1990 as the temperature scale; IPTS-68 was the previous standard. The differences are related to redefining certain triple points and other melt or freeze cells that are used as the fundamental standards for temperature. Over the oceanographic ranges of temperature, a linear approximation is used to convert:
IPTS-68 = 1.00024 * ITS-90
The difference is small, but at WOCE levels it is significant.
Note: Salinity, density, and sound velocity are still defined in terms of IPTS-68 temperature. Sea-Bird’s software uses IPTS-68 temperature to calculate these derived parameters, regardless of which temperature scale you select for outputting or plotting temperature.
Application Note 42: ITS-90 Temperature Scale provides a more detailed description.