Since July 1, 1991, the implementation of the "1990 International Scale", and stipulates that from January 1, 1994 onwards, all the temperature measurement equipment production, use, the value of transmission must be in accordance with the "ITS-90" requirements. The 1993 International Temperature Advisory Committee (CCT) revised the difference (t90-t68,t90-t76) table in the original officially promulgated "ITS-90" text, revising the temperature zone to 630 ℃ to 1064 ℃.