Estimates of the difference between thermodynamic temperature and the International Temperature Scale of 1990 in the range 118 K to 303 K

Abstract

Using exceptionally accurate measurements of the speed of sound in argon, we have made estimates of the difference between thermodynamic temperature, T , and the temperature estimated using the International Temperature Scale of 1990, T 90 , in the range 118 K to 303 K. Thermodynamic temperature was estimated using the technique of relative primary acoustic thermometry in the NPL-Cranfield combined microwave and acoustic resonator. Our values of ( T − T 90 ) agree well with most recent estimates, but because we have taken data at closely spaced temperature intervals, the data reveal previously unseen detail. Most strikingly, we see undulations in ( T − T 90 ) below 273.16 K, and the discontinuity in the slope of ( T − T 90 ) at 273.16 K appears to have the opposite sign to that previously reported.