NPL-75: A Low Temperature Gas Thermometry Scale from 2.6 K to 27.1 K

P-V isotherm thermometry has been used to obtain a value of 20.271 2 K for the normal boiling point of equilibrium hydrogen, the total uncertainty, expressed as a 99% confidence limit, being ±1.0 mK. Based on this fixed point as a reference temperature a constant volume gas thermometry scale, NPL-75, has been produced, which covers the range from 2.6 K to 27.1 K. Corrections for the non-ideality of the thermometric gas, 4 He, were made with virial coefficients derived from measurements of P-V isotherms in the same temperature range. The uncertainty of NPL-75, expressed as a 99% confidence limit, is ±0.4 mK, and it is estimated that the scale does not deviate from thermodynamic temperature by more than 0.3 mK, 0.9 mK and 1.4 mK at 4 K, 20 K and 27 K respectively. NPL-75 has been compared with IPTS-68, T 58 and T XISU (1975).