⁴He interpolating constant-volume gas thermometry in the range 3.0 K to 24.5561 K

The range 3.0 K to 24.5561 K of the International Temperature Scale of 1990 (ITS-90) was realized at VSL with a He-4 interpolating constant-volume gas thermometer (ICVGT). The standard uncertainty of the generated scale ranged from 0.32 mK at 3.3 K to 0.38 mK at 24.5561 K. The ICVGT scale was compared with the classical absolute gas thermometer scales of the National Physical Laboratory (NPL-75, 1975) and the Kamerlingh Onnes Laboratory (KOL, 1986) and was found to deviate from them by a maximum of 1.5 mK. The non-uniqueness of the generated scale was investigated by comparing it with alternative ICVGT scales (but having equal status because still satisfying all the requirements of the ITS-90) obtained from different choices of the lowest calibration temperature and interpolation function. The size of correction effects such as thermo-molecular pressure, aerostatic head pressure and dead volume was calculated and the effectiveness of the ICVGT calibration procedure in compensating for such correction effects was evaluated.