Thermodynamic Effects in a Gas Modulated Invar-Based Dual Fabry-Pérot Cavity Refractometer Addressing 100 Kpa of Nitrogen

Abstract

An Invar-based dual Fabry-Pérot cavity refractometer used for assessments of pressure by the gas modulation refractometry (GAMOR) methodology has been scrutinized with respect to the influence of thermodynamic effects (pV-work) that originates from the gas exchange process when 100 kPa of nitrogen is addressed. It is shown that the actual temperature variation of the cavity spacer solely is a fraction of the previously assessed upper limits (0.5 mK/100 kPa), limited to sub-parts-permillion (ppm) levels. This finding additionally supports the conclusion that the thermodynamic effects will not be a limiting factor when the system is used for assessments of pressure.

‘Quantum-Based Realizations of the Pascal’: Status and Progress of the Empir-Project: Quantumpascal

“…It was found that, due to the small dimensions of the Invar-based FPC system and the thermal properties of Invar, for the case with 30 kPa of N2 being assessed, the temperature of the gas species is within 1 mK of the cavity wall in less than 1 s, that after 10 s an upper limit for the temperature increase at any point of the spacer is 1 mK, and that, at the end of the gas filling, the difference in temperature between the gas and the locations of the temperature probes in the spacer block are less than 0.3 mK [13]. A subsequent study has indicated that the actual fluctuations of the temperature are markedly less than the predicted upper limits, implying that the system has similar properties when 100 kPa of N2 is addressed [14]. This implies that assessments made by use of the GAMOR methodology are not significantly affected by any temperature fluctuations due to the filling and evacuation of gas.…”