Low-pressure performance of a Fabry-Perot cavity based optical pressure standard working at the zero-thermal-expansion temperature

Fabry-Perot cavity (FPC) refractometer based optical pressure standard (OPS) is very promising for a new realization of the pascal in the range 1 Pa-100 kPa. Although FPCs are made using ultra-low-expansion glass, cavity baseline (frequency) shifts due to temperature changes caused by gas pumping and filling are still a crucial factor limiting the accuracy of the OPS developed at the National Institute of Metrology (NIM) of China at the low-pressure end. In this study, the baseline shift in the NIM-OPS is eliminated by setting the working temperature at the zero-thermal-expansion (ZTE) point, which is determined by measuring the coefficient of thermal expansion (CTE) of the FPC in the temperature range 23-40 C-degrees. CTE measurements are obtained under a background vacuum, and also under the condition of remaining gas in order to minimize the temperature gradient. The performance of the OPS for low-pressure measurements is investigated at two different working temperatures, i.e. 30.1 C-degrees used previously and the determined ZTE temperature of 36.8 C-degrees. An indirect comparison between the OPS and a static expansion system is performed at 1 Pa using a capacitance diaphragm gauge as the transfer standard. The disagreement of similar to 254 mPa at 30.1 C-degrees is reduced to within 10 mPa at the ZTE temperature.