Analysis of the Outgassing in an Optical Pressure Standard

Abstract

Outgassing effects in a Fabry-Perot cavity based optical pressure standard (OPS) developed at NIM were studied at low pressures using a pressure rising method. The pressure rising curves were recorded by the OPS and a calibrated pressure transducer simultaneously. The pressure values assessed by the OPS were dependent on the gas refractivity parameters and deviated from the transducer values. From the difference, the effective molar polarizability of the accumulated gases was deduced and found to be close to the parameter of water vapour and increased with time. It supported that the accumulated gas in the OPS was composed of water vapor from outgassing and a small amount of air from leakage. To lower the outgassing effects, an external large vacuum chamber was attached to the OPS manifold. Finally, the pressure increase rate was reduced to ~ 50 mPa/h for a period of ~ 140 h, and ~ 27 mPa/h for the first 20 h.

Working equation for a Fabry-Perot cavity based optical pressure standard

“…As shown in Fig. 3, the pressure difference went to a process of slow drift after 1 h. The drift was mainly attributed to the outgassing of water vapor [14]. The intrinsic pressure difference between the PG and OPS is deduced by the slow drift part with an exponential decay function and extrapolating to the starting time of gas filling.…”