Unsteady Flow of Rarefied Gas in Microseconds Dynamic Vacuum Calibration: A Simulation and Experimental Study

The unsteady flow of rarefied-gas in formation of standard pressure, for the lab-built millisecond level, dynamic vacuum calibrator comprising the calibration/expansion chambers, gate-valve and limiting orifice with constant gas-flow conductance, was mathematically modeled, theoretically analyzed, numerically simulated with COMSOL Multiphysics and experimentally evaluated. The results show that three factors, including the pressure decaying in calibration chamber, gate-valve opening time and orifice conductance, strongly affect the formation time of standard pressure. The largest discrepancies between the measured, calculated and simulated results, in chocked flow mode, were 10% and 4.65%, respectively, suggesting that the lab-built calibrator is capable of generating millisecond dynamic pressure variations; the realistic gas correction factor little affected the standard pressure. Moreover, the assumption of constant orifice conductance was valid because its max-min difference was 0.8. Unlike calibrated gauges, the thermal couple should be installed at the center of calibration chamber because of large temperature gradient.