Use of porous plugs in the transition flow range for the calibration of vacuum gauges

The use of a porous plug for admitting gases in the molecular flow range is well known. In this paper, the authors deal with the transition flow region, for which the pressure of gas leaking through the plug is expressible in terms of a polynomial of nth degree in time unlike the simple exponential relation for the molecular flow region. The conductance of the plug in the transition flow range, determined from the time derivative of this polynomial, varies linearly with pressure. The slopes of the conductance versus pressure curves for helium, nitrogen, and argon when multiplied by respective viscosities are found to be constant for a plug, as expected from theory. The flowmeter thus established has been used for the calibration of vacuum gauges using an orifice flow system both in the constant volume and constant pressure modes.