Molecular and Hydrodynamical Effusion of Mercury Vapor from Knudsen Cells

Abstract

The unilateral flow of saturated mercury vapor through cylindrical channels of stainless steel and a thin‐edged orifice into a vacuum has been investigated at source pressures ranging from 10^—6 to 1 atm. The results generally agree with the semitheoretical equation derived by Knudsen in a study of the bilateral flow of permanent gases under small pressure gradients, but significant differences exist. For channels with ratios of length to radius of about 60 to 100 and radii of about 0.02 cm, the agreement is within the experimental errors for pressures ranging from the lowest of 10^—6 to about 5×10^—4 atm. Above this latter value the rate of flow is less than that predicted by the equation, and at pressures near 1 atm the rate tends to approach that value which is expected for a situation in which the channel is filled with the saturated vapor. For the thin‐edged orifice the results display the molecular flow and a transition to hydrodynamic flow which is nearly isothermal. For an intermediate value of the ratio (∼5) the results at the lowest pressures are about 6% less than those predicted by molecular flow and show a transition to a flow which tends to approach isothermal hydrodynamic flow and which is greater than molecular flow divided by the transmission probability.