Wrede-Harteck gauges

Wrede and Harteck first applied the property of effusion to the measurement of atom concentrations. A catalyst, effective for the atom recombination reaction, is placed within an enclosed volume which is connected to the reaction system by one or more small orifices. Molecules and atoms effuse into the enclosed volume, while only molecules effuse out. Thus an equilibrium is set up, and mass balance requires a pressure difference, Ap, between the two sides of the orifice, given by 

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The Wrede—Harteck gauge consists of a small chamber in communication with the atom—molecule mixture via a hole whose dimensions are small compared with the mean free path. The chamber is coated with an efficient catalyst so that, ideally, every atom which enters is recombined. Because the atom flux is unidirectional, a pressure difference (AP) between the pressure outside and inside the gauge will characterise the stationary condition, such that... 

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