Effusion from Knudsen Triple Cells

A quantitative theory for vapor effusion from Knudsen triple cells has been developed. These cells, when used in conjunction with a mass spectrometer, permit a direct determination of the thermodynamic activity of a binary alloy component. In conventional Knudsen effusion studies, the vapor pressure of an element in an alloy and in the pure state is determined in separate experiments, and the ratio is used to compute the activity. Because of the exponential relationship between vapor pressure and temperature, even small errors in temperature reproducibility result in large errors in the calculated activities. The Knudsen triple cell avoids these errors but is subject to other limitations that are discussed.