The Knudsen effusion method is a dynamic gravimetric technique based on the rate of escape of vapor molecules through an orifice of known dimensions in a Knudsen cell into a vacuum at a known temperature. The rate of mass loss through the orifice is measured by the Surface Measurement Systems’ UltraBalance within the VPA system. Sample masses from 1 to 100mg can be studied typically in the temperature range from 20 to 400 °C.