Chapter 1.4 - Throughput, Pumping Speed, Evacuation Rate, Outgassing Rate, and Leak Rate

Abstract

The process of evacuation is in effect the removal of mass from the vacuum vessel, thus the rate of mass removal (the mass flow) determines the rate at which pressure falls. This chapter presents the relationship between the throughput Q and the corresponding volumetric flow S to the gas pressure. The number of molecules per unit volume is proportional to the pressure and the volumetric flow rate, and is usually defined as the product—Q = SP. The speed of a pump is defined as the volumetric rate of flow into the pump where the volume is measured at the pressure at the pump inlet. A leak rate is the throughput of a small hole from atmospheric pressure to the vacuum space and an outgassing rate of a system is the rate at which gas is desorbed from system walls to be pumped away by the vacuum pump, and is expressed as a throughput. Construction materials, especially glasses and elastomers, are permeable to the light constituents of the atmosphere. Permeability rates are expressed in the same terms as outgassing rates.