Ion Gauges

When operating below the Pirani gauge range, an ion gauge can be used to measure pressure. The ion gauge consists of three distinct parts: the filament, the grid, and the collector. The filament produces electrons by thermionic emission. A positive charge on the grid attracts the electrons away from the filament; they circulate around the grid passing through the fine structure many times until eventually they collide with the grid. Gas molecules inside the grid may collide with circulating electrons. The collision can result in the gas molecule being ionized. The collector inside the grid is negatively charged and attracts these positively charged ions. Likewise they are repelled from the positive grid at the same time. The number of ions collected by the collector is directly proportional to the number of molecules inside the vacuum system. By this method, measuring the collected ion current gives a direct reading of the pressure.