Open ion source

OIS is the most widely available type of RGA. Cylindrical and axially symmetrical, this kind of ionizer has been around since the early 1950s. The OIS type is usually mounted directly to the vacuum chamber, exposing the filament wire and anode wire cage to the surrounding vacuum chamber, allowing all molecules in the vacuum chamber move easily through the ion source. With a maximum operating pressure of 10 − 4 Torr and a maximum detectable partial pressure as low as 10 − 14 Torr when used in tandem with an electron multiplier.

OIS RGAs measure residual gas levels without affecting the gas composition of their vacuum environment, though there are performance limitations which include:

* Outgassing of water from the chamber, H2 from the OIS electrodes and most varieties of 300-series stainless steel used in the surrounding vacuum chamber due to the high temperatures of the hot-cathode source (> 1300°C).
* Electron Stimulated Desorption (ESD) is noted by peaks observed at 12, 16, 19 and 35 u rather than by electron-impact ionization of gaseous species, with the affects similar to outgassing effects. This is frequently counteracted by gold-plating the ionizer which in turn reduces the adsorption of many gases. Using platinum-clad molybdenum ionizers is an alternative.