A weak beta source and a high potential raise the helium atom of the carrier gas to a metastable state. All other substances having an ionization potential lower than \(18\ \rm{eV}\) are ionized, and the current that results is used to measure the components. The detector is usually employed to measure inorganic compounds at concentrations between \(0.1\) and \(10\ \rm{ppmv}\). It has a linear range of about \(10^{2}\) but is somewhat unstable and requires great care to ensure the helium purity and to eliminate all leaks in the system.