Function
A regulated electron current (typically 10 mA) is emitted from a heated filament. The electrons are attracted to the helical grid by a dc potential of about +150 volts. In their passage from the filament to the grid, the electrons collide with gas molecules in the gauge envelope, causing a fraction of them to be ionized. The gas ions formed by the electron collisions are attracted to the central ion collector wire by the negative voltage on the collector (typically a minus 30 volts). Ion currents are on the order of 1 mA/Pa. This current is amplified and displayed by a high-gain-differential amplifier/ electrometer.
This ion current will differ for different gases at the same pressure; that is, a hot filament ionization gauge is composition-dependent. Over a wide range of molecular density, however, the ion current from a gas of constant composition will be directly proportional to the molecular density of the gas in the gauge.
Read more about this topic: Hot-filament Ionization Gauge
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