A source can work without any magnetic field like in a hot filament ionization gauge.
2.
Ionization gauge calibration is very sensitive to construction geometry, chemical composition of gases being measured, corrosion and surface deposits.
3.
This ion current differs for different gases at the same pressure; that is, a hot-filament ionization gauge is composition-dependent.
4.
The composition of gases at high vacuums will usually be unpredictable, so a mass spectrometer must be used in conjunction with the ionization gauge for accurate measurement.
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