All quadropole mass spectrometers that I have seen provide a total pressure reading in addition to partial pressure readings for the selected masses.
The mechanism of the total pressure measurement is not described anywhere I have looked.
Does the mass spectrometer use its quadropole and faraday cup or is there simply a hot cathode vacuum gauge somewhere inside?
Looking for this answer in the context of RGAs (rest gas analyzers), I was able to find an answer:
Leybold answers the question in an application note
The total pressure is measured by a hot filament ionization gauge, where the cathode is the filament of the mass spectrometer and the anode is created by an aperture in front of the entry to the quadropole mass filter.
A "total pressure plate" is placed in front of the entry to the quadropole section of the mass spectrometer. Ions that are used for partial pressure measurements pass through a hole in the plate into the quadropole section of the mass spectrometer. Another portion of the ions collide with the plate. The ion current from the plate can be measured and is correlated to the total pressure.
