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Subject: Re: Idea - Cheap Pressure Gauge
Date: Aug 09, 2:51 am
Poster: Dave Cooper

On Aug 09, 2:51 am, Dave Cooper wrote:

>I can't put my hands on it at the moment, but there is a simple calibration equation for the standard Bayard Alpert type triode Ionization gage. Will try to find it..
Found it.
Granville-Phillips shows a calibration constant of 100 uamps/um Hg (for N2), for both tungsten and thoriated irridium filament gages. The only difference is in grid current. Tungsten filament uses 8mA grid current, Th-Ir filament uses 10 mA.

Based on this, a 1 nanoamp current (0.001 uA) on the meter would indicate 10^-8 Torr in the tube for N2. For other gases, pressures should be proportional to the sq root of mass, based on N2 as reference.

Since currents indicate charges per second, lightweight molecules such as H, D, and He will have higher velocities and hence higher ion currents for the same gas density (aka pressure). Thus Bayard-Alpert gages need to be adjusted for the actual gases in the system.

Dave Cooper