The short answer is to use another gage tube that you have reason to believe is accurate and adjust your unknown TC gage circuit (tube heater and/or output current meter shunt) to give a similar set of readings over the range of say...10 microns to 1 or 2 mm Hg. I remember the Veeco evap unit I used, had a simple technique... set the max pressure with the unit vented... (TC gage set to read 1-2 mmHg)at 1 Atm. Then with the system under diffusion pump vacuum, bring the chamber down below 1 micron Hg pressure and set the the TC gage meter to read "0" . In other words set the Full Scale and Zero points and the gage scale will be good enough in between.
If you don't have a calibrated readout, then you DO need to get a comparison gage of some sort. I use a McCleod gage (because I have it), but it is tedious to use... however as long as you don't have condensible vapors in the residual gas,and the gage plumbing is clean, it does provide a nearly absolute pressure reading.
Dave Cooper
Created on Tuesday, April 17, 2001 9:02 PM EDT by David Cooper