I tended to think of the Pirani gauge as the thing we used for softer vacuum really. It wasn't a lot of use below 10^-3 to 10^-4 Torr. When we were getting down to operating pressures on the electron-beam diodes (10^-6 Torr or better, for preference) we used commercial Penning gauges or hot filament ionisation gauges. The readings on these do depend strongly on the species being ionised. But since this whole debate is just about orders of magnitude that doesn't matter so much here.
The easiest way to measure the pressure inside a valve has to be by using ionisation, since you've almost always got all the electrodes you need already in situ. The issue of course is calibration. I guess you have to make up a dozen special valves, identical to the normal ones but with a side-tube on the envelope. Then you could cross-calibrate the ionisation current in the valve itself against an externally attached gauge. Once you know how the current in the valve's electrode structure varies with independently measured pressure you can apply the same calibration factor to valves without the side-tube.
I take your point about the pressure that can be achieved during pumping on the exhaust machine. But that's not the final pressure in the valve of course. The open envelope is pumped when everything inside it is extremely hot - much hotter than in normal valve operation. So whatever pressure is achieved at seal-off will drop very substantially as the metalwork cools and gas adsorption back onto the metal and, more importantly, gas reaction with the getter starts to work.
The pressure reduction due to gettering is what gets (no pun intended) us down to the vacuum we need. I see that in slide no 16 here
http://www-project.slac.stanford.edu...pec-060204.pdf, magnesium is ruled out for gettering in small vacuum tubes because its vapour pressure of 10^-5 Torr is too high. Barium (often as part of an alloy) is used instead - one more indication of the very low ultimate pressures which have to be achieved if the valve is going to work well for an acceptably long time.
If it's any consolation I worked for a few years on this project
https://en.wikipedia.org/wiki/ALICE_(accelerator). The whole machine was under high vacuum, but the electron gun was a particular challenge. The electron emitter (photocathode) was GaAs activated with essentially a monolayer of Cs atoms on the surface. To give this a reasonable lifetime the gun had to run below 10^-10 Torr (ideally around 10^-11)
https://accelconf.web.cern.ch/accelc...rs/mopc062.pdf. That took about a month to pump down, using some pretty serious pumps. But that sort of vacuum is not a job for part-timers. The project had a group of vacuum specialists who did that and only that.
VB