Cheap vacuum gage

[Doug Coulter]

Here is my original writing on this. They work, and in fact are in use all the time here, so reliable for what they are and trouble free I forgot the topic for awhile.
Pirani gage.

Since that covered the basics pretty well, I'll only add a little update here. One is that we ran out of those bulbs, but did find a decent substitute surplus. I normally eschew surplus for things like this because of how flakey most sources are when you want more later -- but in this case we managed to get a few hundred, so I'm set.

I have since laid out a PCB for this design, no big changes other than I added an adjustable volt divider on this version so you can set fullscale to the 2.0v most panel meters like to see.

I never did get around to absolute calibration on these, it's not either easy, nor is it really necessary. They will read down a bit lower than you can get a fusor to run, and being repeatable, they are really all that's needed for that. They are reading about a quarter volt at the normal running pressures of that system, on a 2 v full scale calibration. The resulting significant digits are actually better than that delivered by my PKR-251 gages from Pfeiffer!

Here is a great use for these....if your pumping station doesn't already allow this (mine do) use one to control an intermittently running forepump, based on foreline pressure.
Saves power and noise! And life.

I do a similar thing with my turbos, but it's done a little differently there. The control electronics allow the forepump to be turned on and off based on the power it is taking to keep the turbo spinning at the desired speed, so all I had to add there was a big solid state relay to take their logic level signal and change it into forepump on/off power.

Having not had enough experience with diffusion pumps to know -- you may want a reservoir in the foreline, and perhaps a solenoid valve to shut down the line at the forepump to keep backstreaming down?