Thanks everyone for the useful comments and suggestions on this system! Finn, I intend to rig up the system with another valve to connect the roughing pump to the high vacuum side for quick cycling like you mentioned. It will be very nice to be able to cycle the high vacuum without having to spin down the turbo each time.
I finally had some time this past weekend to measure the high vacuum with a Granville-Phillips 343014 Mini Ion Gauge that I purchased from eBay. I also got a couple more bellows valves, so I traded out the constriction-inducing straight valve for a bellows valve. I interfaced with the gauge through a DB9 cable and breakout board.
- The vacuum gauge attached through a bellows valve
- Model number information
The gauge couldn't be simpler to use! It requires 24VDC to operate. After connecting the ON/OFF pin to ground with the gauge under vacuum from the turbo, the analog output from pin 3 indicates the vacuum. It goes from 0-10V and logarithmically indicates the pressure with 0 volts (theoretically) corresponding to 1E-9 torr, 1 volt = 1E-8 torr, 2 volts = 2E-7 torr, etc. With the turbo at full speed, the gauge initially showed 4.2 volts indicating a pressure of about 2E-5 torr. The pressure dropped to the xE-6 range only 30 minutes after turning on the turbo. After allowing the system to pump for four hours, the analog output read 3.8 volts corresponding to 6E-6 torr. I am satisfied with this considering that the chamber was open to atmosphere prior to the run! I'm sure the pressure would continue to drop if I let things outgas longer. However, a pressure in the XE-6 range is sufficient for my vacuum tube experiments already.
- Graph from the gauge manual indicating pressure vs. voltage
