The reason why I wanted to lower the base-pressure was because I thought it was much higher in the order of hundreds of millibar. I thought this because my lowest-pressure Baratron refused to zero. You see, Baratrons are the best vacuum gauge you can use, if you can use it. This is because Baratrons measure the pressure directly - they are in essence mechanical gauges that measure the deformation of a membrane capcitively. Accurate, not dependent on the gas-type, long-living, can handle corrosive gases, and you don't have to calibrate - except to zero them. Each Baratron will have a slightly different zero-point, so you have to pump down to about 1% of it's maximum range (or lower, if you can manage it), and then zero the signal. Then you can go on and use it from 10% of its maximum range up to its... wait for it... maximum range. Of course, that means that since I want cover a wide pressure range from 0.1 mbar to 100 mbar, I have to use 3 Baratrons: a 100mbar one, a 10 mbar one and a 2 mbar one (Those are the ones that were lying around in the lab).Recently I wanted to go below 1 mbar for the first time, but couldn't get the 2 mbar Baratron to zero. This meant, or so I thought, that my base-pressure was above 0.02 mbar. I searched for a leak, I applied vacuum grease to all seals - no joy. Also, the seals were now angry and refused to play ball. Finally I dug up a cold cathode gauge... a tenth of a microbar? Conclusion: The Baratron is broken, against all odds. Those things almost never break. Luckily I "found" another 2 mbar Baratron on an experiment that was, ahem, unattended. And whadda ya know, zip, zap, zero! Everything works just fine now.
Stuff breaks. Keep that in mind.
Agreed. At least you figured it out. I used to be the communications engineer at a large facility. Whenever we'd run a test, some of the ancient comm stations would fail. When I got it all under configuration control, there was supposed to be no more willy-nilly swapping.
ReplyDeleteInstead of waiting for paperwork, the techs just swapped out a failed comm station and would not switch it back. I was told "it happened on 3rd shift" like that made it OK. When I was on the phone to complain to the boss, they switched it a second time.
"Why can't we do it our way?" they whined when I called 'em on it again. "Because, if you put three different comm stations in the same spot and none of 'em work, that's not where the problem is."
"Oh."
Ah, good times.
Ha, those pesky engineers! Ahem, present company excepted, of course ;)
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