The last couple of days I have been fighting to get the base pressure of my reactor down. The fight was successful - a tenth of a microbar (about a tenth of a millionth of an atmosphere, or only ten times higher than the part of the thermosphere where the ISS is orbiting) is quite good for not having a turbomolecular pump running - and useless.
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.
1 week ago