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Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615
I guess the problem is there are so many commonly used units for pressure : Bar, Pascal, PSI, inchs of Hg, Torr, microns. The first definition I saw in a search referred to microns being um of Hg. um of water would be a much smaller unit. Vaccum seems to mostly use Torr or microns, which are both metric systems based on Hg. So i'd be inclined to use um of Hg, as it would seem the more common definition.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
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That topic has been beaten to death at the fusor.net forum.
I've never heard of water column heights used for anything but differential pressures (e.g. for air condtioning or equipment cooling).
The pre-SI traditional unit is the torr. Originally 1 mm of mercury, it's now exactly 1/760 of an atmosphere, which is exactly 101325 pascals. I think the gas pressure in neon and fluorescent lamps is around 20 torrs. Partial pressure of mercury if present is much lower (see vapor pressure).
Some equipment vendors, and writers of scientific papers, give vacuum pressure in pascals, the natural SI unit (newtons per square meter). Maybe its use in vacuum will take over faster than the use of "mebibyte" for 1048576 bytes, since mega stands for 1000000.
We also sometimes see vacuum pressures given in millibars. 1 millibar is exactly 100 Pa, and is about 3/4 of a torr. Millibar has long been the standard unit in weather science. Like the liter and the hectare, it's metric but not really SI.
Back to traditional vacuum jargon in torr territory. Between 1 and 0.001 torr it's common to state pressures in "microns". Vapor pressure of Hg at room temperature is about 2 microns. Below 1 micron it goes back to scientific-notation torrs. Half a micron is the same as 5 x 10^-4 torr, sometimes written as 5e-4. There's no confusion when somebody on a vacuum forum says their system gets down to low minus 6's. In my unpracticed opinion, that means 10^-6 torr or mbar, the difference commonly being of little consequence. Guess a purist could say 100 micropascals.
Deep in the minus 7 decade we find 1.5e-7 torr = 2.0e-7 mbar = 20 uPa. That pressure value is noteworthy because, as a RMS variation on ambient air pressure, it's the zero-dB reference for sound pressure. At some frequencies it's marginally detectable by human ears in like-new condition.
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Vacuum Measurement in the Lab and Academia.
