> Have it your way, Karl. But try sometime, just sealing up a glass
> full of water and evaporating it using your compressor. It will appear to
> at first, as you pull the dissolved air and so on out of the water. But
> will stop boiling and just look at you. Leave it overnight, tomorrow you
> see a small drop in level but there will still be plenty of liquid water
OK, Randall - but another thing to remember about all this is that the
absolute volume of water in an HVAC system should be miniscule - enough that
the receiver/drier should cope with a number of openings with minimal
vacuuming. In fact, although it's common practice to change the drier every
time a system is opened, GM actually advises to only change the
receiver/drier if it's physically damaged (this is a sealed receiver/drier,
nnot one where you can change the desiccant bag). As much as anything, you
need to draw out air and evaporate any flushing compounds, etc.
And I'm not sure about the conversion to microns as I don't have any need
for that and didn't bother to work out the factor, but any normal gauge set
can indeed show the difference between 29.0 and 29.8 inches of mercury.
Laboratory accurate? Probably not - but plenty good enough for HVAC work.
Bottom line, a salvaged refrigerator, room air conditioner, dehumidifier,
etc. compressor is quite adequate as a service tool for occasional HVAC work
and indeed will draw a deeper vacuum than most of the vacuum pumps marketed
to pros for that very purpose. Its main drawback is low volume - that's why
the ones most HVAC guys carry are rotary-vane compressors. They're far less
capable in terms of deep vacuum, but they draw down a big system quickly.
And besides, Chad has his answer already ;-)