> And I'm not sure about the conversion to microns as I don't have any need
> for that
Exactly. You have no idea how much vacuum it takes to boil water, and don't
> 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.
Ok, maybe my gauge set is "abnormal". I've never seen one that even _claims_ to
be accurate to less than 1 percent ... and the accuracy we need to distinguish
500 microns from 1500 microns is more like .0005%. Not to mention the fact that
your refrigeration gauges are referenced to atmospheric pressure, which changes
by many times that amount. So even if your gauge was perfectly accurate, you
would still have to know the current atmospheric pressure to better than 1
millibar in order to know if your vacuum was good enough or not.
> Bottom line, a salvaged refrigerator, room air conditioner, dehumidifier,
> etc. compressor is quite adequate as a service tool for occasional HVAC work
As I said before, only if you don't care about getting all the water out.
> and indeed will draw a deeper vacuum than most of the vacuum pumps marketed
> to pros for that very purpose.
Simply not true. Here's a nice little example, made by Robinair (one of the
larger names in the business)
Note how it will pull to 40 microns ?
Look at it this way, atmospheric pressure is roughly 750000 microns. Every HVAC
compressor I've ever seen is a single stage, piston type compressor. For it to
compress 500 microns at the inlet to the 750000 microns at the outlet (ignoring
things like leakage past the rings), it would have to have a compression ratio
of 1500:1 !
> And besides, Chad has his answer already
Indeed he does, so I'll shut up. I just hate to see myths perpetuated.