> I can pull a vacuum equal to whatever the ambient pressure is, according to
> my gauge set.
That's true. But your gauge set won't accurately read the difference between
25000 microns (which is about 29" of Hg) and 1500 microns (about 29.8" as I
recall), which is what it takes to pull water out of the dessicant ... good
practice is about 500 microns. Ideally, you have a gauge that will read the
difference between 500 and 1500 ... after pulling for as long as you think it
takes, you shut off the valve and watch to see if the pressure rises. If it
rises to 1500 and stays there, there is still water in the system. (If it
continues to rise, there is a leak.)
> Heck, some people use vane
> pumps or even those venturi gadgets - now THOSE can't pull a deep vacuum.
And those people say exactly the same thing you are saying : "It works fine for
> But a real HVAC compressor will pull as deep a vacuum as you need to boil
> off any water.
Have it your way, Karl. But try sometime, just sealing up a glass container
full of water and evaporating it using your compressor. It will appear to boil,
at first, as you pull the dissolved air and so on out of the water. But then it
will stop boiling and just look at you. Leave it overnight, tomorrow you may
see a small drop in level but there will still be plenty of liquid water left.
(BTW, if you try this trick with a real vacuum pump, you need to change the oil
afterwards. That much water vapor will almost certainly contaminate the oil.)
> Of course, I do leave it on for at least an hour before
> charging - water will evaporate at ambient pressure, so a deep vacuum just
> speeds things up.
Not if it's adsorbed into a desiccant/dryer ! And what's the point of having a
dryer in the system, if it's already saturated with water from being exposed to
the air (and not pulled down hard enough to purge it) ?