"Bill" wrote in message
oups.com...
Refrigeration pumps are designed to pump a much better vacuum than most
of the pumps used for vacuum chucks. The oil is part of the rotary seal
of the pump. The oil absorps moisture which creates vapor pressure when
the oil heats up. This VP competes with the ability of the pump to suck
a lower vacuum. Atmospheric pressure at sea level is 14.7 psi, which we
express as 0 psi guage. This 14.7psi is equal to a column of mercury 30
inches tall. We inverse this to say that the lower the pressure the
higher the vacuum. Therefore 0 psi absolute is 30 " of vacuum. 29.9" of
vacuum equals 0.1" of Hg(mercury) pressure. Refrigeration pumps can go
as low as 50 micons of Hg. This low of a pressure is necessary to boil
water at room temperature. This is how the pumps remove liquid water
and other contaminants from Freon systems. Water boils when it's vapor
pressure equals the atmospheric pressure above the surface of the
water.

Bill in WNC mountains

Not very knowledgeable about this subject, but did you mean 50 "microns" of
Hg, i.e., 50 millionths of Hg?

Leif