Leo Lichtman wrote:

wrote in message
oups.com...

A certain valve has a rating of 29"/hg. What is the P.S.I. Equiv. of this?
(clip) I just need them for 20 lbs of air pressure.

^^^^^^^^^^^^^^^^
A rating of 29"Hg is meaningful only in terms of vacuum. The equivalent
rating in PSI would be 14.7, but may not be a valid conversion. A valve
intended for vacuum service at 29" of mercury is probably designed to have
less than a certain specified leakage. This would be of no particular
interest if you are going to use the valve under pressure.

Not only that, most vacuum valves I've looked
at seemed to be distinctly one-way. By that
I mean that they expect vacuum on a specific
side and atmosphere on the other. A vacuum
gate valve might be happy with 20 psig on the
atmosphere side and atmosphere on the vacuum
side, but not the other way around.