At 05:13 PM 9/7/01 -0700, Matt Gewain wrote:
>One atmosphere of pressure is equal to 14.7 PSI and equal to 29.92 in of Hg.
>So (18 /29.92) * 14.7= 8.84 PSI
>Generally dividing by 2 is close enough.
Definitions of standard atmosphere -- OK.
Unit conversion OK, but concept missing.
Close enough?!?!? I think not.
Atmospheric pressure at sea level will support a column of mercury
approximately 29.92 inches high, as in a barometer. The actual height of
mercury depends on weather conditions and decreases with altitude.
A vacuum gauge reads approximately 29.92 inches of mercury (chemical symbol
Hg) when open to atmosphere. Start to draw a vacuum, and the gauge reading
drops toward zero. These are absolute pressure measurements.
Pressure can also be measured relative to atmospheric pressure. This is
commonly expressed as "XX inches Hg vacuum." For example, 10 inches Hg
vacuum = 19.92 inches Hg absolute pressure.
The pressure on a partially evacuated volume is equal to the difference in
internal and external pressures, which is the relative measurement. Using
the example above and converting units, (10 inches Hg)(14.7 psi)/(29.92
inches Hg) = applied pressure in psi.
Vacuum -- much ado about nothing.
Matt Olson
Glendale, AZ USA
>-----Original Message-----
>From: Ed Berris [mailto:[EMAIL PROTECTED]]
>Sent: Thursday, September 06, 2001 3:45 PM
>To: [EMAIL PROTECTED]
>Subject: [RCSE] Vacuum Pressure
>
>How do you calculate pounds per sq inch applied to a wing when vacuum
>bagging if you know the inches of hq.
>
>For example, what would 18 inches hg equal in pounds per square inch and how
>do I calculate it?
>TIA,
>Ed
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