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Re: shock wave as pressure builds in a vacuum?



You may want to look into the design and operation of the Baratron pressure
sensor:
http://www.mksinst.com/barainfo1.html
http://www.mksinst.com/techbaratronfaq.html#applications
http://www.mksinst.com/PMC1.html

Larry Woolf;General Atomics;San Diego CA
92121;Ph:858-526-8575;FAX:858-526-8568; www.ga.com; www.sci-ed-ga.org

-----Original Message-----
From: Stefan Jeglinski
Sent: Tuesday, May 27, 2003 9:44 PM
Subject: shock wave as pressure builds in a vacuum?


I have a real-world situation that I am idealizing for the purposes
of discussion. Consider an extremely thin membrane that is capable of
withstanding an atmospheric pressure differential under steady-state
conditions. IOW, one side of the membrane can be at high vacuum,
while the other is at 1 atm. The membrane is likely flexed concave
somewhat in steady state with a 1-atm differential (center deflection
x toward the high vacuum, let's say), but it has a wire mesh on the
high vacuum side to provide mechanical support.

Now let's say that under normal operation, the high vacuum side is at
10^-9 Torr while the other side is at 10^-6. Consider what happens
when the 10^-6 side undergoes "catastrophic venting." That is, it is
opened suddenly to atmospheric pressure. Air "rushes in," but it is
my contention that there is no "front" associated with it that would
cause a sudden overpressure at the membrane. It is tempting to make
an analogy to water rushing into an empty vessel and the force it
creates when it crashes against the inside walls, but I'm not sure
the analogy is appropriate here. I know little about shock wave
physics, don't know how that might enter in.

It is experimentally observed that under these conditions
("catastrophic venting"), the membrane is sometimes broken. One
colleague argues for some kind of shock wave, essentially an
overpressure transient at the membrane as the air rushes in. I argue
that there is little or no physics to support that, and that the
membrane failure is due to an excessive deflection rate (dx/dt, as
the pressure increases rapidly) that it cannot withstand from a
mechanical standpoint.