Hey guys,

We've been talking about these pop-off valves as safety devices for some

but the problem always seems to be that the available crop of these things
will only work at sea level.  This is an obvious shortcoming for airplane
applications, as the MAP will always be decreasing with altitude.

I've been kicking around some ideas for awhile and came up with a little
drawing of one of these things that, I think, addresses the problem.  See

attached picture.

My thinking is that if you create a chamber where you carry around your

level altitude with you, close it off with a piston, the exposed side of

is open to the atmosphere, then as your altitude increases the piston

move down an amount equal to the change in atmospheric pressure.  It then
applies the required additional force to our spring and keeps a steady

of force on the valve at the bottom.  (This would be the equivalent of 40"

MAP, but you could make it be whatever you want by changing the spring

 As you can see, the exposed sides of both the movable piston and the

are seeing atmospheric pressure... in this example drawing the piston and
valve's areas are both one square inch, just to keep things simple.

Now, for the $64,000 question... what's wrong with this?  If it won't

why not?  What do we change to make it work?  This can't be that hard.
 Suggestions????........

      <Marv>

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