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Animator,
I'm not a Pnuemattics expert either, but I do spend a lot of time around really big pressure vessels (15'x45' autoclaves), and some time running them.
You are correct in saying that it is not proper design to use a relief valve as a "pressure sensor", or even as a "control".
However, in Dave's system the relief valve should never need to do it's thing. By setting the system timing so that at optimal conditions it just barely reaches 120 psi from the threshold pressure, you pratically ensure that it never quite does so under competition conditions. What really happens is that when the pressure drops, the compressor runs for X seconds and then stops. The final pressure will be close to but always less than 120psi. Remember, we are less than optimal here. If at the end of the cycle, the pressure is less than your threshold then the compressor can restart and try again. If the pressure is greater than the threshold but less than 120, it will just sit there until something changes.
I think that part of Dave's point is that the extra psi or two you can get with two sensors probably won't make a difference. If you're drawing down the air system that far you don't have enough margin to start with.
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Christopher H Husmann, PE
"Who is John Galt?"
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