Will the Real Compression Test Please Stand Up!

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It takes x PSI to open it, but once it's open, what's there is there.

Not so, the spring is still exerting it’s pressure to close the valve. But I agree that with the right valve who cares - you use the gauge reading.

This is close to becoming an oil thread. So I am agreeing to disagree and I am out. Not mad or anything - just have more productive things to do.

Ron
 
Not so, the spring is still exerting it’s pressure to close the valve. But I agree that with the right valve who cares - you use the gauge reading.

This is close to becoming an oil thread. So I am agreeing to disagree and I am out. Not mad or anything - just have more productive things to do.

Ron
But once you are above that pressure it's open. At spring pressure and above, is it not open? No problem here, enlighten me.
 
It takes x PSI to open it, but once it's open, what's there is there.

Nope, that’s the point: It doesn’t open and stay open, it needs x psi differential across it and once flow stops into the gauge, the valve closes, holding that differential pressure. The gauge never reaches tghe cylinder pressure.

Here’s an experiment: put a tire valve into the compr tester and do a test again, It will be way low on the gauge.

If the valve truly stayed open, there would be no need for the special schrader valve.

In practice, it doesn’t matter as the valve spring is so small with the proper valve. But this is the reason you can’t use tire stem valves, and have to buy the $2 ones specifically for compression gauges.

For compr gauge, just use the right valve. But understand the concept if you set relief or pressure regulating valves.
 
It takes x PSI to open it, but once it's open, what's there is there.

Not so, the spring is still exerting it’s pressure to close the valve. But I agree that with the right valve who cares - you use the gauge reading. This is close to becoming an oil thread. So I am agreeing to disagree and I am out. Not mad or anything - just have more productive things to do.
Ron

I think Brad may be referring to the tire valve which is pushed open by the mechanical force of the hand holding the tire gauge. Once open and HELD open, it's as if the tire valve was not even there. This situation is quite different from that where the compression gauge valve is being opened (but not held open) by the compression pressure within the cylinder. Does that make it clearer?
 
I think Brad may be referring to the tire valve which is pushed open by the mechanical force of the hand holding the tire gauge. Once open and HELD open, it's as if the tire valve was not even there. This situation is quite different from that where the compression gauge valve is being opened (but not held open) by the compression pressure within the cylinder. Does that make it clearer?
What would make it clear would be does a compression gauge give the exact correct numbers that are in the cyl?
 
What would make it clear would be does a compression gauge give the exact correct numbers that are in the cyl?

Let the pile continue:
Compression gauge reading = Actual cylinder pressure minus opening pressure of gauge Schrader valve.

This is the reason for the special Schrader valve . . . to make the opening pressure approach zero.
 

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