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Compression Testers - Schrader Valve Pop-off Testing

SteveSr

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Hello,

There has been a lot of discussion lately about the effect of different Schrader valves on compression measurements. The physics says that the Schrader opening pressure is subtracted from your compression reading. In other words if your compression tester is reading 145 PSI and the Schrader valve requires 15 PSI to open then your real compression is 160 PSI.

I just bought a Mityvac 5530? compression tester and also a 10mm spark plug adapter with a Schrader valve from www.TinyTach.com and wanted to know the likely error caused by different Schrader valves so I set up a test with my air compressor as shown in the photos below

IMG_5979.JPG

Test setup. The Mityvac pressure tester is there only for a low pressure gauge to read the result.


IMG_5978.JPG
The test valves and the TinyTach adapter that I used for the test. The test subjects and results are as follows:

Clear - TinyTach valve - 10-12 PSI
White - Mityvac valve - 5-8 PSI
Red - bicycle tube valve - > 30 PSI!

Note: To get good measurements you really need a good secondary low pressure regulator. The standard on in my Makita compressor was rather fiddly at these lower pressures. Also while these valves will actually "pop" open the reading that you want is when the valve just starts to open and leak pressure.

I hope that this will encourage others to measure their Schrader valves and post their results. This way we can find the best valves and also know their effects on our compression reading results.


BTW, someone in another thread was wondering about adding a Schrader valve to adapters that didn't already have them.

IMG_5982.JPG

Here I sectioned the bicycle valve so that you can see how one fits into the housing. Looks like one would have to have the proper tools and do some pretty good machining to get the valve centered and to seal properly around the colored gasket. Any leakage around the gasket would obviously invalidate the effort.
 

breese

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The physics says that the Schrader opening pressure is subtracted from your compression reading. In other words if your compression tester is reading 145 PSI and the Schrader valve requires 15 PSI to open then your real compression is 160 PSI.
I would love to know more about this "Physics".

Just because it takes 15 PSI to open, once open the pressure equalization to the gauge has stabilized between the gauge and cylinder and the Schrader closes The reading should be spot on (depending on the length of the hose between the two).
I do not understand how a 15 PSI Schrader would cause the gauge to read 15 PSI less (i.e. Remove pressure built up behind the Schrader).

The 2 area's of concern when dealing with this type of measurement is Volume.
The volume of air being compressed within the cylinder, transferred to the hose, and then transferred to the gauge.
As the compressed air in the cylinder is released into the hose and gauge there is a natural pressure drop. If X amount of pressure is being released to pressurize the gauge, then that is the amount lost. Not the Schrader PSI.
 
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Lightning Performance

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More trouble than it is worth imo. I just build by the math. Low compression can be felt with a simple pull on the rope.
 

breese

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I read thru the arborist site thread and that went Way off track.
The trying to compare a tire pressure gauge and a compression tester is like trying to compare grapes and grapefruits!

A tire pressure gauge holds the valve open (as designed) for the time period someone holds the gauge on the stem there-by giving an accurate reading (i.e. Equal pressure in both locations).

I do however see a simplistic way of expressing what they Might be trying to explain.

Because the hose and gauge most likely have a greater volume than a single turn of a 2 cycle cylinder, it is required to do multiple pulls (revolutions of the motor) in order to equalize the pressure between the 2.

As such (because the cylinder will go to Zero between revolutions) the gauge vers cylinder compression will be different. Would it be equal to the Schrader # or the physical effort required to break the seal the Schrader is holding?

All great questions but without the proper tools / devices to measure it all, I would have no idea how to completely say Yes or No to the Schrader # is the "missing poundage that should be added to your readings".
 

wcorey

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Note: To get good measurements you really need a good secondary low pressure regulator. The standard on in my Makita compressor was rather fiddly at these lower pressures. Also while these valves will actually "pop" open the reading that you want is when the valve just starts to open and leak pressure.

I hope that this will encourage others to measure their Schrader valves and post their results. This way we can find the best valves and also know their effects on our compression reading results.

All that’s really needed is to look at the pressure differential between an input gauge and an output gauge with the schrader in the middle, then the input pressure doesn’t matter so much.

I’ll rig up something quick and dirty...

An alternative may be to just measure the relative spring tension pushing the valve down on a scale. The cross sectional area of the valve seat looks to be about the same on the different valves so tension alone should be a good indicator.
 

wcorey

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All great questions but without the proper tools / devices to measure it all, I would have no idea how to completely say Yes or No to the Schrader # is the "missing poundage that should be added to your readings".

That would be a “Yes”...
 

wcorey

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295E991A-035B-4676-89A1-2B25D2F0C25E.jpeg


Without valve

4FF2E4BC-FAA3-417A-B785-D41A6C395AB5.jpeg


With automotive (red) valve

159B6866-A11A-4BA0-AAA9-CB17EDB19F4E.jpeg
 
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