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I'm getting ready to build my own cylinder leakdown tester through average parts found at local hardware stores and so forth. But the whole concept of the measurment of air percentage lost has me confussed.

The tester is required to be set at a desired psi using a regulator. Then a quick connect is supplied to the hose within the cylinder being tested. If my compressor is set at 120 psi and I set my gauge to test for 100 psi i'm going to inject 100psi continuesly into the cylinder. Regarless if its leaking my compressor will kick on and continue to compress air into the cylinder. So how am I supposed to measure the percentage of air lost if my compressor is going to keep 100psi within the line?

Yeah doing a leakdown test will allow you to hear the probably cause but I also want to see the percentage or amount of air lost. If someone can elaborate on the theory of how this test works I would greatly appreciate it. I know the process is simple I'm just lost on how it actually reads percentage of air lost.
 

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Look at how the leakdown tester is setup. You set the regulator on the compressor to 120psi. You set the regulator on the tester to 100psi and the second gauge is used to measure the pressure between the 1st gauge(inlet) on the tester and whats held in the cylinder. If the cylinder is leaking, it will stay less than 100psi.

Be sure to calibrate your gauges so that they read the same 100% of the time.

In all honesty, I'd just buy a leakdown tester unless you just happen to have all of the parts lying around. I bought a tavia leakdown tester a few years back for $65 or so.

 

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Yeah I can understand that.... But my compressor still has 20 psi to try and push into the cylinder. ALSO I am going to lose pressue because I would be connecting the tester to 2-2.5 ft of none pressured air line that goes to the cylinder. So of course there is a pressure drop....

I know I'm taking this a little far because if there is a problem you usually hear it and the need to look at the "percentage reading" is absent.

I build a lot of mini trucks so I do have all the things laying around to throw one together....
 

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a leak down tester simply measures the cylinders ability to seal. the lost air is read on the second guage as the amount of air flow it takes to maintain the test pressure in the cylinder. for the process, there is an article in wikipedia how this measures.
 
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