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Volume / Pressure Conversion

Robert Leach
All,

I know this is not necessarily PCDMIS related but it is job related.

The note on the print says:

Max leakage at 5.5 bar: 5 cubic mm per second at atmospheric pressure.

I have a tester that can input 80 psi and I can measure the drop in pressure. For instance I can watch for 1 min and see that it drops to 70psi. Is there a way to convert this into something I can use to satisfy the print callout? Any help would be greatly appreciated.
  • Yes, that is correct. That is why you have to use the ideal gas law to find the difference in molar mass of the chamber air. Using this difference the only now is the volume of the evacuated air which you would then use atmospheric state variables to calculate along with the "missing" molar mass from the chamber. Divide this volume by time and you have your volumetric flow rate.

    right, all I was saying is that it's impossible with provided info. The theo volume of air chamber does not reflect the actual and the leakage allowance is too small, you are talking about 5 cubic mm per second, that's not even a fart.
  • right, all I was saying is that it's impossible with provided info. The theo volume of air chamber does not reflect the actual and the leakage allowance is too small, you are talking about 5 cubic mm per second, that's not even a fart.

    I do not see how it is impossible to calculate- I completely agree with jhingle. True, the theo and actual volumes are going to be slightly different, but the difference should be so small you can use the theo with very little error. If you want more info on the ideal gas law and how to use it, check out https://www.thermofluids.net. It's a website created by one of my old thermodynamics professors at SDSU. It has calculators (damons) that will do all the calculations for you if you input your info correctly. But you may have to pay a little bit of money to use the damons. Either way, there is plenty of info on there that should help you out
  • I do not see how it is impossible to calculate- I completely agree with jhingle. True, the theo and actual volumes are going to be slightly different, but the difference should be so small you can use the theo with very little error.

    That would be a neat trick. We could assume and speculate all day long.
  • That would be a neat trick. We could assume and speculate all day long.

    C'mon man, you can't be serious. It's called measurement uncertainty and you deal with it everyday. Not knowing the "exact" measurement of something is a fact of life. The fact that the allowance is so small is that much more of a reason to use calculations because there would be no real way to measure it.
  • I attempted to post my formula above. I was unable to enlarge it. Check it out and thanks for all of the help everyone.
  •

    Or equivalently your volume change over time is equal to chamber volume times pressure drop over time divided by atmospheric pressure. I didn't think to work through the gas law with variables, and after seeing that formula I did it and arrived at the formula I stated above which is equivalent to your formula. Anyway, seems like you have it figured out.
  • C'mon man, you can't be serious. It's called measurement uncertainty and you deal with it everyday. Not knowing the "exact" measurement of something is a fact of life. The fact that the allowance is so small is that much more of a reason to use calculations because there would be no real way to measure it.

    We do both pressure test and vacuum test here, from light to heavy. Things that are associated with atmospheric pressure we calibrated them at that. As required per contracts, we spent millions on test equipments over the past 2 years. We just finished installed 4 air pressure tanks, 40 feet high x 12 feet in diameter ea (max 600psi) on 6 feet thick concrete...

    The way we callout is a bit different; we have close to 50 different type of valves, with different type of seats, different requirements. Leakage allowance depends on the function of each valve, some constant, some are average on x amount of time, some are not and should not exceed the max allowance at any point during the test cycle. All that saying is that we cover wide range of pressure test, I am not sure if we have the proper equipment to check what the OP asks, but I’m very certain that there is something out there will do just that. Based on the max given value, I have to think that whatever he’s testing is critical, it should not be taken lightly.
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