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Thread: Pressure drop due to temperature change?

  1. #1
    DIY Senior Member Noth Jersey's Avatar
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    Default Pressure drop due to temperature change?

    What is the magnitude of the pressure drop in a gas line that can be attributed to temperature change? I pressurized the line with air for a leak test. The pressure dropped from 17.5 psi to 17 psi over about three hours. I noticed the temperature of the attic, where about 2/3 of the line runs, dropped around 15 degrees during this time. If you're doing a 24 hour test, what kind of pressure change could be chalked up to temperature?

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    Plumber jimbo's Avatar
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    This pretty well sums it up:

    http://en.wikipedia.org/wiki/Ideal_gas_law

    The pertinent equation in this case is for contstant volume:

    P2= P1(T2/T1)

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    Plunger/TurdPuncher kingsotall's Avatar
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    Default nano plumbing

    And in laymens terms...
    I just post cuz I like to see my avatar.

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    Plumber jimbo's Avatar
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    P1 is your starting pressure. Multiply it by the ratio of the new temperature divivided by the original temp. to get the new pressure P2..

    I have to double check what units the temp. must be stated in. It might need to be kelvin.


    edit: yes, temp. is in K. So, lets take an example:

    starting Temp. 80 Ending temp 65
    starting press. 15

    ending pressure = 15 X ( 291.98/299.81)

    ending pressure appro. 14.6 PSI



    note: I hope PSIG is the proper unit for pressure. Might it have to be PSIA?? I have to sort that one out!
    Last edited by jimbo; 06-07-2009 at 04:46 AM.

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    Moderator & Master Plumber hj's Avatar
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    Default pressure

    One the temperature stabilizes, the pressure should also. That is why the inspector usually only checks it during a 15 minute period, assuming you have a very delicate gauge.

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    Plumber Winslow's Avatar
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    I use a manometer to test gas lines. It is by far the best method. I use a mercury manometer to test rough piping (as per UPC) and water manometer to test final. It let's you know instantly if you have any leaks.

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    DIY Senior Member Noth Jersey's Avatar
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    Quote Originally Posted by hj View Post
    One the temperature stabilizes, the pressure should also. That is why the inspector usually only checks it during a 15 minute period, assuming you have a very delicate gauge.
    I kept the line under pressure over weekend. After it bottomed out, it rose half a PSI by Monday morning, so I feel pretty confident I'll pass my inspection.

    I should have thought of using the manometer I bought for my HVAC projects. I was just using a 25 psi gauge with .5 psi increments, which is what the propane company and the inspector will look at.

  8. #8
    Moderator & Master Plumber hj's Avatar
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    Default gauge

    Here, it has to be a 10 psi test using a 15 pound gauge with 1/10 psi graduations. Even a manometer will respond to the pressure variation from temperature changes.

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