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  1. #1

    Nitrogen pressure testing

    If I put 250psig of dry nitrogen in a suspected leaker, would the pressure drop overnight if there was not a leak and the ambient dropped from 88deg to 74deg? I know temperature and pressure are directly related, but how does that apply to nitrogen?

  2. #2
    Join Date
    Apr 2005
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    http://webbook.nist.gov/cgi/fluid.cg...79&Action=Page

    you can punch in the numbers on the link above...you may get a pressure drop, but not much you could actually measure....
    Jason J Saylor
    Pinellas County Schools
    HVAC Tech
    Pinellas County Florida

    "You will encounter many distractions and many temptations to put your goals aside: The security of a job, a wife who wants kids, Whatever. But if you hang in there, always following your vision, I have no doubt you will succeed.
    Larry Flynt quote

  3. #3
    I have about 12 years in the field, but not in thermodynamics. I am looking for something like a T/P chart.

  4. #4
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    Marc O'Brien has a handy calculator for a change in temperature's effect on nitrogen pressure on his Fridgetech website:

    http://www.fridgetech.com/calculators/nitrogen.html

    I get a result of 243.2 psig.

  5. #5
    Join Date
    Nov 2000
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    Lancaster, Pennsylvania
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    I would put 100# more in to start with. I know the pressure will drop a little bit. Good question.
    KX500......the original big green meanie

  6. #6
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    Quote Originally Posted by icemeister View Post
    Marc O'Brien has a handy calculator for a change in temperature's effect on nitrogen pressure on his Fridgetech website:

    http://www.fridgetech.com/calculators/nitrogen.html

    I get a result of 243.2 psig.

    thanks for posting that.

  7. #7
    Join Date
    Aug 2004
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    North Richland Hills, Texas
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    To do the calculation, you need absolute pressure and temperature values. It is ok to round decimals to the nearest whole number.
    For the pressures, just add 15 to psig to get pisa. For temperature add 460 to the temperature to get the absolute temperature in the Rankine scale, assuming you are working with F to start.
    If you are using C, convert it to Kelvin by adding 273 to the temperature.

    P1/T1 = P2/T2

    P2 = (P1/T1)*T2

    P1 = 250psig + 15 = 265psia
    T1 = 88F + 460 = 548R
    T2 = 74F + 460 = 534R

    P2 = (265/548)*534

    P2 = .489*534

    P2 = 258 psia

    265 - 258 = 7

    The pressure drop would be about 7 psi.

    Or just use one of the handy calculators.
    If more government is the answer, then it's a really stupid question.

  8. #8
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    Quote Originally Posted by dna View Post
    I would put 100# more in to start with. I know the pressure will drop a little bit. Good question.
    Why would you tell someone to put in 350psi of nitro on a system that we know nothing about?

    This is why there is no DIY.

    I've done all my leak testing and pressure testing with 150psi. I don't see a need for anymore than that.

  9. #9
    Join Date
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    Charm City--the city that bleeds
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    If it is 100% nitrogen, does a "standing pressure test" ever change much in a short amount of time? Usually not.
    Now if someone had been leak checking with a refrigerant and there was some left in the system before the test, then there would likely be a change...
    It's great to be alive and pumping oxygen!

  10. #10
    Join Date
    Jan 2010
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    Quote Originally Posted by jdblack View Post
    Why would you tell someone to put in 350psi of nitro on a system that we know nothing about?

    This is why there is no DIY.

    I've done all my leak testing and pressure testing with 150psi. I don't see a need for anymore than that.
    Most low side non 410 coils show test pressure @ 150 psig.

  11. #11
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    Quote Originally Posted by The Doctor View Post
    If it is 100% nitrogen, does a "standing pressure test" ever change much in a short amount of time? Usually not.
    If the temperature is changing one way or the other, you will see a noticeable pressure change, even in a very short time.

    I use digital gauges that read down to the tenth of a psi. I've watched the pressure go down a small amount, then back up, just from a gust of wind.
    My first generation Testo 523 actually goes down to the hundredth of a psi in its temperature compensated pressure test mode.

    I've also had the outdoor temperature literally drop by >20 in just a few minutes when a storm front rolled in while pressure testing a unit, quite a few times over the years.
    If more government is the answer, then it's a really stupid question.

  12. #12
    I'll rephrase. I know the system leaks because it didn't have enough gas in it to register in my high side gauge. I started out pressure testing at 200psig and waited for about 15 minutes with no noticeable change. The pressure went up from there because eventually a leak will be found whether it was already there or developed from too much pressure. Safe to say it will probably be easier to find with a little gas and a sniffer.

  13. #13
    Join Date
    Mar 2008
    Location
    SW Ohio
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    [QUOTE=mark beiser;
    My first generation Testo 523 actually goes down to the hundredth of a psi in its temperature compensated pressure test mode.[/QUOTE]

    Is this the gauge that Jim Bergmann is referring to in his article in RSES Journel / July pg46 1st para under "Why digital does it better"?


    referrnr, there is a great article in that issue called "the pressure is on" that is a good read on the subject. If your not an RSES member you should consider it. Best return on $100 investment I've ever made. For what's available it's hard to believe it's only that for a year.

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