# Thread: Nitrogen pressure testing

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## 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. 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....

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

4. 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. I would put 100# more in to start with. I know the pressure will drop a little bit. Good question.

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Originally Posted by icemeister
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. 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 ºF to start.
If you are using ºC, convert it to Kelvin by adding 273 to the temperature.

P1/T1 = P2/T2

P2 = (P1/T1)*T2

P1 = 250psig + 15 = 265psia
T1 = 88ºF + 460 = 548ºR
T2 = 74ºF + 460 = 534ºR

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.

8. Originally Posted by dna
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. 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...

10. Originally Posted by jdblack
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. Originally Posted by The Doctor
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 >20º in just a few minutes when a storm front rolled in while pressure testing a unit, quite a few times over the years.

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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.

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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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