Pressure drop due to temperature change?

[Noth Jersey]

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?

[jimbo]

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)

[kingsotall]

And in laymens terms...¿

[jimbo]

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 ºK. So, lets take an example:

starting Temp. 80º Ending temp 65º
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!

[hj]

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.

[Winslow]

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.

[Noth Jersey]

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.

[hj]

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.