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# Thread: Pressure drop due to temperature change?

1. ## 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?

2. 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)

3. ## nano plumbing

And in laymens terms...¿

4. 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!

5. ## 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.

6. 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.

7. Originally Posted by 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.
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. ## 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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