Nitrogen pressure

Techman

Does the pressure of the nitrogen change (overnite) in relation to ambient temp, this is on a residential unit. 80*f, press test is 400psig and in the morning it's 50*f. Now , it's only vapor ,right?, so the p/t chart does not apply. 

ChrisJ

No answers

Techman, did you see much of a change if any at all?

Techman

nitro press

Hay ChrisJ. I'm not sure, I want to know if it's a leak or a vapor press drop due to " The Shrinkage Factor".  How do you check what the press should be? Is your workshop KindaSorta the same temp all/most of the time?

ChrisJ

No

Hi Techman,



Not sure what you mean? The temp in the shop stays the same for the most part.

I was wondering if you saw much of a change in pressure when the ambient went from 80F to 50F?

Techman

NitroPress

There was a press drop from 400 down to 397 . With your shop,  the Vapor press had better be the same press because the temp is the same or you do have a leak. I looked in my 1943 book and nothing!

ChrisJ

Ah

After re-reading I'm guessing it did change.



That's a good question and I don't know the answer. I had the same problem when I pressured tested my natural gas line. I didn't know if I had a leak, or if it was temperature change. I found out after just letting it sit there for 2 weeks but there must be a way to know how much dry nitrogen should change.

Techman

Nitro press.

I feel your pain bro!

R Mannino

Short Answer, Temp will change Pressure

http://en.wikipedia.org/wiki/Charles%27s_law



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

Techman

Press Temp

Hi R Mannino,I agree. But at what rate does the press drop? Some here,in past posts , have said there should be no press change because it is nitrogen, but I believe that's at the same temp..I'm trying to understand if I have a leak or just a press drop as you said. Is there a chart KindaSorta like a P/T chart? Thanks!

Eugene_Silberstein

Pressure Testing with Nitrogen

There can indeed be a pressure difference between the time you pressurize the system and the time you check the pressure. There is a neat little formula that will give you the acceptable pressure loss on a system. The formula is:



0.8 (Tp - Tc) = Pressure Loss



Where Tp is the ambient temperature when the system was pressurized and

Tc is the ambient temperature when the system pressure is checked.



For example, a system is pressurized to 200 psig when the ambient temperature is 100 Degrees and the system is checked the next day, when the ambient temperature is 80 Degrees, and the pressure in the system is 160 psig.



The acceptable pressure loss is 0.8 (100 - 80) = 16 psig, so the system pressure should be 200 - 16 = 184 psig. The system pressure of 160 psig indicates that there is a leak in the system, as the pressure is too low.



As an aside, we have to be careful that we use "dry" nitrogen to pressurize our systems. The nitrogen we get from welding supply houses is NOT DRY. Be careful!

ChrisJ

Dry or not

When I bought my tank I specifically said "dry nitrogen".



Is there anyway to be sure you get the right thing? I bought from an Airgas distributor.

Eugene_Silberstein

Dry Nitrogen

Many people call nitrogen "dry" if it is not liquid. However, the term "dry" refers to the moisture content in the nitrogen. Industrial nitrogen, which is what most air conditioning guys use for system pressure testing, can have moisture contents over 400 ppm.



What we should be using is medical grade, NF, nitrogen, which can have moisture contents lower than 10 ppm. Of course, NF nitrogen is much more expensive than industrial nitrogen, but it will hep reduce evacuation times as well as contribute to extended equipment life expectancy.

ChrisJ

Medical grade

Ah.

I wonder if I can get that in a size 60 cylinder which is what I bought. The refrigerators I work on use refrigerants that are very sensitive to moisture so the last thing I want to do is expose the oil and the rest of the system to any more moisture than necessary.



The next time I get a refill I will see if I can get medical grade nitrogen.



Thank you Professor Silberstein.

Techman

Dear Professor

Nice "Formula" ,I will certainly remember that!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! But, do you have a formula for the #'s that I supplied ? I had a similar situation this very day, at 30*f ambient!. Humbly Yours, again, Terry.

Eugene_Silberstein

You are Fine

If the temperature dropped 30 degrees, you should have a pressure drop of no more than 24 psig 0.8 (80 - 50) = 24 psig. You mentioned that you lost 3 psig.



I would call that a homerun.

Techman

Nitro PPM of moisture

If I'm using that crummy moisture laden nitro w/ a water ppm of 400 and the MoistureIndicator/SightGlass is sensitive to/at 65-75 ppm then ,,,,,,,,,,,,,,,what?

ChrisJ

Can

He said it can have moisture as high as 400ppm. Doesn't mean it necessarily does though.





Personally, I'd rather be safe than sorry.

Techman

OK ChrisJ

399 sounds better! LOL!  You were doing 66mph in a 65mph zone and YOU get a ticket. LOL!  Happy T'giving Day!