# Thread: Nitrogen pressure changes with temperature

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Nitrogen pressure changes with temperature

Posted by someone on another HVAC message board:

The company I work for just changed out a 15-ton split
system. We left it pressured with nitrogen overnight to
make sure we didn't have any small leaks. It was
about 80 F ambient when it was pressured, and the
temp dropped to about 50 F that night. The next day
we had lost 10 psig. Leak check all welds. After finding
no leak we contacted the supplier. They said it was
possible that with that much of a temperature difference
it could have changed the pressure of the nitrogen. It
was something we had never heard, and has never been
brought up before. I was just wondering if anyone had
some insight.

My response:
Re: Nitrogen pressure change with temperature

You want to use Gay-Lussac's Gas law:
P1 / T1 = P2 / T2 (constant Volume and moles)
P = Pressure T = Temperature P1 is first temperature, etc.
or
Second Pressure = (first pressure x second temperature) divided by (first temperature)

The night temperature doesn't matter. The only thing that
matters is the temperature of the gas at the time you
measured the second pressure.

So the second pressure should equal the first pressure
multiplied by the second temperature all divided by the
first temperature.

In order for the equation to work, you have to convert from Fahrenheit to
some temperature scale that is zero at absolute zero. You can convert
to Kelvin degrees or Rankine degrees, then back to Fahrenheit. If you
have Java on your computer here is a website that will do the conversion
for you.

http://www.csgnetwork.com/tempconv.html

To convert from Fahrenheit to Rankine subtract 459.67

To convert from Rankine to Fahrenheit add 459.67

Here is another site that will do the conversion:

http://www.metric-conversions.org/te...fahrenheit.htm

Cheers,
Michael

2. Better blow some N2 through the lines before you pressure check too, you want to get all the air and moisture out that you can because it will throw your reading's off too.

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Hi amickracing,

Originally posted by amickracing
Better blow some N2 through the lines before you pressure check too, you want to get all the air and moisture out that you can because it will throw your reading's off too.
You don't need to get air out of the lines or
for that matter any gas. So long as there is
just gas in the line pressure will vary in
direct proportion to temperature increase or
decrease. Liquid moisture turning to vapor
could theoretically influence pressure, but
unless there is standing water, I doubt that
mere moisture turned to water vapor would do
much to increase internal pressure.

If you are vaccuming a system with moisture,
you are not going to see the moisture create
a system pressure of even 1 psig.

Small amounts of moisture in refrigerant oil
are not going to change the pressure reading
even 1 psig.

Cheers,
Michael

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nitrogen pressure does not change with temp change, you have a leak, even if it changed 1lb you have a leak.
the only way the pressure changed is if there was another gas in the system.

5. Originally posted by airworx
nitrogen pressure does not change with temp change, you have a leak, even if it changed 1lb you have a leak.
the only way the pressure changed is if there was another gas in the system.
not true .. it will change .. the other day we did a coil & cond unit change out .. i did od unit & when it was time to pressurize .. i allowed it to hold so i can spray my fittings & so he can inside ... well i went & took a dump & came back & my suction pressure was up real high ... i left it at 90 .. it was past 100 .. i was like wtf .. lol then i immediately knew he disregarded my standing pres. test & either put on the fan or the heat .... anyway it will change

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Dear airworx,

Originally posted by airworx
nitrogen pressure does not change with temp change, you have a leak, even if it changed 1lb you have a leak.
the only way the pressure changed is if there was another gas in the system.
Nitrogen pressure does change with temperature
change as does the pressure of every gas in this
universe. You might want to review the Perfect
gas law. PV = nRT Then Charles gas law. And
Boyle's gas law.

I'd invite you to review my inlaws gas, but I
wouldn't be that crass.

There is a direct mathematical relationship
between the pressure of every single gas in
the universe and its temperature so long as
gas volume remains constant. With volume constant,
increased pressure leads to increased temperature
and vice versa. The increased temperature of a
compressed gas is called heat of compression.
Compressors add heat to refrigerant by
compressing refrigerant gas.

On the other hand if you raise the temperature
of a gas, while its volume remains, that means
the atoms and/or molecules of gas will move
with greater velocity and exert a greater force
on the sides of the container. Temperature
is really just a measure of speed. As atoms
of a gas move around faster, they strike the
walls of their container with greater force,
and this exerts greater pressure on the container
walls.

My college major was mathematics and physics.
I took second year college chemistry at Wayne
State University during the summer following

Cheers,
Michael

7. Doubt you would see a 10PSI drop in pressure with just a 30° temp change.

Nitrogen has a very small thermo expansion rate.

8. Well they are going from 80 to 50 F, be about a 5.6% drop in absolute pressure.

(50 + 460)/(80 +460)=0.9444444444444444444

so the pressures would drop in a similar manner.

10 psi is a big drop though.

10/.056=178.6 psia absolute or about 164 pounds of gauge pressure. So maybe it could drop from 164 to 154 psi because of the temperature. They pump it up that high in the first place?

9. Originally posted by cobaltfjord
Dear airworx,

Originally posted by airworx
nitrogen pressure does not change with temp change, you have a leak, even if it changed 1lb you have a leak.
the only way the pressure changed is if there was another gas in the system.
Nitrogen pressure does change with temperature
change as does the pressure of every gas in this
universe. You might want to review the Perfect
gas law. PV = nRT Then Charles gas law. And
Boyle's gas law.

I'd invite you to review my inlaws gas, but I
wouldn't be that crass.

There is a direct mathematical relationship
between the pressure of every single gas in
the universe and its temperature so long as
gas volume remains constant. With volume constant,
increased pressure leads to increased temperature
and vice versa. The increased temperature of a
compressed gas is called heat of compression.
Compressors add heat to refrigerant by
compressing refrigerant gas.

On the other hand if you raise the temperature
of a gas, while its volume remains, that means
the atoms and/or molecules of gas will move
with greater velocity and exert a greater force
on the sides of the container. Temperature
is really just a measure of speed. As atoms
of a gas move around faster, they strike the
walls of their container with greater force,
and this exerts greater pressure on the container
walls.

My college major was mathematics and physics.
I took second year college chemistry at Wayne
State University during the summer following

Cheers,
Michael

So you quit your day job at NASA to fix window shakers?

10. Professional Member*
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NASA... too funny

11. Regular Guest
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WTF... Nitrogen is what is referred to as an inert gas... Pressures don't change regardless of temperature... For the people that have seen pressures rise, have you replaced your manifold set recently, if you have an old set and close them off with 250+ pounds of bottled nitrogen on one side and 100+ pounds system nitrogen on the other, you will bleed through... Secondly, if you put nitrogen into the vapor side of the system, it may take time to show up on the liquid side of the system depending on the TXV... So, if you put in 100+ pounds, you leave for the night, you come back and you have lost pressure and you don't have a leak, it probably took some time to bleed through the remainder of the system...

12. Sorry, but I gotta disagree. Inert gas means that it is not reactive under normal circumstances. Hence, nitrogen is a popular choice to flush a system prior to brazing to prevent the oxidation reaction. Also good for food preservation, etc.

I gotta go with Mr. Science, too.

See http://en.wikipedia.org/wiki/Gas_laws

13. ## If

you do not "chase" the air out of the refrigerant lines and indoor coil with nitrogen and just dump it in, the pressure will flucuate in the system you are testing. Trust me, I was faced with this same scenario this weekend on a Tempstar 4 ton install. Pressure dropped 3psi when the attic dropped 20 degrees overnight. Next day, conditions were identical and magically I got my 3psi back. Vacumned the system down to 500 microns and opened the service valves.

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