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Static pressure + air in system

swalve on Sat March 20, 2010 12:58 PM User is offline

Year: 1997
Make: Pontiac
Model: Grand Prix
Engine Size: 3.8
Refrigerant Type: r134a
Ambient Temp: na
Pressure Low: na
Pressure High: na

Hello- I've been a big fan of the forum for a while now, read quite a lot of it to learn about ac. Thanks!

My question is more of a theoretical kind of question.

I understand that the static pressure of a system is only valid if there is enough charge in the system so that some of it is in the liquid state. It is the vapor pressure of the compound that causes this- a liquid can only boil off if the pressure around the liquid is lower than the vapor pressure. Once they are in equilibrium, no more liquid will boil off unless the pressure is vented or the temperature is raised.

(I actually learned this in a different context a long while back- when I worked at McDonald's, they needed to do a repair on the bulk CO2 tank. The CO2 is in a liquid state in the tank. Instead of trying to pump out all the gas, their procedure was to flash off the pressure in the vessel so that the liquid CO2 actually turned into a chunk of dry ice and the valves could be removed. But I digress.)

Anyway, what effect does having air or moisture in the system have on the static readings? I can't remember enough chemistry to work this out on my own. If we assume that a system has enough charge to have some liquid refrigerant, and that it is at a steady, measurable temperature, would the static pressure be different in a contaminated system?


TRB on Sat March 20, 2010 11:55 PM User is offlineView users profile

Static pressures mean little to me other than to tell if the system has any refrigerant. I know that does not answer your question! But I find static pressure useless for the most part with auto a/c systems.


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bohica2xo on Sun March 21, 2010 2:17 AM User is offline

A badly contaminated system could show the same static pressure as a clean system under the right conditions. A static pressure reading will not tell you if the system is wet, or contaminated with non condensible gas.

A static pressure significantly ABOVE the expected static pressure indicates a large quantity of non condensible gas. Pressure testing with an ounce of 134a & 200 psi nitrogen for instance.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

mk378 on Sun March 21, 2010 11:22 PM User is offline

When you have gases that don't interact with each other, such as air and refrigerant, the total pressure (absolute) is the sum of the (absolute) partial pressures of all substances. For example leaving a system open to air at sea level will fill it with air to 14.7 psi absolute. If refrigerant is then charged in without removing any of the air, the static pressure will be 14.7 psi higher than expected by the vapor pressure of the refrigerant. But in order to know that expected pressure for pure refrigerant, you would need to know the temperature quite precisely. Because of that, the concept is really not actually useful as a field test. Moisture won't noticeably change the static pressure at all, because the vapor pressure of water at ambient temperature is very low.

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