AC "Resting Pressure" question.... - PeachParts Mercedes-Benz Forum

JimmyL · #1 05-13-2007, 09:41 PM

What I am calling "resting pressure" is the freon pressure read on the gauges when both the high and low side have equalized.
The wagon has not done well this week with the temps in the mid 80's. Only getting about 50 degrees out of the vents. Running Freeze12 unfortunately, aux fan working.
At the end of last summer, last time measured, my "resting pressure" was in the 95-100 range. Temps were in the high 90's at least.
Does temperature affect this "resting pressure"? Temp outside is around 80 degrees, and the pressure was only about 75. Does this indicate some of the freon leaked out, or is the "resting pressure" less at lower temperatures?
Thanks

jbaj007 · #2 05-13-2007, 11:07 PM

Resting pressure is directly related to temp. PV=nRT

Each refrigerant has a different coefficient fot this relationship. It's cool to fiddle with the Ideal Gas Law, but resting pressure tells you next to nothing as far as practical A/C repairs go.

#3 05-13-2007, 11:10 PM

JimmyL · #4 05-14-2007, 09:29 PM

So, the higher the outside temp, the higher the nominal {resting} pressure will be, given an equal amount of freon in the system?

#5 05-14-2007, 10:25 PM

Quote:

Originally Posted by JimmyL

So, the higher the outside temp, the higher the nominal {resting} pressure will be, given an equal amount of freon in the system?

Correct, but it's actually a little more complicated than the ideal gas law because it's a two phase system (liquid and vapor) in equilibrium in a constant volume. The pressure should go up as the temperature goes up.

vstech · #6 05-14-2007, 10:34 PM

this is true, however, if the system is equalized and you are reading 75psi at 80°F, then you are certainly low.
at equalized 80°F you should be around 85 to 87 psi depending on the mix of chemicals in the Freeze 12. this is only true, if the car is not hot inside, and it's not been running for long enough to totally soak the temp throughout the system.
so, while it is a pretty useless #, the resting pressure, can tell you at least if you have some liquid in the system. in your case, I doubt it does.
John

sailor15015 · #7 05-14-2007, 10:37 PM

Should we use that chart as a guide to charge our systems? My resting pressure at equilibrium was about 65 psi at 75*F and that chart says it should be 77 psi. However, I see no bubbles in my receiver/dryer when the a/c is running. Should I add a can or two?

*Edit*

Its an R-12 system, by the way.

JimmyL · #8 05-14-2007, 10:39 PM

Thank yall very much. I was looking at this number as merely an indication of a possible low charge, and not in any "absolute" condition.....
I kept pretty good records from last summer with temp, resting pressure, low idle high and low side, and high idle high and low side pressures.

vstech · #9 05-14-2007, 11:16 PM

Quote:

Originally Posted by sailor15015

Should we use that chart as a guide to charge our systems? My resting pressure at equilibrium was about 65 psi at 75*F and that chart says it should be 77 psi. However, I see no bubbles in my receiver/dryer when the a/c is running. Should I add a can or two?

*Edit*

Its an R-12 system, by the way.

not a guide at all. merely a quick and dirty way to know if there is liquid in the system if the numbers are low.
it only works, when exact temps are known, and boiling is occurring. in otherwords, liquid needs to be present, and the inside and outside coils need to be in exact same conditions.
for instance, if you have your guages hooked up to a bottle, that chart will tell you exactly what temp the bottle is, if there is liquid in it.
however, if you have liquid in the low side, and the high side. (evap and condenser) and the car is in the sun, and the windows are up, the inside will be hotter, than the outside, so a mixture of the two temps will show.
also the liquid will flow to the cooler side, so it's nowhere near the guide you want. it's only an indicator if the temps show there should be much higher pressures at the temp measured, this shows there is either some boiling off through a leak, or there is no liquid present.
in other words, running is the only way to tell the charge other than weighing in the charge.
John