From: jgd@dixie.com (John De Armond)
Newsgroups: rec.autos.tech
Subject: Re: A/C manifold gauges question
Date: 11 Sep 92 04:37:20 GMT
Organization: Dixie Communications Public Access.  The Mouth of the South.

mikeb@nowaksg.chem.nd.edu (Michael George Buening) writes:


>>The R12 pressure (or is temperature the correct word?) in a
>>non-operating system should be the same no matter what the total
>>charge is, as long as there is still liquid R12 in the system.  For
>>example, the pressure in a propane tank will read the same at a given
>>temperature no matter how much propane is actually in the tank. 

>Whoa!!! What is the logic behind this statement.  I'm not sure what
>you mean here but I can't make sense of this.  As many of you know from
>a simple chem/phys text:              PV=nRT
>If you have more propane in a tank at the
>same temp you will have more pressure.  Because this is related to
>n, or the amount of propane, you can correlate pressure with concentration
>if you know what the volume is.

Gad, and that coming from a host with "chem" in the address.  

Gas law has nothing to do with neither the air conditioning system nor 
the propane tank.  In both cases, as long as there is liquified gas present 
in the system, by definition the liquid and vapor are in equilibrium with each
other and the pressure is the vapor pressure of the liquid at the
prevailing temperature.  In both cases the pressure has nothing to do 
with the quantity of substance in the system.

To the original problem, a quiesent pressure check on a refrigeration
system tells you essentially nothing except that there either is or is
not some refrigerant in the system.  The pressure depends on the 
distribution of volumes across the system and whether there is any 
liquid remaining while stopped.  Late model GM systems with the 
suction pressure control canister, for example, likely will NOT have
any liquid left because of the suction side volume and lack of liquid
accumulator.  My old Fury III on the other hand, probably will because
of the huge receiver and no suction control.

Even when running the suction pressure tells you little about charge 
conditions.  As long as the expansion device is supplied with more or
less enough liquid, the evap pressure will sit at the vapor pressure 
prevailing in the evap plus superheat.  There will be no change as 
charge is added until the system is so grossly overcharged that the 
excessive high side pressure starts reducing the volumetric efficiency
of the compressor so it cannot keep up.  By then the high side cutout
will have tripped if one exists.

There are only 2 and a half correct ways of charging the system.  With
a sight glass, simply charge til the glass is clear plus an ounce or two.
Lacking a sightglass, you must evacuate and charge by weight.  The half
correct way is to use some method to infer the status of the condenser
exit line.  Ultrasonic bubble listeners work sometimes by listening
for bubbles in the line but only if there is not too much noise from
the compressor.  Feeling the condenser is a somewhat workable method.
The liquid flooded rows of tubes are relatively cooler than the part
actively shedding heat.  A last method that works fairly well with 
skill and experience is to briefly heat the condenser exit tube with 
a high intensity flame, say from a MAP gas torch.  If the tube remains
cool, it contains liquid.  If it gets warm, it contains mostly gas.

John
-- 
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