From: jgd@dixie.com (John De Armond)
Subject: Re: A/C manifold gauges question
Date: Sat, 12 Sep 92 03:16:26 GMT
Organization: Dixie Communications Public Access.  The Mouth of the South.

plumpe@oasys.dt.navy.mil (David Plumpe) writes:

>    I generally keep track of refrigerant levels in both my car and
>house A/C by feeling the temperature of the suction line where it leaves
>the evaporator.  

>    When adding refrigerant, I do it through a gage set, but use the above
>"feels good" method to tell me when enough's enough.

>    Is there anything wrong with what I'm doing (and have done for ~20
>years)?


It only tells you the system condition in the broadest terms and then
only under certain conditions.  For example, if the heat load is mild,
the suction line will sweat even if the system is low on refrigerant.
It will also sweat if the humidity is high.  The line will start sweating
before the system is fully charged and will not stop sweating until the system
is grossly overcharged.  As an excess of refrigerant is added, the condenser
will start filling up.  As it does, the available condensing surface is
reduced, the temperature and therefore the pressure rises. Only when the 
pressure rises to the point where the compressor loses efficiency, the 
liquid remains hot enough to affect the evaporator or the high pressure
cutout operates will the suction line stop sweating.  The situation gets
even more complicated when you start to consider the modern design that 
typically has some kind of suction line control valve such as a POA (Pilot
Operated Absolute) valve that regulates the evaporator pressure.

Older systems with lots of receiver capacity and large bore liquid lines
are much more tolerant to overcharging.  Newer systems that run with 
minimal refrigerant are much less tolerant.  Example.  The 68 Fury III
I just bought specifies 3.5 lbs of freon.  My wife's toyota specifies 
14 ounces.  Both cool about the same.  I stuck an extra half pound of freon
in the Fury because I knew it would be stored in the receiver.  The only
receiver on the Toyota is the volume of the liquid line so any extra
refrigerant fills up the bottom of the condenser and reduces its efficiency.

I look on the "feel" method as I do tuning an engine by ear.  It'll get you
in the ballpark but for optimum performance you have to instrument.  If you're
working on an older system and using a 14 oz blowoff can, you'll have to
work to screw it up regardless.  Dumping that same blowoff can in a modern
system that doesn't need it will at least reduce the efficiency and may
cause the high pressure cutout to operate.  Since blowoff cans are 
history, you'll no longer have that safety margin as you work with a 30 lb 
can.  It's trivially easy to dump in too much refrigerant from one of
these.

John
-- 
John De Armond, WD4OQC               |Interested in high performance mobility?  
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