Manifold
pressure gauges will never tell you how much refrigerant is in the A/C system,
nor will they ever tell you if the system is fully charged or not. Anybody
who tells you that the pressure gauge readings will show if the system is
full or how much refrigerant is in the system is just lying!
In
fact, it’s an old misbelief that somehow, using some magic formula that
an A/C technician can tell how much refrigerant is in the A/C system just
by looking at the gauges. First of all, understand that the pressure gauges
are showing refrigerant pressures. Every A/C system is properly recharged
by weight of refrigerant. There is absolutely no correlation between the
pressure and the weight.
To
better understand this, you should first of all realize that refrigerant
does have a pressure/temperature relationship. What ever change occurs to
one (either pressure or temperature) a similar change will occur to the other.
In other words, if you do nothing more than increase the pressure of the
refrigerant, you will automatically increase it’s temperature, without
adding any outside source of heat.
Likewise,
just increasing the temperature of the refrigerant will automatically cause
the refrigerant’s pressures to rise. Remember, in both of these cases,
the changes occur automatically.
You
should also remember that in both cases, the opposite holds true as well.
That is, reduce the pressure of the refrigerant and you automatically reduce
it’s temperature. Cool the refrigerant, and you automatically reduce
the refrigerant’s pressure.
For
openers, you should start to realize that this one simple factor alone means
that pressure gauges could never tell you anything accurate about the amount
of refrigerant, because the pressures would constantly be changing with the
temperature of the ambient air.
As
an example, take a typical small can of refrigerant and connect it to a pressure
gauge, just like when recharging an A/C system. Assuming an ambient air
temperature of 70°F (21°C), the refrigerant’s pressure would
be about 70 psi. However, if the ambient air temperature was 100°F
(38°C), then the refrigerant’s pressure would be 124 psi. Now remember,
these are refrigerant pressures in the can, just as they would be in an A/C
system without the compressor operating.
In
the above example, there is a difference of about 55 psi for the same can
of refrigerant. Was the can full at 70 psi.? Yes, it was. Was the can full
of refrigerant at 124 psi? Sure was! But the difference in pressure readings
means that it’s really impossible to tell!
Additionally
you should realize that when you’re recharging an A/C system, you know
that the last portion of the can is more difficult to get into the system.
That’s because when the can gets close to empty, the pressure is certainly
reduced. But typically, that is when the can is far under 1/2 full.
The
only guideline that you should follow regarding pressure gauge readings and
the amount of charge are as follows:
1-
Static pressure gauges don’t really tell you anything. If the pressures
are close to the ambient air temperature in °F, then you only know that
there is some refrigerant in the system and you can proceed with your
A/C performance
test.
2-
If static pressures are under 50 psi, you should consider that the system
is basically empty. It will have to be recharged before doing any accurate
performance testing or diagnosis. Just adding refrigerant to a system that
is so low is not a good idea. If there are other contaminants (like air)
in the system, those contaminants will effect pressure gauge readings and
your diagnosis can not be accurate.
Besides
the two guidelines mentioned above, the only real way to understand
exactly how much refrigerant (or freon) is in the A/C system is to recover
all of the refrigerant, pull deep vacuum and then recharge it with
the factory specified amount of refrigerant. |
This
was probably one of the best books on recharging air conditioning that I
have seen...
