The
reasons are that, first of all, you can not know how much refrigerant is
in the system, so you never know how much to add. There are no pressure gauge
readings (regardless of the amount of experience servicing and repairing
auto A/C systems a technician has) that will ever provide an accurate reading
of how much refrigerant is in the A/C system.
Understand
that pressure gauge readings are telling you the pressures of the refrigerant.
The automotive A/C system is always charged by weight. Therefore, connecting
a set of pressure gauges on a typical summer day with ambient air temperatures
of about 70°F and getting pressure gauge readings of about 70 psi only
tells you that there is some refrigerant in the system. That’s all.
Just some refrigerant. In no way should it ever be considered that the 70
psi on the pressure gauges means that the system is full.
As
an easy experiment, you can start with a typical small ‘one pound’
can of refrigerant. Upon inspection you will find that those small cans contain
12 ounces of refrigerant (when they are full). If you connect your low pressure
gauge to a full can of refrigerant, you will find that the pressure is about
equal to the temperature of the day (in degrees F). That is, if the ambient
air temperature is about 70°F, the refrigerant pressure will also be
about 70 psi. Now, as an example, remove 2 ounces of refrigerant. That would
leave 10 ounces in the container. If you do the math, those 10 ounces are
just over 80% of the ‘full’ can. When you check the pressures of
the container with only 10 ounces in it, you will find that the pressures
remain pretty close to the 70 psi that you started with. If you only knew
the pressure of the can was 70 psi, how much refrigerant would you add to
the can to ‘fill it’? The truth is, you wouldn’t know. Both
the full 12 ounce can and the can with 10 ounces of refrigerant will show
about the same pressures. One of the cans is only 80 of the ‘full’
capacity.
The
same applies to an A/C system, but it becomes even more difficult. Typical
auto A/C systems hold about 2 to 3 pounds of refrigerant. That’s about
36 to 48 ounces. When the A/C system is low just a few ounces, it is impossible
to determine that by the pressure gauge readings.
What
becomes a bigger problem is that when adding refrigerant, it is very easy
to overcharge the system. An overcharge of refrigerant will cause the pressure
gauges to increase as the system operates. That increase in pressure could
be confused with other problems on the A/C system. However, because you just
added refrigerant, you still do not know if the system is fully charged and
free of all air and moisture.
One
other consideration is the total size or charge capacity of the A/C system
you are working on. Most of today's vehicles use very little refrigerant.
In fact, in some applications, the entire A/C system charge is only about
a pound of refrigerant or just a little more. That means adding 4 ounces
of refrigerant (about 1/3 of a small can) to a small system that only
holds 1 pound (16 ounces) would mean that the system could be overcharged
by 25%! That amount of refrigerant overcharge will most certainly cause excessive
high side pressures and possibly cause safety relief valves to discharge
the system.
For
those reasons, you should never consider adding refrigerant or topping off
A/C systems as a good or final repair. The addition of refrigerant should
only ever be used as a method of testing the system operation. |
This
was probably one of the best books on recharging air conditioning that I
have seen...
