Refrigerant level is measured in weight, not pressure.
Umm, what?
You most certainly have to know how much pressure is in the system.
Your goal isn’t to add the right number of ounces; it’s to have the correct
pressure. The entire
system is based on pressure, which determines how cold it can get. Too much pressure
and it doesn't evaporate correctly. So, the more you add, the higher the pressure will
get. That's why you need a pressure gauge.
Refrigerant Under Pressure:
If you want to know why having too much refrigerant results in hot air, you have to
understand a little bit about how an air conditioner works. The R-134a refrigerant
used in car air conditioners would normally be a gas (it boils at a frigid 15 degrees
below zero). If you compress it and pressurize it enough, though, it turns into a
liquid. The compressor in your vehicle’s A/C does exactly that, so you have liquid
R-134a pumping through part of the A/C system. When that liquid reaches the
in-dash cooling unit (called an evaporator coil), it passes through a valve that
lowers the pressure. The lower pressure allows the liquid to boil and evaporate
back into a gas. It absorbs heat from the air in order to boil. With the heat
removed, the air becomes cold. If you don’t have enough refrigerant, it won’t
be able to cool the air. If you have too much, though, the pressure will be too
high for the refrigerant to boil or for the compressor to work. Which means the
system won’t cool at all, and you’ll still be left with hot air.
Sometimes people wind up with too much refrigerant because they try to add a
certain number of ounces or cans to the system without checking the pressure.
Your goal isn’t to add the right number of ounces; it’s to have the correct
pressure.
What Could Possibly Go Wrong?
Understanding how the A/C system works can help you understand what causes it to sometimes
not work.
If your A/C is blowing hot or lukewarm air, the most likely culprit is low refrigerant. That’s because, except
for the compressor, there aren’t really any moving parts in an A/C system; the refrigerant is the moving part.
Since it’s a closed loop, your A/C doesn’t ever "use up" refrigerant. And refrigerant can’t wear out any more
than you could wear out water or air. The only way your refrigerant can be low is if it has leaked out.
So, as long as the compressor works, a broken car A/C can usually be fixed by sealing the leak and
refilling the system with R-134a refrigerant. The easiest way to do that is by adding R-134a. If the leak
is in a metal part instead, it needs repaired. Don't use sealants from cans...
The BEST way, is to a shop, where they can evacuate the system, and based on the SPECS, add the
correct amount without using a pressure gauge, because it's only supposed to have so much in it.
Even after a complete evac and refill, they use a pressure gauge to see where it's at, then ADD more
if the pressure isn't right. If you have to add refrigerant, then there's a leak, it must be fixed.
The weight is based on an empty system, to full. For example, on a ford, right under the hood
it actually states 33 ounces or 30-38psi. This is from a completely empty system to recharge.
As a DIY'er, you can't know how many ounces your adding, so you have to go by the pressure
gauge.
AS STATED, the only way to do it right, is to have it evacuated, repaired, then the OEM ounces
added to the system, then test. No one should be adding refrigerant to their cars, it means
there is a problem that needs to be addressed.