R12 systems have a receiver drier at the outlet of the condenser. The sight glass is either part of the receiver drier, or part of the liquid line near the receiver drier. When the compressor clutch engages, bubbles appear in the sight glass and they should clear within 2 seconds. CCOT, (clutch cycling orifice tube) systems do not have a receiver drier. The liquid line runs from the condenser outlet directly to the orifice tube. As a result, there could be bubbles at any time in the liquid line, so a sight glass would be meaningless. CCOT systems operate on a "critical charge". This means that as little as 2 ounces too much or too little refrigerant, will greatly affect performance. That alone is a perfect example of why NOT to use the little cans that the auto parts stores sell... there is no way of precisely measuring the refrigerant.

If the system has a VOV, (instead of a fixed orifice tube) and the evaporator is flooding, (freezing over) the system is overcharged with refrigerant. The VOV is designed to allow more refrigerant flow to the evaporator at lower speeds when ambient temp is high. It comes very close to flooding the evaporator, (32psi). If the system is even slightly overcharged, the evaporator is likely to freeze up, especially if ambient humidity is high. Using a VOV you can actually under charge the system, (~ 2 ounces) and still achieve excellent results.

 

From what I read after my little struggle last spring the VOVs almost have to be undercharged a little.
I had very good luck with the small cans, was lucky enough to have a restaurant scale set on a saw horse table that I could very accurately deduce proper charge. I could sure see how could be a bit of a bitch to get an exact weight without some kind of a rig.


You are so correct about even a little too much. Now when a friend comes over and says I just charged my system and it's all froze up I just start bleeding, oh I mean recovering, till it works.


Too bad about the no sight glass thing, thought they were just getting cheap!