I'm curious about the sight glass as it would be useful in diagnosing my A/C.
When I look at the glass it just looks black! How should it appear, and what
should you see if system pressure is OK?
1988 S4 Auto Marine Blue
The sight glass doesn't tell you anything about system pressure or state of charge when the system is not running.
When the system is operating correctly, and the load and all the approach temps are within the design parameters used originally, the condenser should deliver barely-liquid refrigerant to the drier. The drier is in fact an accumulator of sorts, a reservoir for liquid refrigerant. The liquid is delivered to the expansion valve under pressure. The expansion valve meters liquid refrigerant into the evaporator, where heat from the car boils or evaporates the liquid. It's this transition from liquid state to gaseous state that takes heat, and the source of heat is air from the cabin. So delivering liquid to the expansion valve is good, sending refrigerant that's a gas is bad, since there no state change requiring addition of heat.
So one might be tempted to just add lots of refrigerant to try and keep a lot of liquid available to boil and suck heat away. But there's a catch, and that's the way the compressor/pump works. Since it only pumps vapors, the amount it can pump is related to the pressure/density of the vapor that comes into it. Then, it squeezes it a fixed amount and discharges the hot but higher pressure vapor to the condenser, where the vapor condenses back to liquid. Back at the evaporator, we find that the amount of heat needed to boil the refrigerant is very directly connected to the pressure in the evaporator. Lower pressure means lower temperatures. So there's a bit of a fight between getting the compressor to pump hard enough to condense the gas in the condenser, and at the same time suck hard enough to hold the evaporator pressure and temperature low. Alas, the pump can't do both.
So the ideal charge is "just enough" for the ambient conditions and load. That -generally- means a few bubbles in the sight glass. If you get a full liquid stream showing there on a hot day, you may be overcharged. A lot of bubbles on a cold day, you may be undercharged. That handy expansion valve is all the time doing its best to deliver just the right amount of liquid to the evaporator, so the sight glass doesn't really show state of charge until conditions exceed its ability to adjust. So with the correct charge on a design-conditions day, the drier/sight glass should show a flowing stream of liquid, in which you might see a little trail of oil, and you should see a few but not many bubbles in the liquid flow.
You can probably appreciate why systems are charged by weight these days!
This is a new drier of a GTS. The earlier models (pre-R134a) have a different one.