Proper Reading of the Sight Glass

On R-12 systems, look at the sight glass of the receiver/dryer. This will provide preliminary information on the condition of the refrigerant. With an R-12 system, there will be differences between the cold and hot weather appearance of the sight glass. Generally speaking, bubbles tend to appear in hot weather and are slow to appear in cold weather.







- A few bubbles show up 2 - 3 seconds after the compressor cycles on.
- High-pressure side is hot and low-pressure side is cold.
- Refrigerant is sufficient.







- Bubbles flow continuously; oil streaks.
- Almost no difference in temperature between low - and high-pressure sides.
- Likely to be very little refrigerant.







- "Mist" -like flow, with bubbles totally absent.
- No difference in temperature between the low- and high- pressure sides.
- Probably means no refrigerant.







- A few bubbles show up intermittently, at intervals of 1 - 2 seconds.
- High-pressure side is warm and low-pressure side is fairly cold.
- Refrigerant likely to be insufficient.

The sight glass should not be used as a true indication of refrigerant quantity on R-134a systems.