The sight glass is a small window in the liquid line that shows whether liquid refrigerant is flowing and whether it contains moisture. Steady clear flow is normal. Constant bubbles can mean low charge or a restriction, and a moisture indicator changes color when the system is wet.
Operators sometimes see bubbles and assume the system needs refrigerant. Sometimes that is right, and sometimes bubbles come from a restricted drier, a cold snap or a fast pressure change. Understanding the glass helps you describe what you see.
What it does
The glass sits in the liquid line between the receiver and the expansion valve. When the system is properly charged and the liquid is sub-cooled, the glass looks clear because it is full of liquid. A moisture indicator inside the glass changes color when water is present. Technicians use it together with pressures and temperatures, not on its own.
Signs it is failing
- Steady stream of bubbles or foam.
- Flashing bubbles that come and go.
- Moisture indicator showing wet or caution.
- Cloudy or oily glass.
- Glass cracked or leaking at the fitting.
- Bubbles only at start-up, which can be normal.
Common causes of failure
| Cause | What happens |
|---|---|
| Low refrigerant | From a leak. Bubbles are constant. |
| Restricted drier or line | Pressure drop causes flashing before the glass |
| Cold outdoor air | Head pressure falls and liquid flashes |
| Undersized or long line | Pressure drop along the pipework |
| Moisture | Indicator changes color as the desiccant saturates |
What you can safely check
- Note the pattern. Constant, flashing or only at start-up?
- Check the indicator color. Compare with the key printed on the glass.
- Record outdoor temperature. Cold weather matters.
- Take a photo. It helps the technician see what you saw.
Do not open the fitting or attempt to add refrigerant. Only a technician with the right equipment and license should charge a system.
What a technician does
- Reads pressures and calculates subcooling and superheat.
- Checks the drier for a temperature drop.
- Searches for leaks with a detector.
- Checks head pressure control operation in cold weather.
- Charges by weight after repairs.
Keeping it healthy
- Photograph the glass in normal operation for comparison.
- Ask the technician to note the moisture indicator color at each visit.
- Keep the glass and fitting clean and unstrained.
- Report any oil around the fitting promptly.
A photo taken when the system is healthy makes it much easier to tell later whether what you are seeing has really changed. Also tell the technician about any recent cold snaps, repairs or added stock, since these change what the glass shows.
Repair or replace
The glass itself rarely fails, but a leaking or cracked fitting is repaired. The bigger question is what is causing the bubbles, and that decides whether the fix is a drier, a leak repair or a control adjustment.
Related reading: the parts and components guide, sight glass bubbling and winter operation.
Questions about this part
Do bubbles always mean low refrigerant?
No. A restriction, cold weather or a control problem can cause them.
What does a wet indicator mean?
Moisture is present. The drier usually needs replacing and the system may need evacuation.
Can I top off if I see bubbles?
No. Adding refrigerant without finding the cause can overcharge the system or hide a leak.