The Sight Glass protect the refrigeration system and provide an important indication of any problem involving the refrigerant, moisture, or contamination in the system.
The function of the sight glass is to observe the liquid level of the refrigerant and detect the presence of moisture in the back end of the dry filter in the system. In a system with an expansion valve, a sight glass is usually fitted in the liquid line immediately after the filter drier. But In theory, the sight glass can be fitted at any desired location on the discharge line. However, placing the sight glass close to the expansion valve is especially advisable.
The colour indicator in the sight glass shows the moisture content.
Green -refrigerant contains no water vapor that can be dangerous.
Yellow - The water vapor content in the liquid line at the front end of the expansion valve is too high.
If you see a bubble through the mirror means the following conditions exist: