What could bubbles in a sight glass indicate?

Bubbles in a sight glass are an important indicator of refrigerant flow and system health. When you observe bubbles in a sight glass, it typically signifies that the refrigerant in the liquid phase is mixed with vapor. This condition can arise from two main issues: a low refrigerant charge or a restriction in the system, such as a clogged filter-drier.

When there is a low refrigerant charge, the system doesn't have enough refrigerant to ensure a proper liquid phase, which results in the formation of bubbles. This condition can lead to inadequate cooling and potential damage to components due to insufficient lubrication.

On the other hand, a restriction in the filter-drier can prevent the flow of refrigerant, thus not allowing liquid refrigerant to move continuously through the system. This blockage causes a buildup of vapor and subsequently results in bubbles appearing in the sight glass, as some liquid refrigerant is not able to flow through as needed.

Therefore, the presence of bubbles indicates that the system may be facing significant issues, either related to insufficient refrigerant or obstruction due to a blockage, making the selected answer a comprehensive explanation for the observed phenomenon.