Basic Refrigeration and Charging Procedures Practice Test

When refrigerant is flowing through the system and the load on the evaporator increases, more liquid refrigerant is pushed toward the evaporator and the expansion device causes some of that liquid to flash into vapor. That flashing shows up as visible bubbles in the sight glass. If the system is properly charged, you’ll typically see the sight glass bubble more with higher loads and stay relatively clear at light loads. If the system is undercharged, bubbles can appear even at lower loads because there isn’t enough liquid to absorb the heat and prevent flash gas.

So, seeing bubbles in the sight glass under higher loads is the normal sign you’d expect when the system is not undercharged. Frost or a hissing sound aren’t the typical indicators in this situation, and remaining bubble-free isn’t reliable at higher loads.