Liquid Line Sightglasses - Are They Good For Anything?

Many technicians believe that a bubbling sight glass means nothing but an undercharge of refrigerant. This is simply not true. On start-up on some refrigeration systems when there is a large load on the system, bubbling and flashing could occur in the sight glass downstream of the receiver. This bubbling is caused from the pressure drop at the entrance of the outlet tube of the receiver. Bubbling could also occur during rapid increases in loads. The TXV could be opened wide during an increase in load and some flashing could occur even though the receiver has sufficient liquid. Also, sudden changes in head pressure control systems, which may dump hot gas into a receiver to build up head pressure, often will bubble a sight glass even though there is sufficient liquid in the receiver to form a seal on the receiver’s dip tube outlet. 


Many newer refrigerant blends such as R-410A and R-407C will flash some of their liquid when passing through the increased volume of a sight glass in a liquid line. This will appear as bubbles in a sight glass. However, once this small percentage of vapor leaves the sight glass and re-enters the liquid line again, it will form liquid again. So, the service technician should never try to charge a system using certain blends to a clear sight glass. This may only lead to overcharging the system.


Even in R-22 air conditioning applications, technicians often try to charge to clear a sightglass. The system must absolutely be operating a design conditions for this to work, but 99% of the time, this is not the case so I don't recommend it. Technicians must account for evaporator entering wet-bulb and dry-bulb temperatures, condensing temperature, coil condition and more as well to determine if what they are observing in the sightglass is normal or not.


Sightglasses are merely one of many pieces of information technician can use to determine how a systems is operating and should NEVER be used to determine system refrigerant charge alone.