Why do you need to observe the bubbles in the sight glass for refrigerant filling?

Why do you need to observe the bubbles in the sight glass for refrigerant filling?
This method does show that there are bubbles that indicate a lack of refrigerant, but we must understand that the pressure loss of the liquid pipe can also cause bubbles. If the pressure loss of the liquid pipe is large, the liquid Refrigerant charging pipe will flash into a gas, and the flash gas will reduce the flow of the refrigerant when flowing through the expansion valve and erode the expansion valve. If the subcooling of the system is small, the pressure loss can easily generate bubbles seen in the sight glass. Remember, if there is no liquid but steam, it will be transparent when seen in the sight glass.

If the refrigeration engineer with experience or careful observation will find that when the system is charged correctly, bubbles or flashes can also be seen in the sight glass. This is because the resistance in the liquid pipe in front of the sight glass causes a pressure drop, causing the refrigerant to flash. If the expansion valve is unstable or fluctuates with liquid, the expansion valve opens sharply, and the flow rate increases to produce enough pressure drop to cause the outlet of the liquid reservoir to flash. The sudden change in condenser pressure is also the cause of flash.
For example: a sudden change of the fan speed in a cold storage can easily change the condensing temperature from 5.5°C to 8°C. At this time, the liquid temperature in the liquid reservoir will be higher than the saturation temperature corresponding to the changed condensation pressure, and evaporation will occur at this time, and the evaporation will not end until the liquid temperature drops below the saturation temperature again.