After the initial vapor stage, which type of charge should be performed through the liquid line service valve?

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In this charging sequence, the practical goal is to top off the system with the exact amount of refrigerant needed and place it into circulation in a controllable way. After you’ve established some refrigerant flow with a vapor stage, adding the remaining refrigerant as a liquid through the liquid line service valve is the best way to finish the charge. Liquid refrigerant delivered this way will flash to vapor as it passes through the metering device and evaporator, allowing the system to reach the correct charge and operating pressures without the unpredictability of delivering more refrigerant as gas. Using a gas charge wouldn’t give you precise control over the total charge, and a vapor charge alone isn’t suitable for finishing the charge. Dry nitrogen isn’t a refrigerant and is used for purging or leak testing, not for charging the system to its operating charge. So, topping off with a liquid charge through the liquid line service valve is the appropriate step.