This method is widely used in commercial refrigeration since the exact back-pressure requirement is known. (This is explained in “Saturated Vapor Refrigerant Pressure and Temperature Relationship.”) The correct low-side pressures of all residential units (made in the USA) can be obtained from the Tech Master publication mentioned on page 112. All that is needed is the name and model number of the unit.
a. Install a piercing valve on the compressor suction side and leave it closed.
b. Connect the piercing valve to the compound gauge manifold and the middle line in the manifold to the refrigerant tank as shown in figure 82. Leave all valves closed.
c. Turn on the unit.
d. Turn on the piercing valve, the compound gauge valve, and the refrigerant tank.
e. Keeping a hand on the compound gauge valve, watch the compound gauge, and every few seconds, turn off the valve and check the gauge for the desired low-side pressure.
f. Upon reaching the correct low-side pressure, turn off the manifold valve, the piercing valve, and the refrigerant tank valve.
g. Disconnect the lines, put a little Teflon tape on the threads of the piercing valve, and replace its cap.
Make it a habit of running an electronic leak detector over all the tubing and valves in the system after recharging. Make certain there is no refrigerant leakage. A mixture of one-half cup of liquid soap to one gallon of water can also be used as a leak detector when a leak is suspected in a particular location. Simply apply the mixture to the tubing or the connections and watch for bubbling.
To increase the pressure and cause the liquid (or gas) to be expelled faster, hold the container under your coat next to your body or put it on a condenser to warm it. In severely cold weather, use a refrigerant-container heating blanket such as the one pictured above. If an electric blanket is not available, it is also possible to increase the pressure in the refrigerant container by placing it in a tub of warm water at the time of charging.
Before charging a unit, purge all charging hoses to rid them of moisture and air. Commercial systems may also be charged by the high-side method as shown in figure 80a. The system must be turned off when it is being charged through the high side. This method is used only if the entire amount of refrigerant has been removed from the system. The inverted refrigerant container is placed on a scale, and the unit is charged until the proper amount of liquid refrigerant is transferred into the system.