When measuring the charge by an ammeter, bear in mind that with the unit running, the more refrigerant is charged into the unit, the higher the ammeter will read and vice-versa. If the unit is operating properly, any reading other than the FLA amperage (which is printed on the nameplate of the unit) will either be an undercharge or an overcharge.
In any of the above methods, a piercing valve must be installed on the discharge line and the suction line of the compressor. Commercial units already have service valves installed. (Instructions on installation of piercing valves are provided with the valves.)
a. Connect a refrigerant tank to the compound gauge and to the valve (as shown in fig. 78) and leave all valves closed.
b. Connect a clamp-on type ammeter around the line going to the run or common terminal on the compressor.
c. Turn on the unit.
d. Turn on the access valve.
e. Turn on the refrigerant tank.
f. Turn on the compound gauge and watch the ammeter.
As the system is being charged, the amperage will increase. When it reaches the FLA amperage indicated on the nameplate of the unit, the system is charged with the correct amount of refrigerant. Immediately close the compound gauge valve, the piercing valve, and the refrigerant tank. Replace the cap on the piercing valve. Use a little Teflon tape on the threads to prevent a refrigerant leak.