To add small amounts of refrigerant to the low side of an air conditioning system, the refrigerant should be introduced through a particular valve and in a particular state. What valve and state combination is correct?

Correct Answer: Option A — suction service valve as a vapor

Refrigerant must be added to the low side as a vapor to prevent liquid slugging, which can cause catastrophic damage to the compressor valves and pistons. By introducing the refrigerant through the suction service valve as a vapor, the technician ensures the compressor handles only gaseous refrigerant during the charging process.