Auto A/C Terms - Glossary
Charge - Refrigerant Charge

Every A/C system is manufactured to hold a specific amount of refrigerant. The actual amount is always specified as a weight of refrigerant. That total weight or amount of refrigerant that the system holds it the Charge or Refrigerant Charge for that system. It is important to understand that although the total charge is specified in weight, either pounds or kilograms, the system testing is always done using pressure gauges.

That means, when diagnosing or performance testing an A/C system, the pressure gauge readings will never tell you how much refrigerant is in the A/C system. Many people and a lot of A/C service technicians commonly mistake that the pressure gauges will tell them how much refrigerant is in the A/C system. Understand that the only way to know exactly how much refrigerant is in the A/C system is to pull deep vacuum and charge the system with the full factory specified amount of refrigerant.

As an example, take a single full can of refrigerant and attach a pressure gauge to it. Depending on the temperature of the day, your pressure gauge will typically read between 70 and 90 psi. That is because refrigerant has a pressure temperature relationship. Warmer days will cause higher pressures than cooler days. However, if you release some refrigerant from that can, you will still find that the pressure is about the same. It would still be close to what the pressure was when it was full. It is not until you get well below the half way mark that the pressure in the can will start to drop.

The point of that example is that a full can and a less than full can will show the same pressures. The is no correlation between the pressure of the refrigerant and the actual weight of the refrigerant. The same applies to an auto A/C system. If a system is fully charged with 2 pounds of refrigerant, the pressures of a full system will be very close to the pressures of that system if three or four ounces of refrigerant were removed.

The only absolute method to determine how much refrigerant is in the A/C system is to pull vacuum and recharge the system.