How to Use Refrigeration Gauges
Air conditioning and refrigeration systems use evaporation and condensation of a substance, called a refrigerant, to remove heat from the area that needs to be cooled. The evaporation process takes heat energy to complete, similar to the way you must heat water on the stove to make it boil, or evaporate. In a refrigeration system, the evaporated fluid travels to a condenser, where it is cooled and turns back into a liquid, discharging its heat into the atmosphere and starting the process again. The refrigerant is pumped throughout the system by a compressor. Manifold gauges are useful for diagnosing refrigerant problems.
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Instructions
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Shut off the vehicle or the air conditioning system. Locate the service ports for the air conditioning. Most systems have at least two, with one on the high pressure side of the system and the other on the low pressure side. The ports are usually color-coded, with blue denoting the low side and red indicating the high pressure side. Shut off both valves on the gauges before proceeding.
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Connect the hose fittings from the manifold gauges to the air conditioning system. The hoses are color-coded the same way the access ports are. Many gauges come with a wire hook attached to make it easy to hang the gauges and take readings. If the system has been off for a while, both the high and low pressure sides will indicate similar pressures, as the system has equalized at its static pressure.
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Determine the problem based on the gauge's reading. If the system reads no pressure at all on either side, the refrigeration system is probably empty. The most likely cause for a completely empty system is a leak that allowed the refrigerant to escape. This leak must be found and repaired before the system can be successfully charged and work again.
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Start the vehicle or refrigeration system and let it operate. Set the cooling controls to maximum cool, and use the recirculation setting in a vehicle. Once the system has begun to operate, take another pressure reading with the gauges. For an R-134 system, the high side pressure should show 2.2 to 2.5 times the ambient temperature, according to the Aircondition website. At 70 degrees, the high side should read 154 to 175 pounds per square inch (PSI). The acceptable pressure can vary with humidity and heat load. A reading of 30 PSI is considered a good pressure for the low side. Pressure and temperature are related, and 30 PSI means the evaporator is about at the freezing point of water.
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Connect a can of the correct refrigerant to the center service hose. If refrigerant needs to be added to the system, usually denoted by high and low side pressures that are too low, turn the can upside down and open the low pressure hose valve on the gauges, adding some refrigerant. Keep the can on a scale, and monitor how much refrigerant you add in pounds or grams. If the pressures do not change when adding refrigerant, look for another problem in the system.
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Tips & Warnings
If the system is leaking and you are unable to pinpoint the source of the leak, consider adding dye to the refrigerant as you charge the system. The dye will escape from the system through the leak, and an examination with a black light will cause the dye to glow bright green at the source of the leak.
Never open the high side gauge valve when the gauges are connected to a can of refrigerant. Doing so could cause the can to explode.
