What Is an HVAC Pressure Gauge?

Uses

• HVAC gauges indicate the pressure and temperature of coolant used in the air conditioning unit. When the coolant pressure is lower than it should be, the unit needs to be either recharged or inspected for leaks. Two gauges sit side-by-side in a portable manifold: a low-pressure and high-pressure gauge. The gauges are linked to the air conditioning unit with an arrangement of valves, connectors and colored hoses.

Properties

• Manufacturers use standard colors to indicate the type of gauge and hose. The low-pressure gauge and hose are blue, while the high-pressure gauge and hose are red. A yellow hose connects portable refrigerant containers to the gauges, which is necessary when recharging the system. A set of valves on the manifold allows the technician to control the flow of coolant to the gauges.

Low-Pressure Gauge

• The low-pressure gauge -- also known as the compound gauge -- is mounted on the left side of the manifold, has a round face and indicates pressure in pounds per square inch, or psi. Using the blue hose, the technician connects the gauge to the suction line, which is used to draw coolant into the air conditioning unit. Black numbers on the gauge begin with zero and range up to 150 psi, while red numbers indicate inches of mercury, or HG, from 1 to 30, useful when the technician evacuates the system and pressure is lower than atmospheric pressure, i.e., vacuum pressure.

Coolants

• The various coolants, indicated by “R” numbers, have different properties at different temperatures. In a sealed system, for example, R-22 coolant circulates at 58.8 psi when the temperature is 33 degrees Fahrenheit. Technicians will carry small charts with them for reference when checking coolant pressure.

High-Pressure

• The high-pressure gauge is mounted on the right side, is also round and indicates pressure from 0 to 500 psi of coolant in the discharge valves. This gauge is connected to the liquid line, which is smaller than the suction line and is used to remove coolant from the system. The black number on the outside ring of the gauge indicates pressure, while the numbers on the inside ring indicate the temperature of the coolant.

Considerations

• Because refrigerants have different properties, different gauges are used to measure their temperature and pressure. A gauge used to measure R-22 coolant, for example, cannot be used in a system using R-410A refrigerant. Some gauges can measure more than one type of coolant. Operators and technicians must always use the proper gauges when performing maintenance or repairs.