To get a proper reading from an HVAC gauge, calibrate the gauges first. When they have been calibrated, turn off the air conditioning unit, remove the valve caps, and attach the high and low pressure gauge hoses to it. The gauge does not work if the gauge hoses are not attached correctly. Typically, the low pressure hose gauge is blue and the high pressure hose gauge is red. Turn on the air conditioning unit and read the HVAC gauge.
The low and high pressure hose gauges typically attach to valves on the air conditioning unit. The low pressure hose gauge fastens to the larger return valve, and the red line is secured to the smaller line. The low pressure hose gauge shows the negative pressure and should decrease when the unit has been turned on. The high pressure hose gauge shows the compressor pressure and should increase when attached to the air conditioning unit. When turned on, the HVAC gauge readings should change significantly, and if they do not, something is wrong with the unit's pressure or refrigerant needs to be added.
Most HVAC gauges attach directly to the air conditioning unit's compressor. For centralized air conditioning systems, this compressor is separate from the rest of the system. Remove the cover of the compressor to access the ports, and attach the HVAC gauge.