Thread the coupler of the low pressure gauge, also known as the compound gauge, onto the low pressure valve mounted onto the HVAC unit. Although you have several valves mounted on your HVAC unit, the low pressure gauge, which is blue, will only fit on the low pressure valve.
Thread the coupler of the high pressure gauge into the high pressure valve mounted onto the HVAC unit. Again, the high pressure gauge, which is red, will only fit on the high pressure valve.
Inspect the gauges for the high pressure valve and for the low pressure valve. You will see several rows of numbers around the gauges. The numbers you need are labeled in black and red.
Compare the numbers on the pressure gauges to the specified tolerances for your HVAC unit listed in your owner's manual. If the numbers on the gauges read lower than required, consult a trained technician to troubleshoot leaks in your unit.