Shut off the main water supply so the overall water pressure in the building being tested is neutral.
Locate the backflow valve and, using the diagrams on the valve, determine the direction water flows through the valve.
Bleed test cocks to free them of dirt and accumulated water pressure by turning them open using a screwdriver.
Insert the differential gauge's low pressure hose into the test cock closest to the water supply (the upstream cock).
Insert the differential gauge's high pressure hose into the downstream test cock.
Bleed the high-pressure hose and the low-pressure hose to clear any air or water from the gauge. Record the pressure differential between the two segments. A differential pressure of at least 1 psid indicates a working and tightly sealed pressure gauge.
Turn the building's main water supply back on.