Differential pressure gauge
The calibrated field test instrument that measures pressure differences across the components of a backflow prevention assembly to verify they are holding.
A differential pressure gauge is the central instrument in a backflow tester's kit. Rather than reading a single pressure, it measures the difference in pressure between two points, typically across a check valve or between the supply and the relief zone of an assembly. Field test kits combine the gauge with hoses, fittings, and bleed valves that connect to an assembly's test cocks, letting the tester isolate components and observe how well each one holds.
The gauge matters because assembly health is defined in differential terms. A check valve is judged by the pressure difference it can maintain while closed, and a relief valve is judged by the differential at which it opens. None of this is observable from the outside of the assembly, so the gauge is effectively the tester's window into the internals. An accurate reading can distinguish a tightly sealed check from one barely hanging on, guiding repair decisions before an outright failure.
Testers should treat the gauge itself as a precision instrument that requires care. It needs periodic accuracy verification against a known reference, protection from freezing and rough handling, and clean hoses free of debris that could foul both the gauge and the assemblies being tested. Property owners should expect a professional tester to arrive with a calibrated kit and to record readings for each component, since those numbers become the documented evidence that the assembly passed.
Related terms
| Test cocks | Small valved ports on a backflow prevention assembly that give a tester gauge access to the pressure zones inside the assembly. |
| Check valve | A valve that allows water to flow in one direction and closes to prevent flow in the reverse direction. |
| Relief valve | The hydraulically operated valve between the checks of a reduced pressure principle assembly that discharges water to atmosphere when internal pressure conditions become unsafe. |
| Annual testing | The periodic field testing of backflow prevention assemblies, typically annual where required, set by the water authority, that verifies each assembly still works. |
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