Pressure Gauges
The Pulse of the System

The Dial Nobody Really Reads

Every fire protection system is covered in small, round, 3.5-inch Bourdon-tube pressure gauges. NFPA 13 §16.9 tells you where to put them: at each riser, across control valves, on each side of a check valve and alarm check valve, at fire pump suction and discharge. They are cheap, mechanical, and require no power. They are also the single fastest field diagnostic tool you have.

A gauge that reads zero on the system side and 70 psi on the supply side tells you a control valve is closed. A gauge that reads 40 psi on the air side and 0 on the water side of a dry pipe valve tells you the differential has set. A gauge that reads 175 psi where you expected 60 tells you thermal expansion has pressurized a closed section of wet system.

How a Bourdon Tube Works

Inside a typical 3.5″ fire-protection gauge is a curved, sealed brass or stainless tube — the Bourdon tube. The tube cross-section is flattened, not round. When pressure rises inside the tube, it tries to become more round, and that small dimensional change causes the tube tip to straighten slightly. A tiny geared linkage amplifies that motion and turns the needle on the dial.

Bourdon gauges drift over time. Sudden pressure spikes (water hammer), freeze-thaw cycles, and corrosion of the brass tube all shift the zero point. That's why NFPA 25 requires replacement or recalibration every 5 years regardless of whether a gauge appears to be working.

NFPA 25 Schedule

References