How differential pressure meters measure flow
Squeeze a pipe down at one point and the fluid speeds up while its static pressure drops. Measuring that drop and working backwards is how every differential pressure flow meter works. The equation is Q = C ÷ √(1−β⁴) × A × √(2ΔP/ρ), where β is the beta ratio (d/D), A is the throat area and C is the discharge coefficient. A 4 in line with a 2 in orifice and 7.25 psi of differential on 68°F water works out to roughly 203 GPM.
The discharge coefficient depends on the meter shape. An orifice plate sits near 0.61 but is cheap to make and easy to swap, while a venturi tube reaches about 0.98 and keeps permanent pressure loss low, at the cost of length and price. Metering that will be billed or logged should always use the coefficient from the manufacturer's calibration certificate; treat the number here as a screening estimate.
Accuracy depends heavily on straight run. Mounted right behind an elbow or a control valve, the velocity profile is distorted and the error grows. Standard coefficients hold over a beta ratio of roughly 0.2 to 0.75, and compressible fluids such as steam or air need an expansion factor on top of this equation. Have a mechanical engineer or the inspection authority confirm any metering installation before it is finalized.
Frequently Asked Questions
Choose an orifice plate when installed cost and easy maintenance matter most, and a venturi on large lines that run continuously, where permanent loss of only 10% to 20% of the differential saves real pumping energy.
Not directly. It assumes an incompressible liquid, so compressible service needs an expansion factor and the true density at operating conditions, calculated to a standard such as ISO 5167.