How does a differential-pressure transducer measure flow?

• A. Measures the pressure difference from 2 points where a liquid flows through 2 different orifice plates.
• B. Measures the weight of liquid flowing through a pipe.
• C. Measures the electrical conductivity of the flowing liquid.
• D. Measures the temperature difference across the pipe.

Answer: (C)

The flow reading comes from the pressure drop created as fluid passes through a restriction in the pipe. Two sensing ports are placed on opposite sides of that restriction to measure the static pressure upstream and downstream. The differential-pressure transducer then converts this pressure difference into an electrical signal. Because the amount of pressure drop depends on how fast the fluid is moving (and on the restriction’s shape and the fluid’s density), the transmitter uses a calibration relationship to translate ΔP into a flow rate. In incompressible flow through common restrictions, the flow rate is related to the square root of the pressure drop, with factors that account for geometry and density. This method specifically relies on pressure difference, not on weight, electrical conductivity, or temperature differences, to determine flow.