How does a DP flow meter work?

• A. By measuring the speed of sound through the fluid.
• B. By measuring the temperature difference across a sensor.
• C. By creating a constriction and measuring the magnetic field change.
• D. By creating a pressure drop across the meter and increasing with flow.

Answer: (D)

Differential pressure flow meters gauge flow by creating a restriction in the pipe and measuring the pressure drop across that restriction. The fluid speeds up as it passes through the constriction, and according to Bernoulli’s principle, the velocity increase comes with a pressure drop. The difference in pressure between the upstream and downstream taps grows as the flow rate rises, and a transmitter converts that pressure difference into a flow rate. Because the relationship between ΔP and flow depends on density and the specific geometry (often ΔP ∝ Q^2 for a given density), these meters are simple, robust, and widely used with devices like orifice plates, Venturi tubes, or flow nozzles.

Other options don’t fit because they describe different measurement principles: measuring the speed of sound in the fluid is how sonic/ultrasonic meters work; using a temperature difference across a sensor is the principle behind thermal mass flow meters; and measuring a magnetic field change in conjunction with a constriction isn’t how standard magnetic or differential-pressure meters operate.