In a differential-pressure flow meter, what is measured to infer flow rate?

• A. Temperature difference
• B. Electrical resistance
• C. Pressure drop across a constriction
• D. Acoustic Doppler shift

Answer: (C)

Measuring the pressure drop across a constriction is how a differential-pressure flow meter infers flow rate. When fluid is forced through a narrower section, it accelerates, so its static pressure decreases. The difference between the upstream pressure and the pressure at the constriction (ΔP) is related to how fast the fluid is moving, and with the pipe geometry and fluid density, this pressure difference is translated into a flow rate using calibration that accounts for real losses (the discharge coefficient). So, the measured parameter is the pressure drop, and from that you deduce how much fluid is moving.

Other ideas wouldn’t map to flow rate in this type of meter: a temperature difference isn’t what sets the flow in a DP meter, though temperature can affect density and thus the conversion; electrical resistance might be used for temperature sensing or other sensors but not to gauge DP flow directly; and acoustic Doppler shift relies on measuring frequency changes in sound waves to infer velocity, not the static pressure drop across a constriction.