In a differential-pressure flow measurement, what physical quantity does the DP transmitter compare to infer flow?

• A. The pressure difference between two sensing points in the flow.
• B. The electrical resistance of the fluid.
• C. The ambient air temperature outside the pipeline.
• D. The color of the liquid.

Answer: (A)

The main idea is that differential-pressure measurement infers flow by comparing the pressure at two points in the flow path, usually upstream and downstream of a constriction like an orifice plate or venturi. As the fluid passes the restriction, it speeds up and the static pressure drops, creating a pressure difference between the two sensing points. The differential-pressure transmitter directly senses this pressure difference and converts it into a signal that corresponds to the flow rate. In incompressible flow, that differential pressure rises with increasing flow (roughly with the square of the flow), so larger ΔP means more flow. The other quantities listed—electrical resistance of the fluid, ambient temperature outside the pipeline, or the color of the liquid—do not define or measure flow in this setup.