How does an orifice plate establish a differential pressure to infer flow?

• A. Fluid flows through a restriction, creating a differential pressure proportional to flow; a transmitter uses Bernoulli-based relationships to compute flow from ΔP, density, and geometry.
• B. The orifice plate heats the fluid and measures the resulting temperature change to infer flow.
• C. The plate acts as a valve to regulate flow and reduce pressure drop.
• D. The plate emits ultrasound to measure flow velocity.

Answer: (A)

An orifice plate measures flow by creating a measurable pressure drop as the fluid is forced through a restricted opening. The restriction makes the fluid accelerate, converting some of its pressure energy into kinetic energy. That acceleration causes the pressure to be higher upstream and lower downstream, producing a differential pressure that increases with the amount of flow.

Knowing the fluid density and the exact geometry of the orifice and pipe allows you to relate that pressure drop to velocity through the orifice, and then to flow rate, using Bernoulli-based relationships and a discharge coefficient. A differential-pressure transmitter simply measures that ΔP and, with density and geometry, computes the flow rate from it.

The other ideas don’t fit this instrument: heating the fluid to infer flow isn’t how an orifice plate works, nor does it act as a regulating valve, nor does it use ultrasound to determine flow.