Which basic principle does a flow meter based on an orifice rely on?

• A. Electrical resistance change
• B. Differential pressure across an orifice to infer flow
• C. Magnetic field interaction
• D. Temperature change to infer flow

Answer: (B)

Measuring flow with an orifice relies on the pressure drop created when fluid is forced through a restriction. As the fluid enters and then speeds up through the narrow opening, its static pressure falls, so there is a measurable difference between the pressure before and after the orifice. That differential, together with the known geometry and fluid density, is related to the flow rate via Bernoulli’s principle and conservation of mass. In simple terms, more flow causes a larger pressure drop, and the meter translates that pressure difference into a flow value using a calibration for the specific orifice, pipe size, and fluid. This method is popular because it has no moving parts and is robust, though it requires a pressure transmitter and proper installation. The other methods rely on different physical effects—electrical resistance changes are typical of temperature or material sensing, magnetic meters use magnetic field interaction for conductive liquids, and temperature-based methods measure flow through heat transfer rather than pressure changes.