How does an orifice plate measure flow?

An orifice plate creates a restriction in a pipe, causing a pressure drop proportional to the square of flow rate (based on Bernoulli's principle). Differential pressure transmitters measure this pressure drop across the orifice. Flow is calculated as Q = K * sqrt(delta P), where K depends on orifice geometry and fluid properties. Orifice plates are simple, inexpensive, and widely used, though they cause permanent pressure loss (typically 50-80% of measured differential).