How does an orifice plate measure flow rate?

An orifice plate is a thin plate with a sharp-edged hole inserted in the pipe. It creates a pressure drop proportional to flow velocity squared. By measuring the pressure difference upstream and at the vena contracta (downstream constriction), flow can be calculated using: Q = Cd*A2*sqrt(2*dP/rho), where Cd is the discharge coefficient (typically 0.6-0.65). Orifice plates are simple, reliable, and inexpensive but cause permanent pressure loss (40-80% of measured dP).