What is the working principle of orifice meter?

An obstruction (orifice) is placed in a pipe filled with fluid. The pressure of the fluid is measured at two different points: 1) just upstream of the orifice and, 2) close to the contraction of the fluid (vena contracta). The difference in these two pressures is known as differential pressure. The differential pressure across an obstruction (orifice) in a pipe of fluid is proportional to the square of the velocity of the fluid.

Many factors associated with the pipe, orifice and fluid affect the measurement. Satisfactory measurement requires steady-state, homogeneous, turbulent flowing fluids. Other properties which affect the measurement include: the ratio of pipe diameter to orifice diameter and the density, temperature, compressibility and viscosity of the fluid.