An orifice plate is a rigid plate of small thickness and pierced with a hole.
It can be a monobloc orifice, an annular chamber or an integrated plate.This plate is introduced into the pipe perpendicular to flow direction. The liquid whose flow rate is constant, sees its speed increase at the passage of the orifice.
Simultaneously, there is an opposite variation of the pressure (a static pressure drop at the orifice). Pressure measurements on both sides of the orifice allow the measurement of the static pressure difference. We can then calculate the instantaneous volumetric flow rate.
Integrated orifices are highly suitable for very low flow rates.