An orifice plate measures flow by narrowing the pipe at one thin plate. The faster flow through the hole drops the pressure, and the difference between the two taps tells the transmitter the flow rate.
It is one of the oldest and simplest ways to measure flow. A thin, sharp-edged plate with a measured hole is bolted into the pipe, and the pressure drop across it is related to how much fluid is passing.
From outside it is just a pair of flanges in the pipe. The measuring action — where the flow pinches into the vena contracta and the two taps sense the pressure — is all inside, so a cutaway makes the principle clear.
Fluid arrives through the upstream pipe and meets the thin plate, which has a single sharp-edged hole in the centre.
Because the hole is much smaller than the pipe, the fluid must crowd through it. To pass the same amount of liquid, it has to move faster through the small opening.
Whenever a flow speeds up, its pressure falls. So the pressure just downstream of the plate is lower than upstream — a pressure difference appears across the plate.
A high-pressure tap upstream and a low-pressure tap downstream (or at the vena contracta) feed a differential-pressure transmitter. It measures the difference, which grows as the square of the flow.
The transmitter, knowing the plate's size and shape, converts that pressure difference into a flow rate — the faster the flow, the larger the difference it reads.
It creates a deliberate pressure drop. The flow speeds up through the small hole, its pressure falls, and the difference between upstream and downstream pressure is proportional to the square of the flow rate.
This is a basic rule of fluid flow (Bernoulli's principle): where the fluid moves faster, its pressure is lower. The orifice forces the fluid to speed up, so the pressure falls.
It is the narrowest point of the jet, a little downstream of the plate where the flow has squeezed to its tightest. The low-pressure tap is often placed near it for the most reliable reading.
It has no moving parts, is cheap and rugged, and works on a principle anyone can draw. That simplicity keeps it in wide use despite newer alternatives.
The plate permanently adds resistance, so it wastes pump energy, and a sharp edge wears with abrasive fluids, which slowly changes the reading and requires recalibration.