A venturi meter measures flow with a smooth pinch instead of a sharp plate. The flow accelerates through the narrow throat, its pressure falls, and the difference between inlet and throat tells the meter the flow rate.
It is a flow meter shaped like a smooth hourglass. Because there is no sharp obstruction, the pressure recovers well downstream, so it wastes less pump energy than an orifice plate — at the cost of being longer and more expensive to make.
The whole measurement depends on the smooth changes of diameter inside the pipe. A cutaway shows exactly how the flow is squeezed, accelerated and then allowed to expand, and where the two pressure taps sit.
Fluid arrives through the straight inlet pipe as a steady, fully-developed stream. The inlet tap reads this full pressure.
The pipe narrows gradually into the throat. Because the passage is smaller but the same liquid must pass, the fluid has to speed up.
The faster the flow, the lower its pressure. In the narrow throat the pressure is at its lowest, and a second tap reads that low value.
The difference between the inlet tap and the throat tap is fed to a differential-pressure transmitter, which converts it into a flow rate. More flow gives a bigger difference.
After the throat the pipe widens gently. The flow slows again and its pressure rises back toward the inlet value — the smooth shape is why a venturi wastes less energy than a plate.
Both measure the pressure drop across a narrowing, but a venturi narrows smoothly while an orifice plate uses a sharp hole. The smooth shape lets the pressure recover, so the venturi wastes less energy but is longer and pricier.
Because the flow speeds up there. By Bernoulli's principle, energy is traded between speed and pressure: a faster stream has lower pressure. The throat is narrowest, so the flow is fastest and the pressure lowest.
One at the wide inlet and one at the narrow throat. The difference between these two pressures is the measurement the transmitter turns into flow.
Better than an orifice plate, because there is no sharp edge to wear and it is less prone to clogging — though a very smooth surface still helps to keep it accurate.
The diverging cone lets the flow slow down gently, converting some of its speed back into pressure. That recovered pressure is energy the pump does not have to supply again.