Now, for a horizontal pipe where the elevation is different, is constant,

the Bernoulli equation reduces to p over gamma plus v-squared over 2g

equals constant.

From which we can see that where the velocity is high, the pressure is low and

vice-versa.

So if I think about the pressure distribution in a Venturi like this,

then the variation of pressure in the pipe upstream here, assuming there's

net energy losses are negligible where the area is constant the pressure is constant.

Then as the area begins to converge here, the velocity increases,

therefore the pressure decreases, reaches a minimum at the throat where the area

is a minimum, and then, as the pipe gradually expands again,

the pressure recovers back to almost its original value.

And if energy losses are negligible, which they are very small

in a venturi meter like this, we should have a perfect pressure recovery,

the pressure recovers back to its original value.