A Venturi meter is used to measure the flow through a pipeline.

In order to understand the principles of a Venturi meter, we must first review two basic concepts.

Based on the principle of Bernoulli’s equation:

The first term in this equation, , is referred to as the pressure head.

The second term, , is referred to as the elevation head.

The third term, , is referred to as the velocity head.

Head is energy per unit weight, as energy is in units of (weight * length), head has length units.

Pressure head is the energy per unit weight stored in the fluid due to the pressure the fluid is under.

Elevation head is the potential energy per unit weight stored in the fluid due to it's elevation measured above a datum elevation.

Velocity head is the kinetic energy per unit weight stored in the fluid due to speed and direction the fluid is moving.

The total amount of head at point 1 must equal the total amount of head at point 2.

The meter uses a narrowing throat in the pipe which then expands back to the original pipe diameter, by measuring the pressure head at both points on the meter, we can calculate the velocity of the fluid.

Using the theory of continuity, which states that total flow must remain constant:

Q = vA

The Q term being the total flow, it is equal to the velocity v multiplied by the cross-sectional area A.

Since Q_{1} = Q_{2}, v_{1}A_{1}
= v_{2}A_{2}