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A Venturi meter measures flow rate by detecting the pressure difference created when fluid accelerates through a narrower throat section. The meter calculates cross-sectional areas of the pipe and throat, applies the continuity equation to determine velocities at both sections, then uses Bernoulli's equation to relate the pressure drop to velocity changes. The pressure gauge attached to the flowmeter indicates this pressure difference, which reflects the flow rate.

As wastewater flows from the larger-diameter pipe into the smaller throat, it accelerates due to the continuity equation, which requires constant volumetric flow rate. This velocity increase causes a corresponding pressure drop, as described by Bernoulli's equation for flow along a streamline. The pressure difference between the pipe and throat is inversely related to velocity: higher velocity at the throat means lower pressure at that location.

Cross-sectional areas are essential because they determine the velocities at each section through the continuity equation. Since volumetric flow rate remains constant for incompressible fluid, the velocity is inversely proportional to area. Knowing both areas allows you to calculate the exact velocities needed to substitute into Bernoulli's equation and determine the pressure difference.

The continuity equation states that volumetric flow rate remains constant along the pipeline for incompressible fluid. This means the flow rate at the pipe equals the flow rate at the throat. Since area and velocity are inversely related, the smaller throat area forces the wastewater to accelerate, creating the velocity difference that Bernoulli's equation uses to calculate pressure drop.

The pressure gauge indicates the pressure difference between the main pipe and the throat section. This pressure drop reflects the increase in velocity as wastewater flows through the narrower throat, consistent with energy conservation principles. The magnitude of this pressure difference directly correlates to the flow rate, allowing operators to determine wastewater flow from a single gauge reading.

Neglecting viscous effects allows you to apply Bernoulli's equation directly, which assumes ideal fluid behavior without energy loss to friction. This simplification is valid when viscous forces are small compared to inertial forces. Under this assumption, all pressure changes result solely from velocity changes, making the relationship between pressure difference and flow rate straightforward and reliable.

Bernoulli's equation relates pressure and velocity in an ideal fluid: P1 + 0.5ρV1² = P2 + 0.5ρV2². Rearranging gives the pressure difference: P1 - P2 = 0.5ρ(V2² - V1²). After calculating velocities V1 and V2 from the continuity equation and known flow rate, substitute them along with fluid density into this formula to find the pressure difference the gauge will display.