Principles of Velocity and Positive Displacement Flow Meters

Velocity flow meters measure the instantaneous flow rate of a liquid passing through them. Depending on whether they measure mass or volume, they can be further divided into volumetric and mass flow velocity types. The primary goal of these meters is to obtain the current flow rate within a pipe, serving as a reference for system adjustments.

Volumetric flow meters include electromagnetic, ultrasonic, paddle wheel, and turbine flow meters, which measure volume. Mass flow meters, such as Coriolis and thermal mass flow meters, are less common. Their main applications are in fluids with significant volume changes upon mixing different chemicals, where the total volume post-mixing does not equal the sum of the individual volumes before mixing. As volume-based flow meters may not yield accurate results in such cases, measuring mass—which remains almost constant according to the law of conservation of mass—provides the most precise outcomes. This is especially true for gases, which undergo significant volume expansions and contractions, making mass measurement the most appropriate method.

Positive displacement flow meters measure fluid by restricting the total volume that can pass through the meter at a time, ensuring measurement accuracy. Examples include oval gear flow meters. They are primarily used in applications requiring volume-based billing, such as water and gasoline, or precise chemical dosages for specific reactions, like expensive pharmaceuticals.

The principle behind these meters involves blocking the fluid until a predetermined volume accumulates, which is then released and measured by the sensing elements. This approach results in slower flow rates and higher pipe losses. Additionally, the presence of solid components is generally not allowed, as they can damage the flow meter, a critical consideration for users.