The most common types of flow meters are differential pressure flow meters, positive displacement flow meters, velocity flow meters, mass flow meters and open channel flow meters. Each type has several different versions that operate under the same concept. Orifice plates, venturi tubes and flow tubes are all examples of differential pressure flow meters. Reciprocating pistons and rotary vanes are both examples of positive displacement flow meters. Turbines and doppler meters are both common types of velocity flow meters. Coriolis and thermal are both mass flow meters.
Flow meters are used to measure flow rates; there are a couple factors that can affect flow rates. The product of the average flow velocity and the cross-sectional area of a particular pipe equals the flow of liquid through a pipe. The viscosity and density of the liquid are also determining factors in measuring flow rate.
"The basic operating principle of differential pressure flow meters is based on the premise that the pressure drop across the meter is proportional to the square of the flow rate," according to MaxiFlo. Flow rates can be obtained by finding the square root of a pressure differential measurement. Positive displacement flow meters separate liquids into increments that can be counted and provide an accurate measure of liquid flow.
Most velocity flow meters are equipped with fittings that allow them to be directly connected into a pipeline. When compared to differential pressure flow meters, velocity flow meters have a greater rangeability. Open channel flow meters are any conduit through which liquids flow with free surfaces, such as canals, streams and rivers.
The most commonly used flow meter is the differential pressure flow meter. Positive displacement flow meters are ideal for applications involving automatic batching and accounting. Mass flow meters were initially employed when more accurate measurements of mass-related processes were needed.