Liquid Flow Meters and Mass Flow Meters
Liquid and mass flow meters are devices that measure the flow rate of a liquid moving through pipes. Operating in a similar manner to air flow meters, they may be called flow indicators, flow gauges or liquid meters. As there are different types of flow meters, it is important to choose the correct type for the fluid being measured as fluids do not all behave in the same way.Positive Displacement Flow Meters
Positive displacement flow meters, also called volumetric flow meters, do not depend on another variable to measure liquid flow as they directly measure the actual volume. These meters use precise rotors that spin as the liquid flows. The number of rotations is converted into flow rate and volume. Some examples of volumetric flow meters include bi-rotor types and rotary piston models.Mass and Velocity Flow Meters
Mass flow meters measure flow based on the mass of a fluid passing through the meter. Thermal and Coriolis flow meters are examples of this type. The latter can measure flow rate at high accuracy independent of density changes, flow turbulence and flow profile. Second, velocity flow meters, such as turbine flow meters used in petroleum pipelines, determine flow by measuring the speed of the fluid at different points. There are numerous types, including ultrasonic, electromagnetic, target and Pelton wheel models.Differential Pressure Flow Meters
Differential pressure flow meters are some of the most common types. They indicate flow rate by measuring the drop in pressure across the flow meter as fluid flows through it. Some examples include flow nozzles, orifice plates and target meters.Open Channel Flow Meters
Open channel flow meters are used to measure flow in large open channels, such as sewers, tunnels or rivers. Examples include the flume and the weir. A common application is measuring inflow into a reservoir.