Flow meters (flowmeters) are devices for measuring the flow rate or quantity of a moving liquid or gas. There are four basic categories of devices: differential pressure (DP), positive displacement (PD), velocity, and true mass.  Differential pressure flow meters obtain the flow rate by measuring the pressure differential and extracting the square root. Choices for DP meters include cone-type devices, elbow tap meters, flow nozzles, laminar flow elements, orifice plates, Pitot tubes, rotameters, target meters, variable area flow meters, and Venturi tubes. Positive displacement flow meters divide the media into specific increments which can be counted by mechanical or electronic techniques. Examples of PD meters include nutating disc devices, oval gear meters, and piston-based designs. Velocity flow meters operate linearly with respect to the flow rate. Because there is no square-root relationship, their range is greater than DP devices. Choices for velocity meters include electromagnetic meters, paddlewheels, sonar-based devices, turbine meters, ultrasonic meters and vortex or shedding meters. True mass flow meters are used to directly measure the mass rate of flow. These flow meters include both thermal meters and Coriolis meters.