Liquid flow meters are used for measuring the flow or quantity of a moving fluid. They use four basic metering technologies: differential pressure (DP), positive displacement (PD), velocity, and true mass.  Differential pressure flow meters obtain a liquid’s flow rate by measuring the pressure differential and extracting the square root. Examples include orifice plates, Venturi tubes, flow nozzles, Pitot tubes, target meters, elbow tap meters, and rotameters. Positive displacement flow meters divide the liquid into specific increments which are counted by mechanical or electronic techniques. These devices are often used for high-viscosity fluids. 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. True mass flow meters are used to directly measure the mass rate of flow. These types of liquid flow meters include thermal meters and Coriolis meters.