Mass liquid flow switches and mass gas flow switches are devices used for measuring the flow or quantity of a moving liquid or gas in terms of a unit of mass per unit time, such as pounds per minute. These devices may be sensors with electrical output or may be stand-alone instruments with local displays and controls.

The technologies for mass gas flow meters and liquid flow meters vary widely. The most common types are inferential flow measurement, positive displacement, velocity meters, and true mass flow meters. Inferential measurement refers to the indirect measurement of flow by directly measuring another value and inferring the flow based on well-known relationships between the directly measured value and flow. The use of differential pressure as an inferred measurement of a liquid's rate of flow is the most common type of unit in use today. Examples of differential pressure meters include orifice plates, venturi tubes, flow nozzles, cone types, Pitot tubes, target meters, elbow tap meters and rotameters.

Positive displacement meters take direct measurements of liquid flows. These devices divide the fluid into specific increments and move it on. The total flow is an accumulation of the measured increments, which can be counted by mechanical or electronic techniques. They are often used for high viscosity fluids. Examples of positive displacement type meters include pistons, oval gears and nutating discs.

Velocity type meters are devices that operate linearly with respect to volume flow rate. Because there is no square-root relationship, as with differential pressure devices, their range ability is greater. Examples of velocity meters include turbine meters, electromagnetic meters, vortex meters and ultrasonic meters.

True mass flow meters are devices that measure mass rate of flow directly, such as thermal meters or Coriolis meters.  

The most important specifications for mass gas flow meters and liquid flow meters and sensors are the flow range to be measured and whether liquids or gases will be the measured fluids. Also important are operating pressure, the fluid temperature and accuracy. Typical electrical outputs for mass gas flow meters and liquid flow meters are analog current, voltage, frequency or switched output. Computer output options can include serial and parallel interfaces. These sensors can be mounted either as inline or insertion devices. In-line sensors can be held in place by using flanges, threaded connections or clamps. Insertion style sensors are typically threaded through a pipe wall and stick directly in the process flow.