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.  

Specifications for flow meters include pipe diameter, mounting style, end fittings, electrical outputs, and interface options. There are three basic mounting styles: in-line, insertion, and non-invasive. In-line flow meters are installed directly in the process line. Insertion-type devices are inserted perpendicular to the flow path and usually require a threaded hole in the process pipe. Non-invasive flow meters do not require mounting directly in the process flow and can be used in closed piping systems. Fittings can be flanged, threaded, or compression-style devices. Clamps, plain ends, socket welds, tube ends, and hose nipples are also available. In terms of electrical outputs, choices include: analog current, analog voltage, frequency, and switch. Some flow meters provide signal outputs in serial, parallel, Ethernet, or other digital formats. Others format output signals according to industrial fieldbus, networking, or industrial automation protocols.  

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.  

Specifications for flow meters include pipe diameter, mounting style, end fittings, electrical outputs, and interface options. There are three basic mounting styles: in-line, insertion, and non-invasive. In-line flow meters are installed directly in the process line. Insertion-type devices are inserted perpendicular to the flow path and usually require a threaded hole in the process pipe. Non-invasive flow meters do not require mounting directly in the process flow and can be used in closed piping systems. Fittings can be flanged, threaded, or compression-style devices. Clamps, plain ends, socket welds, tube ends, and hose nipples are also available. In terms of electrical outputs, choices include: analog current, analog voltage, frequency, and switch. Some flow meters provide signal outputs in serial, parallel, Ethernet, or other digital formats. Others format output signals according to industrial fieldbus, networking, or industrial automation protocols.  

Flow meters differ in terms of features, applications, and operating performance. A flow meter’s technology determines the type of media that it can measure. Media temperature is largely dependent on construction and liner materials. Flow meters that can measure temperature, density, or level are commonly available. They may include audible or visual alarms, averaging and controller functions, programmability, and recorder or totalizer functions. In terms of operating performance, turndown ratio is the effective dynamic or operating range of the flow meter. For example, a 500 SCCM flow rate device with a turndown ratio of 50:1 will operate effectively and resolve flow down to 10 SCCM. If the same device has a turndown of 100:1, then it will resolve effectively to 5 SCCM.