Liquid Flow Meters Information

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Image Credit: ABB Measurement Products

Liquid flow meters are used to measure the volumetric flow rate or amount of a moving liquid. With some devices, the flow rate is determined by measuring the liquid's velocity, a function of the pressure differential that forces the liquid through a pipe. Since the pipe's cross-sectional area remains constant, an average flow rate can be determined. In addition to velocity, metering technologies for flow meters include differential pressure (DP), positive displacement (PD), and true mass.


How Flow Meters Work

This video explains how a volumetric flow meter works.


Product Types

The GlobalSpec SpecSearch database provides information about many different types of flow meters. When selecting products, buyers should consider the advantages and disadvantages of the metering technology used by each.

  • 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.
  • 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. PD devices are often used for high-viscosity fluids.
  • True mass flow meters are used to directly measure the mass rate of flow. These types of devices include thermal meters and Coriolis meters.

Note that in addition to pipe diameter, factors that affect liquid flow rate include the liquid's viscosity and density, as well as the friction of the liquid in contact with the pipe.

Performance and Features

As a rule, a liquid flow meter's technology determines its ability to measure additional media such as gas, steam, suspended solids, or slurries. Media temperature is largely dependent on construction and liner materials. Liquid flow meters that can measure temperature, density, or level are commonly available. With regard to features, devices may include audible or visual alarms, averaging and controller functions, programmability, and recorder or totalizer functions.

Mounting Styles and Other Specifications

Other specifications for liquid flow meters include end fittings, electrical outputs, and interface options, and mounting styles. There are three basic mounting styles: in-line, insertion, and non-invasive.

  • In-line flow meters are installed directly in the process line.

Image Credit: Aalborg Instruments

  • Insertion-type devices are inserted perpendicular to the flow path and usually require a threaded hole in the process pipe.

Image Credit: Dwyer Instruments, Inc.

  • Non-invasive liquid 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.

Image Credit: Honeywell Process Solutions

Some liquid 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.  



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  • Flow Meters

    Flow meters (flowmeters) and flow sensors are devices used for measuring the flow rate or quantity of a moving fluid or gas.

  • Flow Transmitters

    Flow transmitters provide electrical outputs that are proportional to flow inputs. They use flow meters to measure the flow of liquids and gases.

  • Paddlewheel Flow Meters

    Paddlewheel flow meters have a paddle wheel that is perpendicular to the flow path. The rotor axis is positioned to limit contact between the paddles and the flowing media.

  • Positive Displacement Flow Meters

    Positive displacement flow meters measure the volume or flow rate of a moving fluid or gas by dividing the media into fixed, metered volumes. These devices consist of a chamber that obstructs the media flow and a rotating or reciprocating mechanism that allows the passage of fixed-volume amounts.

  • Rotameters

    Rotameters are typically tapered glass tubes positioned vertically in the fluid flow. A float within the tube rides upward in relation to the amount of flow.

  • Turbine Flow Meters

    Turbine flow meters measure the rate of flow in a pipe or process line via a rotor that spins as the media passes through its blades.

  • Vortex Flow Meters

    Vortex flow meters are flow sensors that detect the frequency of vortices shed by a bluff body placed in a flow stream. The frequency of the vortices is proportional to the flow velocity.

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