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.

Types of Liquid Flow Meters

The Engineering360 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.

Video credit: FlowLineOptions / CC BY 3.0

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

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

Outputs

Signal outputs and interfaces include:

• Analog voltage
• Analog current
• Frequency
• Switched
• Serial
• Parallel
• Ethernet
• Fieldbus

Resources

Liquid Flowmeters

Instant Guide to Flow Profile (pdf)

Image credit:

ABB Measurement & Analytics