Flow transmitters provide electrical outputs that are proportional to flow inputs. They use flow meters to measure the flow of liquids and gases.  Flow transmitters use three basic types of meters: mass, volumetric, and velocity. 

Meter Technology

Flow transmitters use several meter technologies, including differential pressure (DP). The basic operating principle of DP meters is that a pressure drop across a meter is proportional to the square of the flow rate. Examples of DP meters include orifice plates, cone devices, Pitot tubes, Venturi tubes, and flow nozzles.

• Orifice plates constrict fluid flow to produce differential pressure across a flat piece of metal that contains a bored hole.

• Cone-based devices position a cone in the center of a fluid flow to create a differential pressure that is proportional to velocity.

• Pitot tubes are pairs of hollow tubes. One tube measures impact pressure and the other tube measures potential.

• Venturi tubes gradually narrow the diameter of a flow pipe and measure the resultant drop in pressure.

• Flow nozzles are similar to Venturi tubes, but do not provide an outlet area for pressure recovery.

Measurement Techniques

Flow transmitters use positive displacement (PD), true mass flow, ultrasonic, electromagnetic and other measurement techniques.

• PD devices divide liquids into specific increments that are counted by mechanical or electronic methods. Examples of PD meters include piston meters, oval gear meters, nutating discs, and rotameters.
• True mass flow meters measure the rate of flow in terms of mass and include devices such as thermal meters and Coriolis meters.
• Ultrasonic meters include both Doppler-effect meters and time-of-flight meters.
• Electromagnetic devices use the liquid as the conductor and a magnetic field is created by energized coils outside a flow tube.
• Elbow meters operate on the principle that when liquid travels in a circular path, centrifugal force is exerted along the outer edges.

Specifications and Features

Flow transmitters vary in terms of operating conditions, flow range parameters, performance measurements, and optional features. Operating conditions include maximum operating pressure, fluid temperature range, and operating temperature.

Measured in pounds per square inch (psi), operating pressure is the maximum head pressure that flow transmitters can withstand.

Fluid temperature range, a function of construction and liner materials, is usually measured in degrees Fahrenheit; however, metric devices are commonly available. Flow range parameters for flow transmitters include velocity flow rate range, volumetric flow rate range, and mass flow rate range.

• Mass flow rate is the range of flow in mass/time. The specification applies to mass flow sensors and meters.
• Velocity flow rate range is the range of flow in distance/time. This specification applies to velocity flow sensors and meters.
• Gas volumetric flow rate range is the range of flow in volume/time. This specification applies to volumetric flow sensors and meters for gases.

Performance measurements include the maximum pressure drop for media traveling through the device. A meter's performance can be determined by its turndown ratio. Turndown ratio is the effective dynamic or operating range of the flow meter. For example, if a 500 SCCM flow rate device has a turndown ratio of 50:1; it 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.

Generally, the type of flow meter determines available features. Some flow transmitters can measure temperature or density. Others measure fluid levels.

Output Options

Flow transmitters output analog voltages, analog currents, frequencies and pulses.

Analog voltage outputs are simple, usually linear, functions of the measurement. They are continuous rather than pulsed or discrete.

Analog current outputs are suitable for sending signals over long distances. They include current loops such as 4 – 20 mA and use feedback to provide an appropriate current regardless of impedance or noise.

Frequency and pulse signal outputs are encoded via amplitude modulation (AM), frequency modulation (FM), and pulse width modulation (PWM). They are also expressed as sine waves and pulse trains. Some flow transmitters are inserted perpendicular to the flow path and require a threaded-hole pipe for access. Others are inserted parallel to the flow patch between two pieces of process pipes and clamped into place.

Standards

Flow transmitters must adhere to certain standards to ensure proper design and functionality.

AD 63-09-01 - Fuel flow transmitter

MIL-T-26298 - Transmitter and rate of flow

QPL-5913 - Used by the Government in the acquisition of products covered by the subject specification 

Image credits:

Flow Technology, Inc. | The Engineering Toolbox