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Mass flow meters, specifically the Coriolis, measure mass flow directly and can meter down to low flow rates. They work based on when a fluid is in motion, any change in direction will produce a reaction in the system. This reaction is proportional to the mass of the fluid being accelerated.
Mass flow meters are viewed by many as close to the perfect flow measuring device, delivering excellent accuracy and overall performance. However, Coriolis type meters can be very expensive.
Velocity flow meters include Ultrasonic and Electromagnetic flow meters, both of which use full pipe bores, measure the liquid velocity and are inherently bi-directional. For large pipes a multipath meter will give superb velocity information.
Add in measurements or tables for the other fluid variables and an ultrasonic flowmeter is the perfect device for these large conduits. For medium size tubes a clamp-on ultrasonic meter will give flow readings with no additional pressure drop to the system, although set-up and calibration is required for really accurate readings.
For tubes down to 1mm bore, in-line volumetric meters are available with excellent accuracy when using the latest available technology. Such ultrasonic flow meters are independent of Reynolds numbers and can therefore operate from laminar flow up to turbulent flow. This makes them highly commercial being able to accurately measure liquids ranging from water to high viscosity oils. As through-flow devices, they can also be tolerant to impurities in the system which would cause havoc to meters with moving parts.
Ultrasonic flow sensors use high frequency sound waves to measure flow. The ultrasound is injected with the direction of flow into the fluid by one transmitting sensor and is received by a second sensor further down the tube. The second sensor then sends a second ultrasound signal back against the flow to the first sensor. As one sound pulse is accelerated by the velocity of the liquid and the second retarded, the difference in the flight time is twice the fluid velocity, and as the dimensions of the flowmeter tubes are known the volumetric flow can be calculated.
The Electromagnetic meters are highly accurate and reliable, making them ideal for any conductive liquid. Electromagnetic meters can be used successfully with aggressive and contaminated liquids and are also reasonably priced.
Turbine flowmeters are economic but do require clean homogenous fluids. External instrumentation is often required adding to the apparent meter installation cost. The larger dynamic range and the ability to meter very low flows, make the turbine meters ideal for beverage dispensing or other low-flow dosing processes.
The appeal of differential pressure devices is obvious: simple, usually reliable, and relatively inexpensive. As a technology, they can handle a wide range of operating conditions from laminar flow through to the most turbulent. The most common method using this principle is with an orifice plate, a hole in the bore of the pipe that creates an obstruction to flow. The square root of the pressure differential is proportional to the flow rate.
use the physical properties of moving liquids. These devices, like the vortex shedding meter, require high Reynolds numbers so are rarely used for very low flows or in small pipes.
“Available technologies that measure flow successfully are many and the user must always perform due diligence on the chosen supplier and technology.”
In line with our ethos of knowledge sharing, Titan Enterprises are excited to have articles published in IChemE’s prestigious publication – The Chemical Engineer. IChemE is the leading professional qualifying body for chemical, biochemical and process engineers.
You can read Flowmeters: Making the Right Choice and Pulse Sensors for Flow Measurement Devices and Applications online, or click the link below to download a PDF document of the article.
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