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All About Flow Meters
You may have never considered the metering, or measurement, of a gas or liquid to be important. Knowing the flow rate of a fluid is extremely critical to a number of manufacturing plant operations, as is the measurement of a gaseous substance. Regulatory agencies require standards be applied to the use of many liquids and gasses to ensure the overall safety of the system. Plant operations need to know this data to make certain of the full functionality and efficiency of the systems in use.
Flow meters are used in all sorts of facilities. These include, but are certainly not limited to, sewage plants, laboratories, semiconductor manufacturing, industrial plants and HVAC operations, as well as engine fuel cells. Today’s flow meter may also offer a digital display for convenience when reading the data. Flow meters also encompass a wide range of applications, from turbine and Doppler flow meters to ultrasonic flow meters. The choice of which particular flow meter to employ is determined by use, footprint and budget. At the opposite ends of the flow meter spectrum are devices that use high-end means to accomplish their tasks, or are low-end mechanical devices to measure the flows.
The ultrasonic flow meter is a device used where ultra-pure liquids are in use, such as in semiconductor operations where cleanliness is definitely next to Godliness. Ultrasonic flow meters for use in the semiconductor industry are usually assembled in at least a Class 100 Clean Room. These units can operate under increased pressures (PSI) as well as within high temperature environments. Sensing of liquid flow is managed through the use of an ultrasonic transmitter and a receiving device mounted just behind the deal at each end of a flow tube. Thanks to the technology used, an instantaneous flow rate is obtained and a total flow volume calculated.
With mechanical flow meters, the fluid within the conduit passes over a rotary device, such as a turbine or an anemometer. The flow rate is calculated as the fluid flows over the device. Another mechanical device is called a venturi meter and harkens back to use centuries ago during the time of the Roman Empire. This meter constricts the flow of the liquid and the differential pressure resulting from this constriction is then measured. Gas running through a pipeline often has its flow rate calculated in this fashion. Another flow meter device called an orifice plate also uses flow constriction for the measurement data.
When the flow passes an object in its path, vortexes are created, and this means pressure changes can be measured. This device is appropriately named a vortex-meter and uses a quartz crystal to accomplish the measurement. For use in extreme environments, where damage to a flow meter would be of high concern, are the rugged pitot tube flow meters. This tube possesses an “L” shape and measures the difference between the dynamic pressure and the static pressure. With this unit, the amount of disturbance to the flow is not as severe as might be expected with an orifice plate or venturi meter. Another model of the pitot tube is the use of a heated element inside the device to measure the cooling rate as the liquid passes by.
Flow meters are also able to correct for conditions, such as fluid characteristics that might alter, varying pressure and temperatures, and non-linearities. Such devices fall within the coriolis, magnetic and ultrasound categories of flow meters.
The Coriolis flow meter uses what is known as the Coriolis effect to directly measure mass flow. They have broad use, from measuring hydrogen gas to a viscous mass, such as peanut butter, without the need of recalibration. A Coriolis measurement is extremely accurate.
The magnetic flow meter is common in use and uses the principle of magnetic induction to calculate the data. Magnetic induction is simply the production of voltage across a conductor that’s situated in a changing magnetic flux, which itself is a measure of the quantity of magnetism. A magnetic field is applied to the metering tube. The result is a measurable difference proportional to the flow velocity perpendicular to the flux lines, or the lines of force passing through a specified area in a magnetic field. Back in 1831, Michael Faraday is believed to have discovered the phenomenon known as induction used for these modern-day calculations.
Then there are the ultrasound units that measure the time difference between pulses sent through the liquid. One pulse is sent in the flow’s direction, the other pulse sent from the opposite direction, and the time difference in the speed of the sound in the fluid is the final measurement.
Flow meters accomplish their tasks through the use of a variety of unique technologies. Some are based upon centuries-old technologies while others rely on high-tech methodology for their results. Whatever the system in use, accurate readings and measurements of liquids and gases are critical to the safety and ultimate productivity of facilities.
About the Author: Sierra Instruments of Monterey, California, is a leading manufacturer of mass flow meters, mass flow controllers, and partial flow sampling systems. This successful business is family-owned and is constantly striving towards perfection in terms of quality, price, delivery, innovation, and customer support.