Flow Control - Jan 2012
flow update The terms rotameter and variable area meter have become synonyms over the years It appears though that Yokogawa has the rights to the name rotameter in Germany while Emerson Process has the rights to the name in the United Kingdom Regardless the name rotameter has become a generic term for variable area flowmeters so its status as a true trademark is open to question This is similar to what happened to Xerox which is now often used generically to describe any photocopy Principle of Operation Most variable area VA flowmeters consist of an upright tapered measuring tube that contains a float Fluid flows through the tube from bottom to top The upward force of the fluid raises the float in the tube The float is counterbalanced by the force of gravity The point at which the float stays constant indicates the volumetric flowrate which can be often read on a scale on the meter tube Variable area meters vary according to their float shape and from the material the float is made Construction Variable area meters come in plastic glass and metal Their cost varies with their material of construction Metal tube meters are generally highest in cost and are designed for high pressure and high temperature applications Plastic meters are typically lowest in cost followed by glass flowmeters Some metal tube meters can exceed 1000 in price A group of VA meters called purgemeters have been designed for low flow applications For most plastic and glass VA meters the flowrate can be read from the scale on the tapered meter tube For metal VA meters the position of the flow is magnetically transmitted to an analog indicator These meters also have a scale indicating flowrate The history of variable area meters goes back to 1908 when they were invented by German engineer Karl Kueppers in Aachen Germany At that time they were called rotameters and the position of the analog indicator displays the flowrate These VA meters still rely on a float to indicate flowrate and they also need to be read manually like most of the tube design meters Metal VA meters can be used for liquid gas and steam applications Applications VA flowmeters are used in research and laboratory environments and in the process industries to measure the flow of gases and air at low flowrates They are also used when a visual indication is sufficient and to check on the performance of other meters VA meters including plastic meters are also used for OEM applications Plastic meters are applicable for water gas and air applications The scales are separately marked for water and air applications and they are available in both English and metric units They are most widely used when cost is the main consideration and high accuracy is not required Suppliers KROHNE krohne com began making variable area meters in Duisburg Germany in 1921 Today KROHNE makes a wide range of plastic glass and metal VA meters that can be used for a variety of applications Some typical applications for KROHNEs meters include hygienic and chemical processes water and wastewater and offshore applications KROHNE quotes accuracies ranging from 1 percent of measured value to 4 6 percent of measured value depending on the model Today Yokogawa makes a range of metal glass and plastic VA meters it calls the RA Series Its metal RAMC VA meter has a transmitter that supports both HART and Profibus PA Having a transmitter output is still the exception for most VA meters but is becoming more common as companies want to combine the advantages of low cost with automation In addition to indicating flow at a point in time the RAMC has a totalizing option Brooks Instrument brooksinstrument com is another major Circle 19 or Request Info Instantly at www FlowControlNetwork com 28 January 2012 Flow Control
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