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Variable area flowmeters offer economy and value  05-20-2003 Just because sophisticated 'high-tech' instruments are available, it does  not mean we have to use them: simple meters can offer many advantages over  more costly and technical types. Although flowmeters may seem to be  a modern invention, basic types were evident as far back as the early  Roman era.  They were often used for measuring water flow to households.   The mathematical foundations of flow theory were evolved during the 17th century.  Flow rate is a product of the velocity of a fluid and volume. Today, there are many types of  flowmeters: variable area, positive displacement, ultrasonic, and mass  flowmeters, to name a few.  Each type has its special features.  There is  no universal, or perfect flowmeter appropriate for all industries.  The  right flowmeter for the job is the one that will perform adequately in  that application, and at a reasonable expense.  The variable area  flowmeter offers many advantages over some more costly and technical  types, making it the meter of choice for many flow measurement  applications.   Tapered The meter (fig. 1) comprises a  vertically tapered tube through which the fluid to be measured is passed,  from the smaller end up to the larger.  As the fluid flows through the  tube, it forces an indicator (float) upward.  The clearance space (area)  between the float and tube increases as the float approaches the top of  the meter.  This increasing area requires a larger amount of fluid to  force the float higher.  By varying the taper of the tube, the mass of the  float, and the length of the tube, different flow ranges can be  calibrated. The variable area flowmeter must  obviously be plumbed into a piping system the right way: that is, with the  narrow part of the taper at the bottom.  Usually printed on, or next to,  the tube are the flow increments (scale).  The flow rate can be read by  matching the increments on the tube with the edge of the float.   Competitive In recent years, variable area  flowmeters have become very competitively priced.  Because the meter  offers such value, more and more manufacturers of water treatment systems  are including them as part of their standard package.  The makers of ultra  filtration equipment (reverse osmosis systems) use them to measure output  through the membrane, and also to measure reject.  The meters play a  critical role in helping to monitor the efficiency of the system. Ultra pure water is used  extensively in the manufacture of printed circuitry (cleansing of computer  chips, particularly).  Flowmeters are used both in the manufacturing and  management of deionized water. Solar-panel manufacturers  recommend certain flow rates for optimum performance.  Variable area  flowmeters allow the installer and user to monitor flow economically, thus  enabling critical process adjustments to be made. Flowmeters offer an advantage over  pressure gauges in filtration systems because they measure actual flow; as  the filter becomes saturated (full), the flow rate drops.  Just a glance  at the flowmeter tells the operator if the filter needs cleaning,  replacement of cartridges, or if there may be another problem, such as a  tear in the filter material or a broken pipe. Although variable area flowmeters  offer good value, and are easy to install and use, they may not be  suitable for all flow measurement applications.  The water processing, and  wastewater treatment industries will usually select meters designed for  use with water.  These meters may not be suitable for use with other  liquids. Compatibility Be certain to check chemical  compatibility.  Don't just rely on compatibility charts, do your own  testing.  Many flowmeter manufacturers are willing to provide material  sample kits. Specific gravity ('weight') of the  liquid affects meter reliability and accuracy.  Viscosity, the degree to  which a fluid resists flow under applied force, also affects accuracy, as  does elevated temperatures.  Flowmeter manufacturer Blue-White Industries  recommends that you seek assistance from your supplier, to ensure you  select the right flowmeter for your particular application.  Accuracy & repeatability Without a good working knowledge  of what these terms mean in the real world, you can easily overbuy, or  under buy a Flowmeter.  In plain terms accuracy, really means error. Avoid  being misled by all the different code, or terminologies used in the flow  industry.  Flowmeter manufactures may simply say 2% accurate, you need to  question that statement, 2% of what, indicated flow, or 2% of full scale.  These two seemingly similar accuracies are very different, and could be  costing you extra money. Repeatability is different than accuracy, and in  many cases more important to industry. Repeatability is the meters ability  to reproduce flow rates (consistently) under the same conditions.   Repeatability is paramount in the processing industry where tracking flow  changes is so important.