
The variable area flow measurement technique is one of the oldest known and widely-recognized flow measurement principles. As such, variable area flowmeters, sometimes also known as rotameters, have one of the largest install bases of any flow measurement technology that is being sold today. Key advantages of this technology are that it is reliable, easy to install, and easy to read. These meters are also commonly chosen because the measurement technique is mechanical in nature and may be deployed in situations where there is no power available.
Due to their status as one of the oldest-known flow measurement techniques and their large install base, variable area meters, or rotameters, they can be seen in various industrial and institutional settings where liquid and gas flows are being measured. While the variable area meters that are offered today may have widely differing appearances, they all share the same basic design elements – a flow body that consists of a tapered glass or clear plastic tube or a cylindrical metal tube flow meters with an internal tapered metering pin, an internal moving float or orifice plate, and a printed scale that provides the flow rate reading.
The volumetric flow rate through the flow body is proportional to the displacement of the float. When a fluid flows through the tube from bottom to top, it causes a relatively constant pressure drop across the float, which produces an upward force that urges the float rises to move towards the top of the tube. Increases in flow rate are directly proportional to the square of this pressure drop.
The tapered walls of the glass tube or plastic tube, or in the case of the metal tube, the tapered metering pin will cause the size of the orifice that the flow passes through to increase as the float travels upwards gradually. This changing orifice size will easily linearize the square root relationship between pressure drop and flow to read the printed scale. It is also where the term “variable-area” comes from.
The glass tube indicating type is a widely used form of variable area flow meter, featuring a distinctive design. The float, which can be machined from various materials such as metal, glass, or plastic, smoothly glides within the tube, carefully crafted from robust borosilicate glass.
Since the variable-area flowmeter relies on gravity, it must be installed vertically with the flow tube perpendicular to the base When the vertical installation is not possible, a spring loading the float within the tube wall can be constructed so it has flexible installation.
VA meters are calibrated with water for liquids and air for gasses. For applications involving fluid media other than water or air, the flow scales are corrected based on the operating density of the alternate fluid. The gases or corrosive liquids flow rates are read by aligning the top of the float with the tick mark on the flow tube.
However, a change in operating parameters will compromise the meter’s accuracy, forcing it to be returned to the factory for recalibration. In general, the average accuracy of a variable-area flowmeter is ±2-4% of full-scale flow.
VA flow meter readings must be corrected when the calibration conditions and operating conditions for gas flows vary. Some manufacturers have a correction that can be used. However, this correction factor is based on constant pressure and temperature, just as is for all volumetric flow meters such as vortex, turbine, PD, DP and others. For liquids, the effect of viscosity changes is another potential disadvantage because drag layers of fluid flow will build up on the float. This will cause a slower-moving viscous liquid to yield the same buoyant force as a faster-moving fluid of lower viscosity. The larger the viscosity, the higher the error. The general rule of thumb is as follows—unless the meter has been specifically calibrated for a higher-viscosity liquid, only water-like liquids should be run through a variable-area flow meter. Sometimes, for slightly thicker liquids than water, a manufacturer-supplied correction factor can be used without recalibrating the whole meter. Learn more about Clamp-on Ultrasonic Flow Meter Manufacturers.
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