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, can be seen in a wide variety of 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 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 flowrate through the flow body is proportional to the displacement of the float. When a fluid is moving through the tube from bottom to top, it causes a pressure drop across the float, which produces an upward force that urges the float 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 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 gradually increase as the float travels upwards. This changing orifice size will linearize the square root relationship between pressure drop and flow so that the printed scale can be easily read. It is also where the term “variable-area” comes from.
Since the variable-area flowmeter relies on gravity, it must be installed vertically with the flow tube perpendicular to the base When vertical installation is not possible, a spring loading the float withing the tube can be constructed so it has flexible installation.
VA meters are calibrated with water for liquids and air for gases. 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 gas or liquid 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.