Variable Area Flowmeter
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.
Variable Area Advantages & Disadvantages
Advantages:
- Major advantages of the variable-area flowmeter include it’s relatively low cost and it’s ease of installation. It’s simplicity of design lends to low maintenance operation, and hence, tends to have a long operating life.
- Another advantage is it’s flexibility to be used in different applications having a wide range of chemicals. With the PTFE-lined metal tube construction, variable area meters are able to resist corrosive damage from aggressive chemicals.
- The variable area meter does not require power and will provide accurate flow rate readings much the same as a glycerin-filled pressure gauge reads pressure readings without any electrical power. This allows the meter to be deployed where it is not practical or possible to run power cable to the device.
Disadvantages:
- One potential disadvantage of a variable-area flowmeter occurs in applications where the media temperature and pressure deviate from the calibration temperature and pressure. Temperature pressure variations will cause a changes in density for both gasses and liquids while changes in pressure will cause a gas to expand and contract. For liquids, viscosity will also change with temperature. Since the variable area measurement principle is volumetric in nature and is also density and viscosity-sensitive, these changes in pressure and temperature will lead to calibration shift.
- During operation, the flowmeter accuracy can quickly degrade once the temperatures and pressures start fluctuating from the standard calibration temperature and pressure. VA meters used for water tend to show less variability, since water viscosity and density changes very little with normal temperature and pressure fluctuations. While there is a way to correlate the flow from actual operating conditions back to the calibration conditions, the conventional formulas used are very simplified, and don’t take into account the effect of viscosity, which can cause large errors.
- VA flowmeter 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 a constant pressure and temperature, just as is for all volumetric flowmeters such as vortex, turbine, PD, DP and others. For liquids, the effect of viscosity changes is another potential disadvantage because drag layers of fluid 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 flowmeter. Sometimes, for liquids that are slightly thicker than water, a manufacturer-supplied correction factor can be used without the need to recalibrate the whole meter.
