In this type of orifice meter, there is a fixed aperture and flow is indicated by a drop in differential pressure. In area meter, there is a variable orifice and the pressure drop is relatively constant. Thus, in the area meter, flow is indicated as a function of the area of the annular opening through which the fluid must pass. This area is generally readout as the position of a float or obstruction in the orifice. Show The effective annular area in area meter is nearly proportional to height of the float, plummet or piston, in the body and relationship between the height of float and flow rate is approximately linear one with linear flow curve as well as scale graduations. Area meters are of two general types :
In this meter, a weighted float or plummet contained in an upright tapered tube, is lifted to the position of equilibrium between the downward force of the plummet and the upward force of the fluid in addition to the buoyancy effect of the fluid flowing past the float through the annular orifice. The flow rate can be read by observing the position of the float. Piston Type MeterIn this meter, a piston is accurately fitted inside a sleeve and is lifted by fluid pressure until sufficient post area in the sleeve is uncovered to permit the passage of the flow. The flow is indicated by the position of the piston. Fig. shows the types of Variable area flow meter (a) Rotameter and (b) Piston Type meter. Performance Characteristics1. LinearityThe flow rate (volume) through a variable area meter is essentially proportional to the area and, as a result, most of these meters have essentially equal-scale increments. A typical indicating rotameters scale is non linear by about 5%. 2. DifferentialAn important characteristic of the variable area meter is that the pressure loss across the float is a constant. The overall differential across the meter will increase at higher flow rates because of friction losses through the fittings. 3. AccuracyThe most common accuracy is ±2% of full scale reading. This increases considerably with individual calibration and scale length. Repeatability is excellent. Variable area flow meters are the most commonly used means for measuring low-flow rates. Full scale capacities range from 0.5 cm3/min of water and 30 std cm3/min of air in the smallest units to over 1200 litres/min of water and 1700 m3/h of air in 8 cm height meters. 5. Minimum Piping RequirementAn area meter usually can be installed without regard to the fittings or lengths of straight pipe proceedings or following the meter. 6. Corrosive or Difficult to handle liquidThese can often be handled successfully in an area meter. They include such materials as oil, tar, refrigerants, sulphuric acid, black liquor, beverages, aqua regia and molten sulphur. In general, if the nature of the fluid does not permit the use of a conventional differential pressure type meter because the fluid is dirty, viscous or corrosive, certain area meters have an advantage over other types of meters. By placing very light floats in over sized meters, flow rates can be handled with a combination of very low pressure loss (often 2.5 cm of water column or less) and 10 : 1 flow range. Articles You May Like :Transit Time Flow Meter Vortex Flow meter Animation Ultrasonic Flow Meters Venturi and Orifice flow meter Calibrate Flow meters A rotameter is a device that measures the volumetric flow rate of fluid in a closed tube.[1] It belongs to a class of meters called variable-area flowmeters, which measure flow rate by allowing the cross-sectional area the fluid travels through to vary, causing a measurable effect.[2] The first variable area meter with rotating float was invented by Karl Kueppers (1874–1933) in Aachen in 1908. This is described in the German patent 215225. Felix Meyer founded the company "Deutsche Rotawerke GmbH" in Aachen recognizing the fundamental importance of this invention. They improved this invention with new shapes of the float and of the glass tube. Kueppers invented the special shape for the inside of the glass tube that realized a symmetrical flow scale. The brand name Rotameter was registered by the British company GEC Elliot automation, Rotameter Co. In many other countries the brand name Rotameter is registered by Rota Yokogawa GmbH & Co. KG in Germany which is now owned by Yokogawa Electric Corp. A rotameter consists of a tapered tube, typically made of glass with a 'float' (a shaped weight, made either of anodized aluminum or a ceramic), inside that is pushed up by the drag force of the flow and pulled down by gravity. The drag force for a given fluid and float cross section is a function of flow speed squared only, see drag equation.[3] A higher volumetric flow rate through a given area increases flow speed and drag force, so the float will be pushed upwards. However, as the inside of the rotameter is cone shaped (widens), the area around the float through which the medium flows increases, the flow speed and drag force decrease until there is mechanical equilibrium with the float's weight. Floats are made in many different shapes, with spheres and ellipsoids being the most common. The float may be diagonally grooved and partially colored so that it rotates axially as the fluid passes. This shows if the float is stuck since it will only rotate if it is free. Readings are usually taken at the top of the widest part of the float; the center for an ellipsoid, or the top for a cylinder. Some manufacturers use a different standard.[3] The "float" must not float in the fluid: it has to have a higher density than the fluid, otherwise it will float to the top even if there is no flow. The mechanical nature of the measuring principle provides a flow measurement device that does not require any electrical power. If the tube is made of metal, the float position is transferred to an external indicator via a magnetic coupling. This capability has considerably expanded the range of applications for the variable area flowmeter, since the measurement can observed remotely from the process or used for automatic control.[3]
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