Rotameter
A rotameter is a device that measures the flow rate of liquid or gas in a closed tube.
It belongs to a class of meters called variable area meters, which measure flow rate by allowing the cross-sectional area the fluid travels through to vary, causing some measurable effect. [1]
History
The first variable area meter with rotating float was invented by Karl Kueppers in Aachen in 1908. This is described in the German patent 215225. Felix Meyer found the first industrial 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 Rotameter Co, in Crawley, and still exists, having been passed down through the acquisition chain: KDG Instruments, Solartron Mobrey, and Emerson Process Management (Brooks Instrument). Rota with their "Rotamesser" are now owned by Yokogawa Electric Corp.
Implementation
A rotameter consists of a tapered tube, typically made of glass with a 'float', actually a shaped weight, inside that is pushed up by the drag force of the flow and pulled down by gravity. Drag force for a given fluid and float cross section is a function of flow speed squared only, see drag equation.
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.
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.
Advantages
- A rotameter requires no external power or fuel, it uses only the inherent properties of the fluid, along with gravity, to measure flow rate.
- A rotameter is also a relatively simple device that can be mass manufactured out of cheap materials, allowing for its widespread use.
- Since the area of the flow passage increases as the float moves up the tube, the scale is approximately linear. [1]
Disadvantages
- Due to its use of gravity, a rotameter must always be vertically oriented and right way up, with the fluid flowing upward.
- Due to its reliance on the ability of the fluid or gas to displace the float, graduations on a given rotameter will only be accurate for a given substance at a given temperature. The main property of importance is the density of the fluid; however, viscosity may also be significant. Floats are ideally designed to be insensitive to viscosity; however, this is seldom verifiable from manufacturers' specifications. Either separate rotameters for different densities and viscosities may be used, or multiple scales on the same rotameter can be used.
- Due to the direct flow indication the resolution is relatively poor compared to other measurement principles. Readout uncertainty gets worse near the bottom of the scale. Oscillations of the float and parallax may further increase the uncertainty of the measurement.
- If several streams, controlled by rotameters' valves, merge into a common volume, the stream coming from the highest-pressure source will shut down all the other streams. Electronic mass-flow controllers do not have this disadvantage.
- Rotameters are not easily adapted for reading by machine; although magnetic floats that drive a follower outside the tube are available.
- Usually rotameters aren't made in very large sizes (more than 6 inches/150 mm), but bypass designs are sometimes used on very large pipes. [1]
- Clear glass is used which is highly resistant to thermal shock and chemical action.
If the tube is made of metal, the float position is transferred to an external indicator via a magnetic coupling. In the case of glass and plastic tubes you can simply view the float position to get a reliable reading of the flow rate..
The mechanical nature of the measuring principle provides a flow device that does not require any electrical power supply. However, there are many applications in process plants that do require electronic indication and transmission of the measured flow rate to other associated devices. This capability has considerably expanded the range of applications for the variable area flowmeter.
Railway rotameter
The New South Wales Government Railways constructed in 1903 a device for measuring the length of its lines of railway. That authority named the machine a Rotameter. It consisted of a four-wheel trolley with an additional large fifth wheel which traveled along the running surface of the rail. Its last recorded use was in the 1920s.[2]
References
- ↑ 1.0 1.1 1.2 Robert S. Brodkey, Harry C. Hershey, Transport Phenomena: A Unified Aprroach Brodkey Publishing (McGraw Hill), 2003 ISBN 0-9726635-8-4, pages 471-476
- ↑ "How Far is That? The Story of the NSWGR Rotameter" Australian Railway History, September, 2007 pp333-343
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