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]
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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.
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 results in increase in 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.
Note that the "float" does not actually 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 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]