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 a measurable effect.
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 Managementin Great Britain. Whereas in many other countries
such as Germany, Switzerland, Austria, Spain, Italy the brand name Rotameter is registered
by Rota Yokogawa GmbH & Co. KG in Germany which is 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. The 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,
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.
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.
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. Clear glass is used which is highly resistant
to thermal shock and chemical action. 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. Since the float must be read through the flowing
medium, some fluids may obscure the reading. A transducer may be required for electronically
measuring the position of the float. Rotameters are not easily adapted for reading
by machine; although magnetic floats that drive a follower outside the tube are available.
Rotameters are not generally manufactured in sizes greater than 6 inches/150 mm, but
bypass designs are sometimes used on very large pipes.
References
External links Rota Yokogawa: Principle of operation
eFunda: Introduction to Variable Area Flowmeters KROHNE: Measuring Principle