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. A rotameter consists of a tapered tube of glass with a float inside that is pushed up by flow and pulled down by gravity. At a higher flow rate more area (between the float and the tube) is needed to accommodate the flow, so the float rises. The float hooks into spiral grooves on the glass (to stabilize the float) causing the float to rotate as it moves up or down. One edge of the float is compared with graduations on the glass to measure the flow rate of the fluid. 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. 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 widespread use in places such as third world countries.

Disavantages:

Due to its use of gravity, a rotameter must always be vertically oriented and right way up, with the fluid flowing upwards.
Due to its reliance on the ability of the fluid or gas to displace the float, the graduations on a given rotameter will only be accurate for a given substance. The main property of importance is the density of the fluid. Either separate rotameters for different substances must be used, or the read out adjusted.
Rotameters normally require the use of glass (or other transparent material), otherwise the user cannot see the float. This limits their use in many industries to benign fluids, such as water.
Rotameters are not easily adapted for reading by machine: although magnetic floats that drive a follower outside the tube are available.

External link

*Variable area meters - Rotameter (Fluid Mechanics - Lecture Notes - M.Subramanian)

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