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.

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.

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.
• 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.

http://en.wikipedia.org/wiki/Rotameter