A rotameter is one particular type of variable area flowmeter that measures flow by varying the cross-sectional area a fluid or gas travels through in a closed tube.
Requires no external power.
Is a simple device that can be easily manufactured out of inexpensive materials.
The clear glass tube is resistant to thermal shock and chemical reaction.
Must be mounted vertically, with designated top and bottom, and with the fluid flowing from bottom to top.
Graduations on a given rotameter are only accurate for a given substance at a given temperature. Separate rotameters must be used for fluids with different densities and viscosities, or multiple scales on the same rotameter must be used.
Resolution is relatively poor and gets worse near the bottom of the scale.
Oscillations of the float and parallax lend to reduced accuracy.
Difficult to automate - primarily a manual / visual device