What is the operating principle of a variable area flow meter, such as a rotameter?

Correct Answer: Option B — The float or rotor is more dense than the fluid being measured and is positioned so that the gravitational force down is balanced by the upward motion of the fluid flow.

Rotameters operate on the variable area principle. A float, denser than the fluid, is suspended in a tapered tube. As fluid flows upward, the float rises until the upward drag force balances the downward gravitational force. The float's equilibrium position within the tapered tube directly correlates to the volumetric flow rate.