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

Correct Answer: Option D — 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 utilize a float that is denser than the fluid. The fluid flows upward through a tapered tube, exerting an upward drag force on the float. Equilibrium is reached when the gravitational force acting downward on the float is balanced by the upward drag and buoyancy forces.