Variable Area Flow Meter

Working Principles for Variable Area Flow Meter

The flow meter operation is based on the variable are principle: fluid flow raises a float in a tapered tube, increasing the area for passage of the fluid. The greater the flow, the higher the float is raised. The height of the float is directly proportional to the flow rate.

The float reaches a stable position in the tube when the upward force exerted by the flowing fluid equals the downward gravitational force exerted by the weight of the float. The float then moves up or down, changing the annular area until it again reaches a position where the forces are in equilibrium.

A change in flow rate upsets this balance of forces. The float adjusts itself so that the buoyancy force B acting on it, the wall tube resistance W and its downward gravitational force by weight of the float F are in equilibrium: G=B+W

It is important to note that because the float position is gravity dependent, the flow meter must be vertically oriented and  mounted.