The variable area flow meter consists of a vertically oriented glass (or plastic) tapered tube with a larger inside diameter at the top, and a metering float which is free to move within the tube. Fluid flow causes the float to rise in the tube as the upward pressure differential and buoyancy of the fluid overcome the effect of gravity. The float rises to a level where the area between the float and tube reach a state of dynamic equilibrium between the upward differential pressure and buoyancy factors, and downward gravity force. The height of the float is an indication of the flow rate. The tube can be calibrated and graduated in appropriate flow units. Magnetic floats can be used for alarm and signal transmission functions. Similar designs use a spring load instead of gravity to act against the flow. These designs can be mounted horizontally.

They are widely used because of their low cost, simplicity and low pressure drop. These meters typically have a turndown ratio up to 10:1. The accuracy ranges from 1% to 5% of full scale. Prices general fall in the range of $50 to $1,000.

-Presented by Max Machinery, Inc. the Positive Displacement Meter Experts