Question

Describe rotameter-type flowmeters, explain how they work, and compare them with

Venturi flowmeters

Answer 1

Rotameter

A rotameter, also sometimes called a ‘variable area’ flow meter, is simply a tapered plastic or glass tube with a float whose position in the tube is determined by the flow rate of fluid through the rotameter.

It works on the principle of upthrust force exerted by fluid and force of gravity. The buoyant force exerted on an immersed object is equal to the weight of liquid displaced by the object.

Working

Fluid enters from the bottom of the tapered tube, then some of the fluid strikes directly into the float bottom and others pass aside the float. Now the float experience two forces in opposite direction, darg force upward and gravitational force downward.

Fluid flow moves the float upward against gravity.At some point, the flowing area reaches a point where the pressure-induced force on the floating body exactly matches the weight of the float. The float will find equilibrium when the area around float generates enough drag equal to weight - buoyancy.

As the float weight and gravity are constant, the distance float displaced upward is proportional to the flow velocity of the fluid passing through the tapered tube.

Comparison with venturimeter

• Rotameters can only be installed vertically whereas venturimeters can be installed both horizontally and vertically.
• venturimeters are expensive as compared to rotameters.
• as compared to venturimeters ,rotameters are easy to install.