The rotameter uses weighted float or plummet contained in a vertical tapered glass tube.

It is lifted to the equilibrium position between downward force of the plummet and the upward force of the fluid in addition to the buoyancy effect of the fluid flowing past the float through the annular orifice.

The flow rate can be read by observing the position of the float.

The fluid to be measured enters at the bottom of the tube, passes upward around the float, and out at the top.

When there is no flow through the meter, the float rests at the bottom of the metering tube where the maximum diameter of the float is approximately same as the bore of the tube.

When fluid enters the metering tube, the buoyant effect of the fluid lightens the float, but it has a greater density than the liquid and the buoyant effect is not sufficient to raise it.  

• The float moves up and down in the tube in proportion to the fluid flow rate and the annular area between the float and the tube. • It reaches a stable position in the tube when the forces are in equilibrium. During the upward movement of the float towards the larger end of the tapered tube, the annular opening between the tube and the float. • As the area increases, the pressure differential across the float decreases.

• The float assumes a position, in dynamic equilibrium, when the buoyancy forces are balanced by weight of float.

• Floats can be made from several materials to obtain corrosion resistance or capacity modification.

• Ratings are generally in terms of meter capacity, using a stainless steel float.

•It has been found that the float shape determines to large extent how much a rotameter will be influenced by changes in the viscosity of the measured fluid.

Advantages

• Low cost.

• Low pressure drop

• It is suitable for small flow rates

• It is easily equipped with alarms and switches or any transmitting devices.

Disadvantages

• It is difficult to handle the glass tube type.

•It must be mounted vertically.

•It is not suitable for pulsating services.