•In case of Full-Bore meters, the variation of flow rate through a constant area is related to the flow rate.

•Unlike full-Bore meters, area flow meters are devices in which pressure drop is constant and the area through which the fluid flows varies with flow rate.

•Area is to be related to flow rate through proper calibration.

Rotameter:
 
 

•Rotameter is a typical area meter

•Consists of gradually tapered glass mounted vertically in a frame with the large end up.

•Fluid flows upward  through the tapered tube and suspends freely a float (which is submerged in the fluid)

• Float is the indicating element, and the greater the flow rate, the higher the float rides in the tube.

•The tube is marked in divisions, and the reading of the meter is obtained from the scale reading at the reading edge of the float, which is taken at the largest cross section of the float.

•A calibration curve must be available to convert the observed scale reading to flow rate.
 

Advantages of a Rotameter:

•Can be modified to cater to the needs of individual operation
Eg:In case of a rotameter with a metal case, the float can have an extension.

• The float of the rotameter does not rotate in modern designs as it used to in the early ones.

•Float shapes and proportions can be varied for different applications.
 

Flow rate through a Rotameter:
 
 

                  where   q - Volumetric flow rate
                             umax      - Maximum Velocity of the fluid
                                 Dt - Tube Diameter
                                 Df - Float Diameter