Rotameters are the mostly widely used type of variable area (VA) flowmeter.The movement of a float in a tapered tube provides a measure of flow rate.
A Rotameter is designed in accordance with the basic volumetric flow rate equation of
Where Q is the flow rate
K is a constant
A is the annular area
G is the force of gravity &
H is the pressure drop across the float
All the rest being a constant in a given rotameter, the flow rate (Q) is directly proportional to the area (A) so the height of the tube is a measure of the flow rate.
In PLACKA purge rotameter a constant flow differential pressure flow regulator is used to maintain the purge flow late at the desired level. A needle valve is used for convenient setting of the flow rate.