Rotameter , Construction , Working

Introduction :

- A Rotameter is a device that measures the flow rate of liquid or gas in a closed tube .

- It Belongs to a class of meters called variable area meters , which measures flow rate by allowing the cross sectional area the fluid travels through to vary . causing some measurable effect .

Construction :

- It Consist of a tapered , metered metering glass tube inside of which is located a rotor or active element ( float )

- The tube is Provided a suitable inlet and outlet connection.

- The Float has a specific gravity higher then that of fluid as the fluid flows through the pipe the float rises - Equilibrium is reached when Pressure and the buoyancy of the float counterbalance gravity .

- The Floats height in the tube is then used to reference a flow rate on a calibrated measurement reference .

Working :

- The Rotameter operation is based on the variable area principle : fluid flow raises a float in a tapered tube , increasing the area for passage of the fluid , the grater the flow the higher the float raised .

- The height of the float is directly proportional to the flow rate . with liquid the float is raised by a combination of the buoyancy of the liquid and the velocity head of the fluid .

-  The float moves up or down in the tube in Proportion to the fluid flow rate and annular area between the float and tube wall .

- The Float Reaches a stable Position in the tube when the upward force exerted by the flowing fluid equals the downward gravitational force exerted by the weight of the float .

- A change in the flow rate upsets this balance of forces . the float then moves up or down changing the annular area until it again reaches a position where the forces are in equilibrium .

-  To satisfy the force equation , the rotameter float assumes a distinct position for every constant flow rate . However it is important to note that because the float position is gravity dependent , rotameters must be vertically oriented and mounted .

Discharge Q is given as :

Q = SQRT ( CdAft(2gVf((rhof - rho)/Af(rho))) )

where :

Q = Discharge .

Cd = Discharge Coefficient 

rhof = Density of fluid

rho = Density of the float

Af = Maximum cross sectional area of float .

Aft = Annular area between float and tube .