ROTAMETER (VARIABLE AREA FLOW METER) - PRINCIPLE- WORKING- CONSTRUCTION

Variable-area flowmeters

• An variable-area flowmeter is one where the fluid must pass through a restriction whose area increases with flow rate. This stands in contrast to flowmeters such as orifice plates and venturi tubes where the cross-sectional area of the flow element remains fixed.

Rotameters

• Rotameter is the simplest example of a variable-area flowmeter, which is used to measure fluid  or gas flow by a solid object (called a plummet or float) as a flow indicator, suspended in the midst of a tapered tube.

• A device used to measure fluid or gas flow, in which a float rises in a tapered vertical tube to a height dependent on the rate of flow through the tube. 

Construction

• Rotameter consists of  a tapered glass tube mounted vertically in the frame with the  large ends up

• The tube contains a moving solid float

• The diameter of the float is smaller than the diameter of the bottom of the tapered tube

• The density of the float material is higher than that of the liquid 

• A perforated plate is provided at both the ends of the tapered tube for arresting the float in the tub.

• A guide is provided for the float so it always remains at the center

• In the tube along the axis of the tube 

• Float is usually made from non- corrosive material such as aluminum, bronze, ss, nikal etc.  

• A nearly linear flow scale is marked in the glass tube it is mounted close to the tube so that  the Position of the float can be marked & the flow rate is then obtained from a calibration curve or direct scale o the flow rate can also be provided over the tapered tube or near the tube

• The float is the indication element and the reading edge of the float is taken at the largest Cross-section of the float.  

• The either rends of the meter may be screwed of flanged

• Rotameter do not require straight runs of pipe before & after the point of installation 

Working Principle

• Variable area flow meter which works on the principle of upward.

• force exerted by fluid and force of gravity which uses a solid object (called a plummet or float) as a flow indicator, suspended in the midst of a tapered tube.

• When a fluid or gas flow passes through a tapered tube containing a float,  a pressure difference  is created between the upper and lower side of the float.

• This pressure differential, acting on the effective area of the float body, develops an upward force

• (F = PA). this force exceeds the weight of the float, the float moves upward.

• Due to the construction of tapper tube float moves upper direction as cross-section of tapper tube increases as the pressure differences.

• When the differential pressure dynamically balances the dead load of the float at the some point , at this point in the tube where the float stops moving,

• The flow rate indicating by its position relative to a scale mounted on the outside of the tube.