When fluid or gas flows through a taper tube containing a float, a pressure difference of P1 and P2 is created between upper and lower side of the float. The float moves upwards by a force obtained by multiplying the pressure differential by the maximum cross sectional area of the float.
Due to taper tube, as the float moves upwards, the fluid passing area increases as a result of which the differential pressure decreases. Upward movement of float stops when the dead load is dynamically balanced by the differential pressure. Tapering of metering tube is so designed that the vertical movement of the float becomes linearly proportional to the rate of flow and the scale is provided to read the position of the float, thus giving birth to flow rate indication.
Based on Bemoulli's theorem, the principle mentioned above can be theoretically expressed as follows
FLOW FORMULA
Where
 Q = Volumetric flow rate
 V = Volume of Float
 C = Flow coefficient
 Af = Maximum pressure receiving area of float.
 A = Fluid passing Area
 P = Float Density
 g = gravimetric acceleration
 y = Fluid Density
There are various types of flow meters available namely:
 GLASS TUBE ROTAMETERS
 BYPASS ROTAMETER COMPLETE ASSEMBLIES
 PLASTIC BODY ROTAMETERS GANG / MULTIPLE ROTAMETERS
 METAL TUBE ROTAMETERS WITH DIGITAL FLOW RATE INDICATION
 ROTEMERS AS PER SAMPLE & OR DRAWING
 METAL TUBE ROTAMETER WITH TRANSMITTER i.e. 420 mA output & DIGITAL TOTALISER ( OPERATING ON 420mA OUTPUT)
 Electromagnetic Flowmeter
 Paddle wheel type (Nutating Disk Type Oil Flow Meters)
