It operates on the principle of variable area the fluid in the glass tube raises the float in the taper tube increasing the area of passage of fluid.
The float moves up in proportion to the rate of flow of fluid and get stabilized when the weight of the float is balanced by the upward hydraulic force.
The tube and the float combination determines the flow rate. The float has a sharp metering edge where the reading is observed by means of scale mounted along side of the tube. The taper glass tube is formed of annealed borosilicate glass of extremely high accuracy bore.