It operates on the principle of the variable area the fluid in the glass tube raises the float in the tapered tube increasing the area of passage of fluid.

The float moves up in proportion to the rate of flow of fluid and gets 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 a scale mounted alongside the tube. The taper glass tube is formed of annealed borosilicate glass of extremely high accuracy bore.