Glass Tube Rotameters consist of a tapered glass tube mounted vertically in a frame with the small diameter at the bottom and large end up. The tube contains a free moving float that rests on the stopper at the base of the tube. When the liquid flows, the float rises until its weight is balanced by the up thrust of the fluid and the float reaches a position of equilibrium. At this position it indicates the instantaneous rate of flow. The flow rate can be read from the adjacent scale.
The pressure drop across the float is equal to its weight divided by its maximum cross-sectional area in the horizontal plane. Both in orifice type meter and in the Rotameter, the pressure drop arises from the conversion of pressure energy to kinetic energy and from the frictional losses. Thus Rotameter may be considered as an orifice meter with a variable aperture and the formula derived for the orifice meter is applicable here with only minor changes.
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