Which flow measurement device employs a float within a tapered glass tube?

The device that uses a float within a tapered glass tube for flow measurement is a rotometer. In a rotometer, the flow of fluid lifts a float, and as the flow increases, the float rises within the taper of the tube. The position of the float indicates the flow rate, which can be read against graduated markings on the glass tube. This design allows for a simple and direct visual representation of flow, making it a popular choice in various applications for measuring the flow of liquids and gases.

In contrast, a venturi meter measures flow by constricting the flow path, leading to a drop in pressure, which is related to flow rate. An orifice plate measures flow based on the pressure drop across a sharp-edged plate, while a magnetic flow meter operates based on the principles of electromagnetic induction and does not utilize a float or glass tube system. The unique operation and construction of a rotometer is what distinguishes it in the realm of flow measurement devices.