Variable area orifices
There are other systems for measuring flow, based on the measurement of the speed of rotation of a small turbine or gear cam. These systems are typically used to know the total mass passed in a given time (gas or fuel meter).
A particular type of mass flowmeter, called a “rotameter,” is widely adopted both in the laboratory and in industrial plants, providing a visual indication of the mass flow rate. The instrument consists of a vertical, transparent and slightly conical tube (Figure 2.39), in which the fluid is introduced from below. A floating body is inserted in the tube and the mass flow rate is read on a scale printed on the wall of the tube: the floating body is in equilibrium at the height where the drag added to the buoyancy force equals its weight. As mass flow rate increases, drag tends to increase and the body moves up to a new larger section where speed, and hence drag, goes back to the previous value.
Often a set of floating bodies of different masses is provided with each rotameter in order to use the same tube for different ranges of mass flow rates. Rotameters are available for a large number of gases at different pressures and for many liquids of industrial interest.