One of the most common types of hydraulic flowmeters is the Variable Area Flow Meter.
Variable Area Principle
Fluid moving through the tube form bottom to top causes a pressure drop across the float, which produces an upward force that causes the float to move up the tube. As this happens, the cross-sectional area between the tube walls and the float (the annulus) increases (hence the term variable-area)
Despite the growing popularity of other flowmeter types, the variable area flowmeter continues to command a major portion of the industrial market. Although it has not enjoyed the same degree of publicity granted more exotic meter forms in technical literature.