As fluid flows upward through the tube, a pressure differential develops across the plummet. This
pressure differential, acting on the effective area of the plummet body, develops an upward force
(F = P
A). If this force exceeds the weight of the plummet, the plummet moves up. As the plummet
moves farther up in the tapered tube, the area between the plummet and the tube walls (through
which the fluid must travel) grows larger. This increased flowing area allows the fluid to make it past
the plummet without having to accelerate as much, thereby developing less pressure drop across the
plummet’s body. At some point, the flowing area reaches a point where the pressure-induced force
on the plummet body exactly matches the weight of the plummet. This is the point in the tube
where the plummet stops moving, indicating flow rate by it position relative to a scale mounted (or
etched) on the outside of the tube