8. Variable-Area Meters

In variable-area meters, the water flows vertically upward in a conically tapered tube in which area increases with height. The rate of flow is indicated by the height at which a shaped weight attains stable support from the flow in the tapered tube. Smaller versions are commonly called rotameters because many of the weights are sometimes vaned to rotate for stability in the tapered tube. The weights appear to float freely in the tapered tube. Thus, they are often called floats. When the tube is transparent, the position of the float can be observed directly against graduations on the tube.

Larger meters sometimes have a stem attached to the float, which is linked mechanically or magnetically to an indicator. Auxiliary recording and transmitting of discharge and totalized volume are sometimes incorporated into these meters.

These meters generally have few moving parts to wear or otherwise cause trouble, and the accuracy of the meters can be high. The head losses may be large. These meters have to be installed in a vertical position, making pipe fitting more difficult.

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