Hydrometry - Float Operated Gauges

Float gauges provide a mechanical means to measure water height.

Here are the basic components:

Float
Line (Tape, or Wire)
Pulley (Float wheel)
Counterweight

Recorder

The line is attached to the float on one end and a counterweight on the other. It hangs on a pulley. The recorder keeps record of the pulley shaft rotation, which can be translated into water height. Recorders have traditionally been paper on a drum, but more are being converted to digital recorders that keep track of the pulley shaft rotation.

Typically, float gauges are installed in stilling wells, which help to produce stable measuring conditions.

There are several sources of error that must be considered when using the float operated gauges.

Systematic Effects:
1.) Float lag: The float tends to lag behind the true water level as the float rises and falls. This can be minimized by selecting an appropriate diameter float and sufficiently large float wheel.

2.) Intake pipe effects: The intake pipe (horizontal pipe in figure to the right) diameter can also introduce measurement error. A diameter that is too large can reduce the effectiveness of the stilling effect of the well by allowing water to surge in and out of the well. However, a too small diameter can introduce measurement lag because of head loss in the intake pipe.

3.) Chimney Effect: How far the intake pipe extends into the river cross section also impacts measurement due to chimney effect. This can produce a systematic underestimation of the water height.

Other Concerns:
4.) Counterweight: The counterweight location must be configured so the counterweight does not hit the float as it drops. Ideally, the counterweight is never submerged, as the effect of bouyancy on the counterweight will affect measurement.

5.) Tape/Wire: Kinks or twists in the line can cause measurement error. In addition, surges of water can potentially unseat the wire from the pulley.

Float Gauges in Stilling Wells
(Source: USGS)