UNIDATA
Unidata Manual 6293 - Precision Water Level Instruments Issue 2.1
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10 APPENDIX C - FLOAT SYSTEMS
10.1 Introduction to Float Systems
Float systems used for monitoring water level usually consist of a sealed float
connected to a floatline that passes over a pulley on a measuring instrument. A
counterweight is connected to the end of the floatline to maintain tension. The
pulley over which the floatline passes is connected to a shaft. As the water level
changes, the float moves up and down rotating the
shaft of the instrument. The instrument records the
water level.
Float systems are subject to a range of errors.
These are explained in the following sections.
Floats above 150mm in diameter give good results
(± 1 mm) for small ranges of water level. To
accurately measure larger ranges of water level,
floats of 200mm are required.
It is important that you select a float system that can deliver the level of accuracy
required.
10.2 Sources of Error in Float Systems
All float systems are subject to a range of minor errors. The errors involved in the
recordings made by a properly designed float-operated level recorder are
generally too small to be of great importance. However, the user should be aware
of potential sources of error and how to compensate for them. These errors can
become significant at sites where a large measuring range is proposed.
Significant source of error can occur at sites such as reservoirs and bores where
large variations in waterlevel are recorded. When the float movement exceeds 10
metres, special components or designs may be required. This is because the net
weight of the floatline transfers from one side of the pulley to the other. In
extreme cases the float may be lifted from the water by the weight of the line on
the opposite side of the pulley and
run away
.
The endless floatline is an alternative that eliminates the transfer of weight from
one side of the float to the other, and the errors so induced. It also solves the
problem of line “run away”.
10.3 Causes of Error
The main causes of error in float line level measurement are:
•
Float line shift.
•
Submergence of counterweight.
