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to the wet sides of the bore casing if it touches. This can cause large errors in
small boreholes and will happen if the boreholes have not been drilled vertically
or straight.
6.4
Floatwell Design for Water Level Measurement
A floatwell or stilling well is required at any site where a 6541 instrument is
used. At many sites floatwells are simple devices that can be constructed from
lightweight materials such as PVC pipe. On larger rivers constructing new
floatwells can be uneconomic and alternative instruments such as UNIDATA
Pressure transducers should be considered.
The water surface in a natural channel frequently surges and swirls with the
velocity and is disturbed by waves and ripples. These cause the float to move
and bounce around and this affects the accuracy of the logged waterlevel. A
floatwell creates a still water surface that moves with the major changes of the
waterlevel, but not the minor ones.
Data accuracy and reliability will be improved if:
There is a stable mounting for the instrument. Movement of the 6541 will
•
appear as a water level change
The instrument is protected and kept dry, clean and secure,
•
The cable to the data logger should be protected from damage.
•
6.5
Measuring Rainfall
Float instruments producing graphical charts were historically used for rainfall
measurement. During the last 30 years these were superseded by tipping bucket
rain gauges that produced high resolution digital data. These instruments also
introduced calibration drifts and reliability uncertainties not experienced with
float instruments.
The Model 6541 instrument has the precision to measure rainfall. Fitted with a
100mm circumference pulley the 6541 will measure water level to a resolution
of 0.2mm. An imperial version fitted with a 5" circumference pulley will
measure in points (1/100"). These instruments can be used as a simple
pluviograph to produce accurate and reliable rainfall data.
The rainfall funnel is connected to a tank with a known diameter. A float on the
water surface is connected to the instrument. Rainfall accumulates in the tank
and evaporation is prevented by a layer of light oil on the water surface. The
instrument scaling in the scheme converts changes in water level in the tank to
rainfall in millimetres.
The tank storage capacity should be sufficient not to be exceeded between visits
by servicing staff. A sight tube allows the total rainfall to be read and a drain
Unidata Manual 6293 - Precision Water Level Instruments Issue 2.1
Precision Water Level Instrument
25
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