UNIDATA
Unidata Manual 6293 - Precision Water Level Instruments Issue 2.1
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valve is used to reset the tank water level for another period of data.
An option can be provided to automatically drain the water from the tank
whenever it reaches a preset level.
6.6 Measuring Pan Evaporation
Pan evaporation is frequently measured as the total daily change in the water
level in a measuring pan. A manual observation is generally taken at the same
time each day. Associated readings of rainfall, water temperature and wind run
are usually also taken.
UNIDATA Model 6529 Version 2000 uses the Model 6541-31/C in a USGS
Class A evaporation pan to a resolution of 0.2mm.
The integrated Micrologger computes the accumulated evaporation and rainfall
for logging. It uses an additional card to control the water level in the
evaporation pan.
6.7 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 water level. A
floatwell creates a still water surface that moves with the major changes of the
water level, but not the minor ones.
Data accuracy and reliability will be improved if:
•
The instrument has a stable mounting. (Movement of the unit will appear as
water level change)
•
The instrument is protected and kept clean, dry and secure
•
The cable to the data logger is protected from damage.
6.8 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