Precision Water Level Instrument
Unidata Manual 6293 - Precision Water Level Instruments Issue 2.1
39
•
Instrument lag.
•
Temperature change.
These causes of error are discussed below:
Other minor causes relate to water density and the movements of supporting
structures and the expansion of such structures resulting from changes in
temperature and water content.
10.3.1
Float Line Shift
With every change in water level, a portion of the float line passes from one side
of the float pulley to the other side. The
weight of the line
changing from one side
of the pulley to the other affects the depth of flotation of the float. This causes an
error in the registered level.
The magnitude of this error varies with the amount of line shifted (i.e. the amount
the level has changed since the recorder was set to a known reference), the weight
of the line and the size of the float.
Typical float line weights are:
125mm beaded float line = 6.0 grams/metre
0.4mm float line
= 1.2 grams/metre
For levels rising above a given reference (starting point) this error will make the
level measure slightly high, for a level falling below the reference point the error
makes the level appear low.
The amount of error may be calculated using
Formula 1
on the following page.
The error is inversely proportional to the square of the float diameter. That is, the
larger the float, the smaller the error.
The error is slightly different for a float line submerged in water (as in the case of
a counterweight being below the float).
Formula 2
on the following page should
be used for submerged float line shift corrections.
10.3.2
Submergence of Counterweight
When the counterweight (and any portion of the float line) becomes submerged,
the pull on the float is reduced and its depth of flotation is reduced.
In this case the level will measure slightly lower than it actually is. The amount
of this error may be calculated from
Formula 3
(on the following page).
10.3.3 Instrument
Lag
A certain amount of force is required to move the shaft mechanism of the level
measuring instrument and also to bend the float line over the float pulley. This
force must be supplied by the pressure of the water on the float.
