2.1 STILLING WELL
GAUGES
On a global
basis, probably the most common type of tide gauge consists of a stilling well
containing a
counterbalanced float. This technology was discussed extensively in Volume I of
this Manual.
In this system
the sea level is measured by determining the length of the float wire relative
to a
level fixed to
the bench marks. The advantages of this system are that it is relatively simple
to
install and
operate, and that it provides a very direct measurement of the sea level. The
stilling well
is not left completely open to the sea, but is vented via a small hole near the
bottom of the
well. This method of allowing water to enter the well provides a mechanical
filtering of
high frequency (notably surface gravity wave) variability. This venting method
does, however,
introduce some dynamical errors and other problems, but careful design and
operation of
these systems can partially deal with these effects (Shih & Baer 1991).
A
"perfect" sea level gauge has not yet been achieved and all existing
measurement
techniques can
suffer from errors or instabilities which need to be carefully understood and
controlled when
the highest accuracy measurements are needed.
2.2 DATUM PROBES
The function of
a datum probe or switch is to identify the time when the sea level is at a
known
fixed level
relative to the tide gauge datum. By relating this time to the tide gauge
record any
datum error or
offset in the record can be identified. With a chart recorder a mark can be
made
automatically on the chart at the time the datum switch operates, indicating
when the
sea level rises
or falls past the fixed switch level. At these times the recorded tidal level
should be the
level of the datum switch. If it is not then the tide gauge or recorder is in
error
and an offset
can be applied to the record to compensate for this error. Where a data logging
system is used
the times of switching can be recorded in the data logger for subsequent
interpolation
of the recorded levels to the times of switching.
Ideally the
datum switch should be mounted at about mid-tidal level outside a stilling
well.
However direct
exposure to waves and swell make it impractical to deal with the large
number
of switchings and it is usually necessary to protect the switch within its own
small
stilling
enclosure. Careful design of this enclosure should minimise any errors
introduced,
particularly
if use is only made of sea level crossings in calm conditions giving rise to
only
single
switchings. An acoustic type of switch has been found to be robust, reliable
and to
have
small hysteresis (e.g. Bestobell Mobrey, Appendix 1).