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The system designed to monitor sea level consists of a
combination of a bottom pressure sensor (the tide gauge itself)
and a surface barometer. The combined use of these two pressure
measurements, altogether with the knowledge of the density
profile of the water column above the tide gauge, allows to
infer the water level above the bottom pressure sensor. It will
be shown that for shallow waters, the accuracy will mainly
depend on the accuracy of the pressure sensors.

The used bottom pressure gauge is a WLR7 Aanderaa, which has a
quartz crystal sensor whose oscillating frequency is a function
of the absolute pressure exerted on it. The range of frequency
variations of the sensor determines the range of depths where
the instrument may be used. Since the bottom pressure sensor
measures not only the pressure exerted by the water column
above, but also the atmospheric pressure, the later must be
measured and conveniently subtracted. The barometer used to
measure the atmospheric pressure is an Aanderaa 2810 model
giving the pressure difference between the two sides of a thin
membrane that is exposed to the atmospheric pressure on one side
and to a reference vacuum on the other. Finally, the tide gauge
also has temperature and conductivity sensors. The first is
included in all models, since temperature is required to
calibrate the pressure measurements. Instead, the conductivity
cell is attached as an optional sensor. Pressure measurements
are finally transformed into water level by using the well-known
hydrostatic relationship g P h r = (1)

Where h is the height above the pressure sensor, P is the
pressure difference measured by the gauge sensor and he
barometer, g is the gravity acceleration and r is the density of
the water column above the pressure sensor. In this simplified
version of the hydrostatic equation density is assumed as
constant along the water column above the instrument, a
reasonable assumption for a few meters depths. The time
variation of density is obtained by using the values of
temperature and conductivity provided by the WRL7 sensors. The
complementary approach regarding density measurements would be
to obtain periodical CTD profiles at the instrument site. In
that case, the water column would not be taken as constant with
height, but the time variations would have to be interpolated in
between the CTD measurements. This approach would be more
appropriate for gauges deployed at open sea, at depths of the
order of tens of meters.

[Summary provided by the Institut Mediterrani d' Estudis Avancats]