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Aanderaa Data Instruments AS – TD302
Note! The tide calculations are always based on the latest samples in time, so even though you specify a longer
averaging time, e.g. 8 minutes averaging, the computed value for tide will be calculated from a running sample
set and a new value sent as an output message at the 1s to 255minute interval according to your settings for the
output interval.
The sensor measures the pressure at the installation
depth. The tide pressure is the average of the measured
hydrostatic pressure, refer Eq. 1-1:
atm
P
gh
P
+
=
ρ
Eq. 1-1
Where
P
is the measured averaged pressure
,
ρ
is the
water density
,
g
is the local acceleration due to gravity,
and
h
is the height of the water column.
The tide level above the sensor deployment depth is
calculated from Eq. 1-2:
0
h
g
P
P
h
atm
−
−
=
ρ
Eq. 1-2
Where h
0
is the Installation Depth
Note! P
atm
, and g are sensor properties that must be set by
the user prior to deployment. The water density,
ρ
, is
calculated from the measured water temperature and the
salinity
of the water. Pressure is set to 0. Salinity is set by
the user. If you can find precise values for the input
parameters at the deployment site, the sensor will
calculate the tide level more accurately.
If
P
atm
can be measured at the site the tide level can be calculated more accurately by means of post processing.
The user of an R-version sensor can update the property
P
atm
on request.
P
atm
will not be presented together
with the measured data.
1.4
Wave measurements for sensor 5218/5218R
The wave measurements are calculated independent from tide and you may choose from 256, 512, 1024 or
2048 samples performed at a default rate of 2 Hz or a higher sample rate of 4 Hz (both settings are user
configurable). The pressure time series used to calculate wave statistics are therefore measured over a time
period of 64 seconds to 17 minutes (measuring time is the product of sample set size and sample frequency).
The wave parameters are calculated according to the linear wave theory, refer Appendix 4.
1
Equation of state 1980 UNESCO polynomial implementation. Density calculated with pressure = 0.
2
IEEE Journal of Oceanic Engineering, Vol. OE-5, No. 1, January 1980, page 14
Figure 1-5 Illustration of the tide measurement
situation.
