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0688-SOM-00001, Rev: 02
© Tritech International Ltd.
5.4 Factors that affect measurement
Water Density
A bathymetric sensor measures seawater pressure. In order to calculate depth it must know the density
throughout the water column from the surface to the operating depth.
This value will change as the sensor moves through the water column, because varying temperatures and salinity
will affect the overall density.
Atmospheric Pressure
The pressure measured includes the atmospheric pressure imposed on the surface of the water. A bathymetric
system must know this pressure and subtract it from the total pressure measured (1mbar change is equal to
1cm in the depth measurement). For sudden changes of barometric pressure, for instance when a weather
front moves across, there may be a significant lag (up to 1 hour) in the transmittal of the effect to the pressure
measurements.
The height of barometric measurement above sea level may need to be accounted for if this is significant (10m
air height is equivalent to 1cm difference in depth reading).
Note
Tritech International Ltd can supply barometric sensors as an option. These are
accurate to 1mbar. If the option is not used then the barometric pressure must
be entered into the system manually.
Tidal Movement
Tidal movement does not directly affect the depth measurement from the surface, but is needed to establish the
datum for a survey. Surveyors will need to correct the depth measurements to mean seawater levels.
Local Gravity
Gravity varies with position on the Earth’s surface. To a good approximation it varies as a function of latitude and
can be calculated using known geodetic formulae.
Instrument Accuracy and Stability
The different sensors housed within the SeaKing 700 series units have different levels of accuracy. All the sensors
are fully calibrated when the unit is shipped and fit within the following parameters:
Pressure
Specified as having a repeatability of 0.01% and hysteresis of 0.015% of full range, these figures represent one
standard deviation and are cumulative such that the statistical variability of the calibrated pressure sensor will
result in a depth value uncertainty of 0.015%.
Density
Applicable to SeaKing 704 units only, the conductivity and temperature sensor are used to calculate density
local to the instrument using standard oceanographic equations. The values calculated are valid only at the time
of descent and if water condition change (e.g. varying temperature or salinity) the value becomes historic and
may induce errors. If desired the calculated value can be overwritten with a manually entered value. Generally
for calculated values it is expected that density variations could double the uncertainty of a depth measurement
to 0.03% of full scale.