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JANUARY 2010 - TD 220a DEPLOYMENT GUIDE, RDCP600
1.5.6
Limitations for the surface cell
The surface cell is positioned so that the centre of the cell lies at the level of the surface. Usually
one would think that this would cause all sorts of problems, but initial tests have shown that for
some configurations the surface speed measurements do make sense.
At the water surface, reflections from the main lobe dominate reflections from the lower energy
side lobes; hence the measurements are most often valid. Best results are obtained in a fixed
bottom mooring since the instrument does not tilt.
However, care must be taken regarding surface cells. We have indications that at situations with
steep, rapid changing waves the surface cell holds inaccurate data. Pay special attention to the
direction measurement. Standard deviation noise is usually higher for surface current.
1.5.7
Measuring close to the Instrument
The instrument’s blanking distance and the pulse length limit measurements of current speed
and direction close to the instrument. The minimum start distance is defined as the blanking
distance plus half the pulse length.
The blanking distance depends in general on how fast the electronics and acoustic system settles
after the high power acoustic pulse is switched off.
In the RDCP 600 this settling time is controlled by the acoustic settling time of the transducer
housing.
The 300m-version housing is made of a material that will settle when the pulse has propagated
1m, while the deep-water housing is made of a material that settles when the pulse has
propagated 2m.
Since the position measured is defined as the centre of the pulse, the minimum start distance will
also increase with larger pulse lengths.
Potential problems arise when a surface referred column extends below the minimum start
distance, which could e.g. happen during low tide periods.
In these cases the RDCP 600 will set the influenced cells to zero speed and tag them as invalid.
NOTE! The cell position is taken as the position of the centre of the cell.
