Simrad ER60 - REV B Operator's Manual page 36

Simrad ER60 Operator manual
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For the smaller spheres it may be necessary to add a weight to
keep the sphere stable. This is done via a second line attached to
the three suspension lines. The length of the line must be at least
two pulse lengths, so that the echo from the additional weight
does not interfere with the sphere echo. Immerse the sphere in a
solution of dishwashing detergent and freshwater and lift it
overboard by the fastened lines without touching it. The soap
helps to eliminate air bubbles attached to the sphere.
Lowering the sphere
Lower the sphere beneath the vessel to the desired depth, for
example 25 m, which is determined roughly by counting the
swivels on each line.
The required depth is mainly determined by the transducer beam
width and the vessel geometry. The physical width of the beam,
which increases linearly with range, should be sufficiently great
so that the sphere echo is unaffected by the small movements to
which it is inevitably subjected.
In most cases calibration will be performed at depths larger than
10 m. However, the chosen depth should be limited to ensure
that it is possible to cover the entire beam. The minimal depth
must also be convenient with respect to the vessel geometry. In
particular, if the suspension lines do not hang freely, then
control of the sphere may be hindered by friction or possible
obstructions on the hull. Despite the number and variety of these
considerations, it is seldom difficult in practice to find a suitable
depth, which satisfies all of the above criteria.
Reference target
Simrad supplies copper spheres designed as reference targets for
the calibration of scientific sounders. Copper is selected because
it is a metal which can be made electrolytically with high purity.
The spheres are machined to the perfect spherical form with
great accuracy, and a nylon loop is attached. Except for 12 kHz,
49 kHz and 50 kHz, the sphere diameter is different for each
frequency in order to obtain a target strength with minimum
dependence of temperature (K. Foote 1983). A curve showing
the variation of the target strength follows each sphere.
850-164692 / Rev.B