Simrad EY500, Instruction Manual

Page: 210 / 262

Calibration
8
P2260/E
The purpose of the swivels is threefold:
- to unravel rotation of the nylon line
- to mark distances on the line
- to add weight so that the line sinks in water
Attaching the sphere
Prior to commencing the sphere measurements, a rope should be drawn
beneath the hull from the first winch to the second and third winch before
anchoring. Use this rope to pull the line from the first winch beneath the hull
to the side with the second and third winches.
Attach the appropriate sphere, with affixed loop, to the three suspension
lines, refer to figure 1. 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 distance, for example
25 m, which is determined roughly by counting the swivels on each line.
In general, one should use sphere distances of 15 m or more for 38 kHz or
higher frequencies. This in order to reduce the effect of pulse rise time and
resolution in distance measurements on the calibration results. Software
version 5.30 has corrected for these effects on the TS and s
A
calibration.
Two further considerations in choosing the range are the transducer
beamwidth and 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, perhaps pendular movements to which it is
inevitably subjected. The minimal range 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 consider-
ations, it is seldom difficult in practice to find a suitable range which satisfies
all of the above criteria.