In almost all cases a code cargo which requires a high level
alarm and overflow control also requires a closed gauging device.
A cargo tank containing such a product therefore requires
three sensors:
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.1 level gauging;
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.2 high level alarm;
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.3 overflow control.
The sensing elements for .1, .2 and .3 should be separated
although sensors for .2 and .3 (reed switches, float chambers, electronic
devices, etc.) may be contained in the same tube.
Electronic, pneumatic, hydraulic circuits required for sensors
.1, .2 and .3 should be independent of each other such that a fault
on any one will not render either of the others inoperative.
Where processing units are used to give digital or visual
indication such as in a bridge space the independency of circuitry
should be maintained at least beyond this point. The power should
be supplied from distribution boards.
Where a control room or a bridge space containing a modular
unit is envisaged, separate level indication and visual alarms must
be provided for each of the functions .1, .2 or .3. An audible alarm
must also be provided but since this is not directional it need not
be separate. An audible alarm must also be arranged in the cargo area.
Where there is no control room an audible and visual alarm should
be arranged at the cargo control station.
Testing of sensors should be arranged from outside the tanks
although entry into product clean tanks is not precluded.
Simulation testing of electronic circuits or circuits which
are self-monitoring is acceptable.