Everything else in a navigation aid system depends on knowing the visibility. The fog signal
should sound when visibility falls below a threshold and not otherwise; lantern intensity is
often stepped by the same measurement. Leaving that to a person is unreliable and, on an
unmanned structure, impossible - which is why the visibility sensor is the quiet centre of a
navaid installation rather than an accessory to it.
The MVS20-Ex uses a compact forward scatter design. Rather than measuring how much light
survives a long path, it shines a beam across a short gap and measures how much is scattered
sideways by whatever is in the air. Scatter increases with the density of droplets or
particles, so the sensor infers visibility from it. The advantages are that the instrument can
be small and self-contained, and that it responds to what is actually in the air at that
location rather than needing a distant target. MSM quote a selectable measurement range up to
8 km.
It measures obstruction to vision from dust, fog, rain, drizzle, snow and hail, which is
worth reading carefully: this is not a fog detector in the narrow sense but a general
visibility instrument, and on an industrial site dust and smoke can reduce visibility as
effectively as weather. MSM note it is suitable for petrochemical production sites, refining
facilities and transportation terminals as well as marine structures.
Two design details separate an instrument that works from one that appears to. The optical
sensor hood is heated, for use in extreme environments - without heating, condensation, frost
or salt film on the optics produces exactly the reading a genuine fog would, and the system
sounds its horn on a clear day. And the sensor is specified as not affected by local lights,
which matters enormously on a platform or a terminal where floodlighting, flares and deck
lights are all in view. There is also comprehensive self-test and maintenance data, so a
degraded sensor reports itself rather than quietly misinforming everything downstream.
The housing is rated for Zone 1 with the marking II 2GD Ex d IIB T5/T6 - note the GD, which
covers dust atmospheres as well as gas - and marine grade sheathed cables maintain IP66
performance in all weather. It is designed according to IALA Recommendation G-1162, and MSM
emphasise flexibility to connect to a wide range of data collection and processing units, so it
can feed an existing control panel rather than requiring theirs.
