Campbell CS125 Present Weather and Visibility Sensor

Visibility is the other limiting weather parameter for flight operations, and on a marine or industrial site it is also an operational one - loading, crane work and vessel movements all have visibility limits. Reporting it by eye is unreliable and, at night or on an unmanned structure, impossible. That is what a forward-scatter sensor replaces.

The CS125 measures visibility by shining infrared light across a short path and measuring how much is scattered forward at a 42 degree angle. Campbell are specific that this angle gives accurate estimates of Meteorological Optical Range for fog and snow, and the choice of angle is the substance of the design: scattering behaviour varies with particle size, and 42 degrees is where fog droplets and snow crystals both produce a signal that maps reliably onto M.O.R.

What lifts it above a plain visibility meter is present weather identification. The sensor identifies precipitation particles from their scattering properties and their fall speeds, and combines that with a temperature measurement to determine what the weather actually is. Fall speed distinguishes drizzle from rain; temperature distinguishes rain from snow at the margin. From that it reports 57 SYNOP present weather codes with the associated METAR and NWS codes, plus hail detection and past weather codes - so the output is an aviation and meteorological report rather than a number needing interpretation.

The heating arrangement deserves attention because it is where cheaper sensors fail. The CS125 has low-power dew prevention heaters and, as standard, higher-power anti-icing heaters for the hoods. Both are automatically controlled to ensure operation in all weather, and either can be disabled individually to save power. That last point matters on a solar or battery-constrained installation, where anti-icing may be pointless in the tropics and turning it off recovers a meaningful part of the power budget.

Self-monitoring is the feature that makes an unattended installation trustworthy. The sensor continuously monitors its own status and reports internal faults and contamination or blockage of its lenses - which is the failure that would otherwise be indistinguishable from genuine fog. It also provides two user-configurable alarm outputs able to drive audible or visual alarms through solid-state relays, so a visibility threshold can trigger something directly without a controller in between. Campbell note the CS125 is certified by Deutscher Wetterdienst as suitable for controlling wind turbine obstruction light systems under the German administrative rules for identifying aircraft obstructions - an independent certification for exactly that automatic control duty. It ships with a 5 m cable and DB9 connector and mounts on a vertical pole of 32 to 52.5 mm diameter.