Copyright 2013 Optical Sensors
MiniOFS - a low price fog sensor

The performance of the MiniOFS is not as good as for the OFS or some other visibility sensors on the market. But it will work fine for road saftety and a lot of other applications. It is well known that fog causes many accidents in road traffic and elsewhere. You can read about how MiniOFS can reduce serial collisions in fog.
Several of our customers however, are researchers who for different reasons need information about visibility.
The MiniOFS is probably the cheapest visibility sensor on the market.
We can also quote modified versions for customer's needs like polled data output.
The outer dimensions of the sensor are as small as 68*45*34 mm
The sensor is sensitive for particles in a zone about 25 cm ahead of the location of the sensor who  limit the visibility in the air. These particles are normally the microscopic water particles constituting fog, but they may also be snowflakes, raindrops or air pollutants. But the most indications of low visibility are because of fog or snow.
More information about different optical techniques for measuring visibility etc is available on Measuring visibility.

The ambient solar light is also measured and displayed.
The sensor is heated to a few degrees above ambient temperature in order to keep moisture away.
In order to keep the electronics dry a there is membrane ventilator that keeps the pressure inside at the same level as outside. This prevents liquid water from sucking into the sensor through micro cracks etc at falling temperature - a creative solution to a big problem.

The sensor produces analog and digital signals:
About the analog output:
The microprocessor controls the analog output giving the visibility directly (VIS =1 km gives 1 Volt, and VIS = 500 meters gives 0.5 Volt etc, up to 4000 meters). If the optical receiver is saturated by for instance sunlight or reflections from an object in the sensitive zone like fallen snow covering the front the output will be about 5 Volt.
About the RS232 output:
The calculated data is presented in digital form as an ASCII string on the RS232 output, 1200 baud 8N1, that is transmitted "streaming" every 30 seconds. This string can be received by many loggers with RS232 inputs but also by a PC via the serial port and with a terminal program like Hyper Terminal (part of WINDOWS).
This is an example of an output string:
amb= 100 alfa=0.0012 VIS= 2500
The first figure - the amb parameter is proportional to the measured ambient solar light level, the measurement is not very accurate but it may be useful for instance to tell if it is sunlight or cloudy, and day or night.
The second figure is the so called extinction coefficent often called alfa. The relation between the extinction and visibility is alfa=3/VIS. The alfa parameter can be used for monitoring trends in visibility when the visibility is larger than 4000 meters. But note that the absolute accuracy in alfa is limited in such cases.
The third figure - the VIS parameter - is the measured visibility in meters up to 4000 meters.
If the optical receiver is saturated by for instance sunlight or reflections from an object in the sensitive zone like an insect or fallen snow covering the front the visibility displayed visibility value will be 5000 and the output can be:
amb=050 alfa=0.0000 VIS= 5000.

Some short data of the MiniOFS sensor:

Dimensions:
68*45*34 mm
Weight:
about 170 grams
Warm up time:
60 seconds ( second data display from start )
Current consumption:
<100 mA from a 12 Volt (8-14) supply
Outputs:
analog 0-5 Volt and digital RS232
Update time:
30 seconds
Temp. range:
-20 to +50 deg C
Optical output power:
About 3 mW from an IR LED, eye safe laser, safety class 3R
Wavelength:
850nm
Housing:
Anodized aluminum, openings sealed with O-rings
Visibility range:
Visibilities from 4 km down to 20 meters
about 1 minute
Mounting
The sensor should be mounted so that it "looks" horizontal and roughly north (on the southern hemisphere south). There must not be anything in the sightline closer than 5 to 10 meters. Outside a cone of about 30 degrees angle objects can be tolerated at down to 2 about meter.
There are four M3 holes on a 14*51 mm rectangle in the top surface -see figure below - which can be used for the mounting.
We offer a PMMA plastic mounting bracket, see picture above.
One advantage with this bracket is that it isolates the sensor electrically which reduces the risk for surge currents.
The sensor is delivered with a 6 meter 5-wire cable connected.