Campbell Scientific PWS100 Present Weather Sensor - Manual (Page 98)

Section 8. Functional Description
possible to measure the immediate surroundings, selecting appropriate
parameters, which can be related to the environmental air quality and human
visual perception.
The PWS100 has the ability to define an obscurant type and determine a
visibility value based on the amount of particle scatter calibrated against the
type of particles in the detection volume. The size distribution can also be used
to calibrate the visibility value if such a distribution exists in the present
weather event (i.e., available when the obscurant is drizzle, snow or some other
large particle but not when fog or mist as particle sizes are too small to enable
the distribution to be determined). When the obscurant consists of fog, mist or
some other sub-drizzle sized particles then no visibility range correction is
required.
The ±10% accuracy range of the visibility output from the PWS100 is from 0
to 10,000 m with a 20,000 m cap on the total range. These accuracy figures are
quoted for fog/rain conditions. Accuracy will be lower in conditions of
freezing precipitation and other conditions such as dust.
As with any instrument sampling obscurants over a small portion of the range
given (including transmissometers, forward scatter meters and backscatter
meters) the output is only accurate if the scattering medium is uniformly dense
over that given range. Some time averaging may lead to better agreement with
a human observer during inconsistent events and may avoid highly variable
output not consistent with overall events. In order to reduce noise levels the
visibility measurements (raw voltage readings) are taken for 9 out of 10
seconds of time measurement interval and then averaged. Visibility range (in
meters) is then processed in the statistical output over the required period.
Any accuracy figures quoted by any manufacturer of automated visibility
sensors will consider only uniform events over the range given, even then
differences in obscurant particle observed (including but not limited to particle
density, surface roughness and optical scatter mechanism) can lead to ± 20%
errors (UK Met. Office studies). Only by determining particle type accurately
and having the added information of particle size distribution can these errors
during uniform events be minimised. The PWS100 is capable of determining
this extra information and therefore giving the most accurate visibility
estimates in uniform events.
Currently the PWS100 implements two separate calibrations for fog and rain
events and interpolates between them depending on the rainfall intensity. In the
future other calibrations will be added to give increase accuracy in other types
of events.
8.7 Applications
Because of the amount of information available from the sensor it is capable of
giving detailed analysis of weather conditions suitable for meteorological,
aeronautical, agricultural and transportation applications. Measurements of
visibility are applicable for aviation or roadside weather monitoring. Drop size
and velocity distributions can be used in the analysis of soil erosion, flood
prediction or as a calibration for radar instruments in meteorological studies.
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