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