Campbell Scientific PWS100 Present Weather Sensor - Manual (Page 87)

Section 8. Functional Description
8.1 General
The PWS100 Present Weather Sensor is an optical sensor using the best of
scatter meter and disdrometer techniques to give accurate analysis of weather
conditions including precipitation classification, precipitation intensity, drop
size distributions and visibility range.
Visibility is measured using the forward scatter technique, which gives an
estimation of the meteorological optical range (MOR). The amount of scatter
detected at the sensor heads is proportional to the visibility for small fog
particles. For larger particles, which are detected as various forms of
precipitation, the PWS100 is able to give estimates of visibility using its
particle discrimination as a calibration factor.
Precipitation is detected using an optical technique, originally developed as a
method for the measurement of the size and velocity of fine particles in sprays,
which has been modified to make it applicable to the analysis of precipitation.
Individual precipitation particles are analyzed using the optical arrangement
described Section 8.2, Optical Measurement. As well as the size and velocity
measurements, the instrument also looks at the structure of the signal to
provide further parameters for particle classification. From the measurements,
the volume of particles (or water equivalent volume in the case of snow) is
calculated which gives a measurement of precipitation intensity. Drop size
distributions can be determined from data collected over set periods.
8.2 Optical Measurement
8.2.1 Optical Arrangement
The optical design is such that the system outputs a number of evenly spaced
horizontal light sheets that are parallel to each other in order to define a
structured volume that is consistent. The two sensors are at angles of 20
° to the
optic axis of the light sheet generator (laser unit), one deviated through 20 ° in
the horizontal plane, the other 20 ° in the vertical plane. This defines a good-
sized volume in which to detect particles of all sizes and provide accurate
statistical analysis of events. With the PWS100 optical arrangement good
signal strength from all types of particle (e.g., drizzle, rain, snow, sleet, hail) is
observed.
The laser unit, as shown in Figure 8-1, comprises a laser diode and a number of
optical elements to produce the light sheet pattern as shown in Figure 8-2,
which extends some distance while maintaining a parallel sheet distribution.
Over the volume length there is negligible distortion to the light sheets.
8-1