Campbell CS120A, Инструкция по эксплуатации

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Instruction Manual
3
1.3 Principle of operation
Figure 1-1. Particles in the sample volume scatter light in all directions,
including into the detector
The CS120A and CS125 comprise an emitter and detector aligned as in
Figure 1-1. The emitter produces a beam of near infra-red light pulsed at 1 kHz. A
detector has a field of view which overlaps the beam and is inclined at 42 degrees
to it. Light scattered by a particle (for example a fog droplet or particle of
precipitation) from the overlap or sample volume towards the detector is detected
by a photodiode and recorded as a signal. The size of the signal is therefore
proportional to the extinction of the emitted beam caused by scattering. The
scattering signal averaged over one second is used to calculate an extinction
coefficient or EXCO. Sixty one second averages are then themselves averaged to
give a one minute average EXCO. This is then converted to a value of
Meteorological Optical Range (MOR) using Koschmieder’s law:
MOR = 3/EXCO where MOR is in km and EXCO in units of km
-1
.
The CS125 is capable of identifying weather type in addition to measuring
visibility. It does this by analysing the amplitude and width of spikes in the APD
signal corresponding to particles of precipitation passing through the sample
volume. The amplitude of the signal is a guide to the size of the particle and the
width, because it represents the time taken for the particle to fall through the
sample volume, is proportional to the fall speed, see Figure 1-2.