Vaisala FD12P - Manual (Page 95)

Chapter 5 _______________________________________________________ Functional Description
VAISALA ________________________________________________________________________ 95
CHAPTER 5
FUNCTIONAL DESCRIPTION
This chapter gives a functional description on the product.
General
The FD12P Weather Sensor is an optical sensor that measures
visibility (meteorological optical range, MOR, and precipitation
intensity and type. The FD12P measures visibility using the forward
scatter measurement principle. Light scatters from particles whose
diameter is in the order of magnitude of the light wavelength. The
amount of scatter is proportional to the attenuation of the light beam.
Larger particles behave as reflectors and refractors and their effect on
the MOR must be handled separately. Usually, these particles are
precipitation droplets. The FD12P optical arrangement allows for
individual droplets to be detected from rapid signal changes. The
FD12P software calculates the precipitation intensity by analyzing the
amplitudes of these changes. This intensity estimate is proportional to
the volume of the precipitation droplets.
The optical signal also contains some information about the
precipitation type but not enough for reliable identification. Additional
information is needed, especially in conditions where the precipitation
is very light or the weather is windy. As the extra parameter, the
FD12P measures an estimate of the water content of precipitation with
the DRD12 rain detector. In rain, the water equivalent and the volume
are equal. However, in snow the optical volume estimate is about ten
times larger. This difference of approximately one decade is used to
discern between rain and snow.