Functional Description - Vaisala PWD22/52 User Manual

Chapter 3 ______________________________________________________ Functional Description
CHAPTER 3

FUNCTIONAL DESCRIPTION

VAISALA _______________________________________________________________________ 19
This chapter describes the functionality of the product.
Vaisala Present Weather Detector PWD22/52 is an optical sensor that
measures visibility (meteorological optical range, MOR), precipitation
intensity, and precipitation type. The detector measures visibility using
the principle of forward scatter measurement. Light scatters from
particles whose diameter is in the order of magnitude of the wavelength
of the light. 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 optical arrangement of PWD22/52 is such that
individual droplets can be detected from rapid signal changes. The
PWD22/52 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 sufficiently for reliable identification. Additional
information is needed, especially in conditions such as very light
precipitation or windy weather. As the extra parameter, PWD22/52
measures an estimate of the water content of precipitation with Vaisala
®
RAINCAP Rain Sensor. In rain, the water equivalent and the volume
are equal whereas in snow the optical volume estimate is about ten times
larger. This difference of approximately one decade is used to distinguish
between rain and snow.