User`s guide

Chapter 2 __________________________________________________________ Product Overview
VAISALA_______________________________________________________________________ 69
sensor measures the moisture content of the dowel by its electrical
capacitance.
A thermistor, located in the dowel where it fastens to the base, measures
the temperature of the dowel giving the estimated temperature on the
forest floor.
Visibility Sensors
0407-023
Figure 64 Visibility Sensor PWD10/PWD20
Vaisala Visibility Sensor PWD10 and PWD20 are optical sensors that
measure visibility (meteorological optical range, MOR). The sensor
measures visibility using the principle of a forward scatter
measurement.
With a measurement range of from 10 to 2000 meters, the Vaisala
Visibility Sensor PWD10 offers economical and reliable visibility
measurement for weather applications.
The PWD10/20 output is either a digital serial interface or an analog
current signal. The digital serial interface can be configured into two
different operating modes: the sensor can be set to send a data message
automatically at selected intervals, or PWD10/20 can be polled by the
host computer. The same serial line is also used as an operator interface.
The analog current signal can be used to report the prevailing visibility.
Three relay controls can be used, driven by visibility limits.
The operator controls and checks the operation of PWD10/20 using a
maintenance terminal. A set of built-in commands and test routines are