Vaisala PWD22 User Manual page 37

Chapter 3________________________________________________________ Functional Description
VAISALA ________________________________________________________________________ 35
The alarm and warning limits are given in the configuration session. If
the alarm limit is exceeded, data is set to ///// and an alarm is
generated. The measured values are used only for warnings and
alarms. No compensation for the visibility signal is calculated.
Transmitter backscatter is measured by an analog circuit using the
transmitter LED as a light source. Its identifier is
TR.BACKSCATTER in the status message. TR.BACKSCATTER is
smaller for higher backscatter signals. Receiver backscatter is
measured with the signal receiver using an additional, controlled LED
as a transmitter. The result is in hertz. It is bigger, when more light is
scattered back (REC. BACKSCATTER).