Vaisala FD12P - Manual (Page 116)

User's Guide _______________________________________________________________________
116 _____________________________________________________ M210296en-A
Contamination Monitoring
The FD12P monitors both the transmitter and receiver contamination
by measuring the backscattered signal. The CLEAN command is used
to set the clean reference values of the backscatter signals. The
deviation of the backscatter signal from the clean values is
proportional to the contamination on the lenses.
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 TRB in the status
message. The TRB 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).