Chapter 3 ______________________________________________________ Functional Description
VAISALA _______________________________________________________________________ 29
Monitoring Contamination
PWD22/52 monitors both transmitter and receiver contamination by
measuring the backscattered signal.
The CLEAN command sets the clean reference values of the backscatter
signals. 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 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).
Data Message Formats
PWD22/52 is a fully automatic instrument for continuous weather
measurement. Usually, PWD22/52 is set to either send data messages
automatically or be polled by a host computer.
In addition, a set of user commands is provided for configuring and
monitoring system performance. These commands are given in command
mode (see section Entering and Exiting Command Mode on page 51).
PWD22/52 has seven standard message formats for data message output.
PWD22/52 presents the weather type using:
- World Meteorological Organization (WMO) code table 4680
(WaWa, Present Weather reported from an automatic weather
station).
- WMO code table 4678 (METAR) format.
- United States National Weather Service (NWS) abbreviations.
Appendix C, NWS and WMO Code Tables, on page 113 lists the NWS
list and WMO codes.
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