Applications; Internal Monitoring; Built-In Tests - Vaisala PWD22 User Manual

User's Guide _______________________________________________________________________

Applications

Internal Monitoring

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PWD22 is presented in Appendix C, NWS and WMO Code Tables,
on page 117. The weather type is selected every 15 seconds using the
principles described earlier. Instant weather type will be selected from
the 15-second types after Weather delay number of samples by
weighted majority occurrences. Instant type is reported using both the
NWS codes and the WMO codes.
For the 15-minute and 1-hour WMO weather codes, the latest 1-hour
of instant precipitation types is stored in a buffer. The last 15-minute
and 1-hour parts of this table are then analyzed. The number of each
precipitation type in the buffer is counted.
The counts are stored in a table in a decreasing order (weighing) by
the code value (in other words, the highest SYNOP code number
first). This table is then analyzed from top down by adding the counts
together until a minimum sum is obtained. The code number
associated with the last added count is the 15-minute or 1-hour WMO
code. The minimum sum is different for the 15-minute code and the
1-hour code.
Obviously the (automatic) weather station that uses the PWD22 data
can in many cases also use other weather parameters to select from the
three PWD22 codes or even correct them. For example, humidity
measurement can be used to select between smoke, fog, mist, and
haze.
The 15-minute code might be a good selection for a general-purpose
report.

Built-in Tests

Extensive, built-in tests are included in the PWD22 operation. Various
voltages are measured and corresponding alarm and warning limits are
checked. Optical contamination of both the transmitter and the
receiver is continuously monitored by measuring the backscattered
light. For this purpose an additional transmitting LED is installed in
the receiver.