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Resets due to unknown reasons (such as reset from the watchdog
timer) are detected.
Initiated BIT will examine the RESET EEPROM input. If the
input is active, the STATUS output will be set to FAIL and the
ICE output and probe heater will be disabled. (This feature
allows a factory technician to perform the MSO capacitor
selection process without activation of the probe heater.)
Activation of the Press-to-Test (PTT) input for greater than 100
ms also causes the ice detector to perform Initiated BIT. The PTT
input is ignored when the ice output is actibe. After PTT is
completed, the correlation count is restored to its pre-test value.
Initiated BIT is complete within 3 ± seconds of initial power up.
8
Correlation Counting
The freezing rain sensor tracks the amount of ice accumulation
on the probe during an icing enoucnter. The correlation count is a
value tracked by the freezing rain sensor that indicates the
amount of ice that has accumulated on the probe during the icing
encounter. Each correlation count equals 0.010 inches of ice.
The correlation count, ranging from 0 to 255, indicates the
number of times the MSO frequency decreases by 65 Hz during
an icing encounter. A decrease in frequency of 65 Hz correlates
to an equivalent 0.25 mm of ice that would have formed on the
ice detector probe, neglecting the change in collection efficiency
caused by ice build-up. Upon reaching a correlation count of 255,
the value is no longer incremented.
The freezing rain sensor compensates by adding a value (ranging
from 0 to 6) to the correlation count when the ice detection cycle
is completed, to account for the ice that would have accumulated
if the heater had not been on.
The correlation count is in the serial string, Table 3. Serial String
Format.
The correlation count is initialized to zero at unit power up.