Campbell Scientific 0871LH1 Freezing Rain Sensor - Manual (Page 23)

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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 encounter. 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 0.25mm (0.01”) 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 0.25mm 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.