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sensing probe. The added mass of accreted ice causes the
frequency of the sensing probe to decrease in accordance with the
laws of classical mechanics. A 0.020” (0.5 mm) thickness of ice
on the probe causes the operating frequency of the probe to
decrease by approximately 130 Hz. Freezing Rain Sensor
software monitors probe frequency, and detects and annunciates
this frequency decrease. At the same time, the internal probe
heater power is applied until the frequency rises to a
predetermined set point plus an additional delay factor to assure
complete de-icing.
Once de-iced, the sensing probe cools within a few seconds and
is ready to sense ice formation again. When ice forms on the
sensing probe again to the point where the MSO frequency
decreases by 130 Hz, the sensor de-ices itself again. This cyclic
process is repeated as long as the freezing rain sensor remains in
an icing environment. The ice signal activates at 0.020” ice
accretion and stays on for 60 seconds after the end of the icing
encounter. Specifically, when the output is activated, a 60-second
timer is started. Each time 0.020” forms on the probe, the 60-
second counter is reset. In effect, the output stays on for 60
seconds after the beginning of the “last” icing encounter.
The Status output indicates whether the freezing rain sensor is
functioning correctly using tests that are described in more detail
in following sections of this document.
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