Campbell Scientific 0871LH1 Freezing Rain Sensor - Manual (Page 5)

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1 Purpose
This document provides detailed information about the Rosemount
Aerospace model 0871LH1 Freezing Rain Sensor for use in ground-based
meteorological applications. Topics covered include requirements,
qualification categories and methodology, and detailed design
information.
2 General
The Rosemount Aerospace 0871LH1 Freezing Rain Sensor is a one-piece
unit that detects the presence of icing condition. Twenty-four volts DC
input power is provided to the freezing rain sensor. The freezing rain
sensor outputs include ice detection indication and fault status
indication. These outputs are provided through an RS-422 interface and
discrete outputs. One freezing rain sensor is used on each station and
provides the primary means of ice detection. The ice signal is used to
indicate to the operator that an icing condition exists so that appropriate
actions can be taken.
3 Detailed Principle of Operation
The freezing rain sensor uses an ultrasonically axially vibrating probe to
detect the presence of icing conditions. The sensing probe is a nickel
alloy tube mounted in the strut at its midpoint (node) with one inch
exposed to the elements. This tube exhibits magnetostrictive properties:
it expands and contracts under the influence of a variable magnetic field.
A magnet mounted inside the strut and modulated by a drive coil
surrounding the lower half of the tube provides the magnetic field.
A magnetostrictive oscillator (MSO) circuit is created with the above
components and the addition of a pickup coil and an electronic
comparator. The ultrasonic axial movement of the tube resulting from
the activation of the drive coil causes a current to be induced in the
pickup coil. The current from the pickup coil drives the comparator that,
in turn, provides the signal for the drive coil.
The oscillation frequency of the circuit is determined by the natural
resonant frequency of the sensor tube, which is tuned to 40 kHz. With the
start of an icing event, ice collects on the 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 0.5mm (0.020”)
thickness of ice on the probe causes the operating frequency of the
probe to decrease by approximately 130 Hz. The freezing rain sensor
onboard software monitors the probe frequency, detects and
annunciates any 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