The AWI Model 6495 Freezing Rain Sensor uses an ultrasonically vibrating probe (nominally 40,000 Hz) to detect the presence of icing conditions. The vibrating frequency of the probe (nominally 40,000 hertz) decreases with the accumulation of ice, frost, or wet snow.
The Freezing Rain Sensor uses an ultrasonic axially vibrating probe to detect the presence of icing conditions. This sensing probe is a nickel alloy tube mounted in the strut at its midpoint with 1 inch (25.4mm) exposed to the atmosphere. This tube exhibits magnetostrictive properties and expands and relaxes under the influence of a variable magnetic field. A magnetic bias field is provided by a magnet mounted inside the strut and modulated by a drive coil surrounding the lower half of the tube. A magnetostrictive oscillator (MSO) circuit is created by the addition of a pickup coil and operational amplifier. 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 operational amplifier, which 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 approximately 40,000 hertz.