Weather Instruments like the 6495 Freezing Rain Sensor can be used to detect the presence of ice, frost, or wet snow

Freezing Rain Sensor Model 6495

GENERAL

Advanced Sensing Technology Eliminates False Signals

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.

Efficient Probe Design Melts Ice in Seconds

After ice has accumulated on the probe to a predetermined thickness, the AWOS DCP instructs the sensor to turn on its internal heaters to deice the probe. During deicing, maximum heater power is 400 watts. The deicing system is capable of completely melting approximately 3.8 mm of ice on the probe and strut within 30 seconds at -20 degrees Celsius (°C).

Self Deicing / Water Shedding Capability

The heat sink dissipates the heat from the probe assembly following a deice cycle. The heat sink provides a recovery time (i.e., the time required for the sensor to revert to ambient temperature) of 10 minutes following a deice cycle.

Accurate / Repeatable Measurements

The heat sink also thermally isolates the probe assembly from the electronics, allowing accurate measurement at temperatures at or close to 0 °C (32 °F).

FAA Certified for AWOS Use

The freezing rain sensor is certified for use on the All Weather, Inc. AWOS 900 series weather systems

FUNCTIONAL DESCRIPTION

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.

As the ice detector encounters an icing environment, 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 0.02-inch (0.5mm) thickness of ice on the probe causes the operating frequency of the probe to decrease by approximately 133 hertz.

The ice detector control circuitry utilizes a microprocessor to monitor probe frequency when instructed by the CDP. The ice detector deices itself through internal heating elements in both the strut and probe. After the ice detector is deiced, the sensing probe cools quickly and is ready to sense ice formation again.

Specifications