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An ice detector is an optical transducer probe available for aviation purposes. It has no moving parts, is completely solid and its principle of operation is entirely optical. Intrusive to the airstream and hermetically sealed, it uses un-collimated light to monitor the opacity and optical refractive index of the substance on the probe. It is de-sensitized to ignore a film of water.
The device works as a combined optical spectrometer and optical switch. A change in opacity registers as rime ice. A change in refractive index registers as clear ice. Optical components are made of acrylic glass, which is the material used for aircraft windshields. The wavelength of the transducer's excitation light is not visible to the human eye so as not to be mistaken for any kind of navigational running light.
Optical ice detectors can be installed on any type of air vehicle with enough air speed to keep water from accumulating on the optics, such as rotorcraft, general aviation aircraft and unmanned aerial vehicles. They can be embedded into host aerospace systems such as an altimeter, antenna, anti-icing system, flight data recorder, jet engine inlet, pitot tube, stall warning indicator or weather system.
Optical ice detectors operate on 3.3 V DC at 100 mA. Installation requires the probe to be mounted in the airstream beyond the boundary layer, and in a location easily accessible to the pilot for occasional cleaning with a cotton swab and isopropyl alcohol.
The device works as a rudimentary go/no-go ice indicator or as an icing rate indicator for pilots who may have inadvertently entered icing domains.
Electronic signal-level comparators sense the transducer's output and activate LEDs to indicate relative icing rates to the pilot. The pilot can then judge the rate of ice accumulation and make a piloting decision. Once the aircraft has left the icing region, the ice either ablates or melts and blows away. The icing rate display sequence then reverses itself.
Transducer probe de-icing can be hastened by incorporating turns of resistance wire and dissipating a few watts into the probe, which weighs ¼ ounce. This resets the transducer in anticipation of the next icing event.
Optical ice detectors offer substantial adjustment range of drive level and returned signal amplification. Thus, they can be applied to operate in a wide variety of applications and sensitivities, down to 0.001" of ice.
Testing at NASA Glenn Icing Research Tunnel in Cleveland, Ohio, has verified the performance of optical ice detectors. reference: NASA Lewis (Glenn) video document D040 dated 23 September, 1997, range time = 18:43:37 hours
Other types of ice detectors for aircraft include:
- acoustic - registers changes in the sound wavelength when icing occurs
- nuclear - registers changes in emission level when icing occurs between source of radiation and the sensor