Audi 2003 A8 Technical Features page 83

Operation
Measurement is performed by way of a
special capacitor which can absorb water
vapour. The water absorbed produces a
change in the electrical properties and thus
the capacitance of the capacitor. The
capacitance measurement thus provides
information on the humidity level. The sender
electronics convert the measured capacitance
into a voltage signal.
Measurement of windscreen temperature
Physical principles
All bodies exchange heat with their
environment in the form of electromagnetic
radiation. This electromagnetic radiation can
include thermal radiation in the infrared
range, visible light or ultraviolet components.
The wavelength of the radiation emitted
depends on the temperature of the actual
body. A change in the temperature of the
body alters, for example, the infrared
component of the radiation emitted. The
temperature of the body can be determined
in a non-contacting manner by measuring the
infrared radiation emitted.
Operation
The infrared radiation emitted by a body
(windscreen) is measured by means of a
highly sensitive infrared radiation sensor.
A change in the temperature of the
windscreen also produces a change in the
infrared component of the thermal radiation
emitted by the windscreen. This is detected
by the sensor and converted by the sensor
electronics into a voltage signal.
SSP282_105
SSP282_106
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