Automotive water sensors in one form or another have been around for a long time. Back in 1951, Harley Earl used a moisture sensor to up the gadget factor of his futuristic Buick LeSabre concept car; positioned in the center console, this water sensor and its control system automatically raised the car's convertible top, should rain strike while the owner was away. While this remains a common application, the water sensor has also seen use in other, slightly more important systems.
Probably the most common water sensor application today involves rain-sensing windshield wipers and headlights. Older rain-sensing wiper systems would automatically default to a medium or default mode upon sensing water, but newer and more precise sensor technology allows the sensor to detect both the presence of water and the amount of water falling on the car. The headlights will automatically trigger when the wipers engage, primarily since many state laws require it. Other applications include sophisticated traction control, antilock brake and all-wheel-drive systems.
Impedance sensors consist of two thin, metal plates separated by a hygroscopic (water-absorbing) material like nylon. The top metal plate -- usually connected to a ground -- contains hundreds of tiny holes that allow water to seep through and into the hygroscopic material. Once the hygroscopic layer gets wet, the water it absorbs forms a link between the top ground plate and the bottom positive plate. Electricity passes through the water, closing a circuit that activates the system. Impedance sensors are simple, rugged, cheap, and the heat produced during their electrical transfer causes water trapped in the hygroscopic material to evaporate. This makes the impedance sensor quick to respond if the rain should suddenly stop.
Not often used in automotive applications, chilled mirror sensors are actually more apt for sensing moisture content in the air than the liquid water itself. These sensors consist of metal mirror chilled by a two-stage heat pump, an LED light source and a photovoltaic cell angled to catch the LED's reflected light. Once the mirror gets cool enough, water from the air will condense on it and reduce amount of light going to the PV cell. The higher the required dew-point temperature, the more moisture is present in the air.
Generally used to test for hydrocarbon evaporation and dewpoint measurements, dark spot sensors are functionally similar to chilled mirror sensors. Instead of a flat and polished mirror, these sensor types use an inverted-cone-shaped glass or metal reflector with a matte or acid-etched surface. The matte surface causes thin moisture films to stick in place and condense into thick-blocking drops. The reflector's shape acts like a lens, causing the beam spot to widen relative to its initial size. This also amplifies the droplet's dark-spotting effect, which makes it easier to detect.
