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Leaf wetness sensors are used to detect the presence of surface moisture or soil moisture. They are used mainly in environmental applications. Some leaf wetness sensors are used to study the effects of metered vegetative wetting. Others are used to determine the best planting time for crops. Leaf wetness sensors can also be used to monitor the conditions which result in fungal development and growth on plant surfaces.

There several types of leaf wetness sensors. Examples include external leaf wetness sensors, moisture detection sensors or pole-mounted leaf wetness sensors, and leaf moisture sensors. An external leaf wetness sensor can be used to determine the amount of time during which a leaf’s surface is wet and dry. A moisture detection sensor or pole-mounted leaf wetness sensor can also be used to detect surface moisture levels. Typically, a moisture detection sensor or pole-mounted leaf wetness sensor is coupled with a soil moisture sensor to determine the best time for planting. A leaf moisture sensor consists of a chemically-treated and gold-plated circuit which changes its electrical resistance in proportion to the moisture which accumulates on the sensing surface.

Leaf wetness sensors consist of a leaf wetness sensing grid, a low-voltage bipolar excitation circuit, and a conductivity-sensing circuit. By measuring conductivity across this leaf wetness sensing grid, leaf wetness sensors can provide outputs which express the moisture level on a scale of 0 to 15. A leaf wetness sensing grid short-circuits when wet, triggering the circuit which activates the relay. When the grid dries the relay, the circuit resets. Electrical specifications for leaf wetness sensors may include a supply voltage of 100µA, an output of 2.5 to 3 VDC, and an accuracy of ±0.5 %. Electrical specifications for a moisture detection sensor or pole-mounted leaf wetness sensor may include a sensor output of 1MOhm for dry conditions, a sensor output of 130 KOhm for wet conditions, and a dew range of 0 to 100%. The electrical resistance for a leaf moisture sensor varies from above 3,000,000 ohms when dry to around 1000 ohms when wet.