LWET--Leaf Wetness Sensor

Avoid exposure to agricultural sprays and exhaust fumes. Contaminants deposited on the sensor surface may shift the response curve up, so that a given electrical response is caused by a smaller amount of moisture. Please wash the sensor frequently when it is exposed to contaminated atmospheres. Painting the sensor can allay the effect of contaminants. As the sensor ages, its zero (dry) point may shift up, due to contaminants. The software may need to adjust for that shift of the baseline.

What is wet? The threshold level of electrical signal that corresponds to "wet" must be determined empirically, by observation. A thin film of condensation on the bare sensor will increase the frequency to 300 hertz, (or 0.3 volts for the LWET/V). Similarly for light rain. So 300 to 400 is a reasonable figure for determining a yes/no answer to the question, "is it wet". Only when the sensor is immersed in a cup of water will the signal increase to the highest level (10,000 Hz, 1 milliamp, 1 volt). Only immersion will activate both the top and the bottom grids. Rainwater is usually not very conductive, so a value of 4000 hz (0.5 volt) will be typical in heavy rain. Light sprinkles of rain put drops here and there by chance, and detection only happens when one of those drops hits the LWET grid. A painted surface will spread out the drop and assure the response.

Maintenance: Clean the surface occasionally with a mild detergent solution and rinse with clear water. Mild vinegar can also be used for cleaning. This is especially important if sulfur or other sprays have been applied.

Painting the sensor: You can paint the surface of the sensor with a high quality exterior grade flat white latex paint. Painting will minimize the effect of contaminants, and it will spread out water droplets that fall on the surface. The consensus among agricultural engineers is that this painting makes this type of surface wetness detector better mimic the wetting and drying characteristics of a leaf surface. You can order the sensor prepainted from EME Systems.

Placement of the sensor: The sensor should be placed in a representative location. Sky and wind exposure are very important in dew deposition and in evaporation. Facing the sensor toward the open northern sky (in the northern hemisphere) will maximize the catchment of dew. Place the sensor at a 45