LWET--Leaf Wetness Sensor

In agriculture, warm, wet weather promotes fungus diseases and moisture-loving insects. Direct surface wetness is pertinent, because it is within the leaf micro-environment or on other plant surface that pathogens develop. Wetness data is usually reduced in practice to a yes or no form. For example, the Mills Tables are based on the hours that leaves remain wet while the temperatures are also warm. Danger arises as a period of warm wet weather is prolonged, and the danger passes only after a certain number of dry hours have elapsed. Wetness measurements are also useful when the exact time of the onset of rainfall is of interest, for example, as part of control systems that automatically open and close greenhouse windows or schedule irrigation. A rain gage is not as useful for this purpose, because there is a delay before the rain gage accumulates enough water to signal its first count . Humidity sensors are not so useful either, because there are many factors that determine precipitation. Sensitive wetness measurements are also applicable to other areas of science and industry, such as the study of moisture in breath, or of corrosion on metals.

The LWET from EME Systems is an artificial leaf surface constructed on a fiberglass circuit board. Moisture present on the surface increases the frequency output from an integrated oscillator circuit. The LWET is distinguished from similar products by having its AC excitation built in. It is not necessary to provide special excitation from the data logger. Changes in frequency of the LWET oscillator can be counted easily by our OWL2c data logger or other micro-controller. Alternatively, the LWET  has voltage and current outputs directly proportional to the level of wetness, so the sensor can also be used with data loggers that have analog inputs. The LWET has identical sensing grids on top and bottom surfaces. The grids can be painted with exterior latex paint to make the response more akin to a natural leaf.

Specifications:

• LWET Manual
  
• Supply Voltage: 5--15 VDC
  less than 0.01% per Volt supply variation.
• Frequency output 50 hz dry @ infinite ohms
  10 khz wet @ zero ohms
  open collector square wave.
• Supply Current: 200±10µA dry to 1000µA wet
  Can be used as analog output signal
• Voltage output, 0.2 to 1 volt. 
• Operating Temperature: -0°C to +70°C
  no meaningful signal below 0°C
• connection to data logger, 10 foot cable
  4 wire flat telephone PVC jacket
  red: + 5 to 15 volts DC
  green: frequency signal, open collector, needs pull-up to +V, 50hz to 10khz
  white: voltage output signal, 0.2 volts to 1 volt.
  black: common, or analog current output.
• see SMX documentation for resistance response.
• lifetime: 1 to 3 years (depends on contaminants)
• Top & bottom sensor grids are 1.625" x 0.625"
  Overall size: 4.5" long x 0.75 wide x 0.031" thick, with mounting holes for #6 screw.
  Includes nylon loop for mounting.   Pipe clamp not provided.

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• LWET ....................Base Price..........$95
• LWET/P ................Painted...........no charge
  

Wiring the LWET to your data logger:   

The figure below shows how to connect the sensor for digital frequency output, current output and voltage output.

For frequency output, the resistor (10kohms, value not critical) can pull up to any voltage from 2.5 to 10 volts dc. The output signal  is a square wave, and its frequency varies from 50 hz when the sensor is bone dry, up to 10000 hertz when the sensor is soaking wet. This output can be measured using a COUNT function on the data logger.

The nest figure below shows how to connect the sensor for analog output. The supply current is 200 (±10) microamps when the sensor is bone dry and 1000 (±100) microamps when it is soaking wet. The 1 kohm resistor converts the output to a voltage for analog meters. The analog current does not depend on the supply voltage, so long as the voltage across the sensor itself is greater than 4 volts. The green "frequency signal" wire is not used in this configuration.

And finally, the optional LWET/V has built-in circuitry to provide a voltage output instead of a frequency output. as shown in the third diagram.

Notes on operation and installation

An excellent source for information about leaf wetness issues in agriculture is to be found at:

http://www.nysaes.cornell.edu/pp/faculty/seem/magarey/leafwet/

• 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° angle from the horizontal, so that rain will not pool up. The mounting bracket facilitates the mounting at the 45° angle. Since the LWET has active areas on both the top and bottom surface, you have several options for mounting the LWET on either a horizontal or a vertical support. These diagrams suggest some of these options.

• 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.

• 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.

•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.

• 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.

Circuit description:

The following is the schematic of the LWET, with voltage, frequency and current output:

The power supply voltage is regulated at 3 volts DC by the micropower voltage regulator. The CMOS LMC555 timer operates in its direct feedback mode, with a square wave on the totem pole output (pin 3) charging and  discharging the 0.1 µf film capacitor through the network of fixed resistors in series/parallel with the conductive sensing grid. When the grid is dry, the 150k½ 1% resistor sets a minimum oscillator frequency of 50 hertz. When the grid is wet, or short circuited, the 390½ 1% resistor in series with the grid limit the upper frequency to about 11 khz. The current through the sensing grid is AC. The additional 4.7 uf capacitors in series with the grid assures that leakage currents do not flow through the grid. The output frequency is transmitted to the logger from the open collector DIS output pin. Alternatively, the supply current drawn by the circuit varies linearly with the frequency, so the supply current is proportional to wetness. For voltage output, the supply current is converted to a voltage by the 1kohm 1% resistor.