LWET--Leaf Wetness Sensor
For monitoring surface moisture
deposition (dew, fog, rain) in agriculture and industry
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 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.
Furthermore, the LWET supply current is directly proportional to the level of
wetness, so the sensor can also be used with data loggers that respond to
voltage or current. A special version, the LWET/V, is available that provides a
voltage output of 0.2 to 1 volt. The LWET has an integral mounting bracket that
holds the sensing surface at a 45 degree angle to mounting plane, and 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:
·
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
·
LWET/V
provides 0.2 to 1 volt instead of frequency
·
Operating
Temperature: -0°C to +70°C
no meaningful signal below 0°C
·
connection
to data logger, 10 foot cable
3 wire #22 gage conductors PVC jacket
red: + 5 to 15 volts DC
green: signal, open collector, needs pull-up to +V
(this wire is the voltage output in the LWET/V)
black: common, or analog current output.
·
lifetime:
1 to 3 years (depends on contaminants)
·
Top
& bottom sensor grids are 1.625" x 0.625"
Overall size: 4" long x 0.625 wide x 0.06 thick,with 45° bend, and a
1/4" mounting hole.
Integral mounting bracket that holds the sensor grid at a 45° angle to the
mounting plane.
Wiring the LWET to your
data logger:
The figure below shows
how to connect the sensor for digital frequency output. The resistor (10kohms, value
not critical) can pull up to any voltage from 2.5 to 15 volts dc. The output
signal then 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. Here is how to connect this sensor to an analog to digital
converter.
Notes on operation and
installation
An excellent source for
information about leaf wetness issues in agriculture is to be found at:
• 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.
• 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 standard version LWET, with frequency and current output:
The power supply
voltage is regulated at 2.5 volts DC by the micropower Zetex ZMR250FCT. The
CMOS LMC555 timer operates in its direct feedback mode, with a square wave on
the totem pole output from pin 3 charging or 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 0.1 uf
capacitor in series with the grid assures that even the leakage currents from
the CMOS chip do not flow through the grid. The output frequency is transmitted
to the logger from the open collector DIS output pin. Normally a pullup
resistor is provided to give voltage transitions at the logger. The current
drawn by the circuit varies linearly with the frequency due to the charge and
discharge cycles. The supply current is proportional to wetness.
The following diagram
shows the circuit diagram of the LWET/V. The supply current is converted to a
voltage by the 1kohm 1% resistor. The cable is connected to provide the voltage
output signal on the green wire. The ordinary LWET can be wired in this same
manner, by providing an external 1kohm, 1% resistor. The 2.2uf capacitor can be
increased externally to a larger value, to smooth the ripple in the output.
