Campbell LWS-L User manual

LWS-L Dielectric
Leaf Wetness Sensor
Revision: 3/09
Copyright © 2008-2009
Campbell Scientific, Inc.

Warranty and Assistance
The LWS-L DIELECTRIC LEAF WETNESS SENSOR is warranted by
CAMPBELL SCIENTIFIC, INC. to be free from defects in materials and
workmanship under normal use and service for twelve (12) months from date
of shipment unless specified otherwise. Batteries have no warranty.
CAMPBELL SCIENTIFIC, INC.'s obligation under this warranty is limited to
repairing or replacing (at CAMPBELL SCIENTIFIC, INC.'s option) defective
products. The customer shall assume all costs of removing, reinstalling, and
shipping defective products to CAMPBELL SCIENTIFIC, INC. CAMPBELL
SCIENTIFIC, INC. will return such products by surface carrier prepaid. This
warranty shall not apply to any CAMPBELL SCIENTIFIC, INC. products
which have been subjected to modification, misuse, neglect, accidents of
nature, or shipping damage. This warranty is in lieu of all other warranties,
expressed or implied, including warranties of merchantability or fitness for a
particular purpose. CAMPBELL SCIENTIFIC, INC. is not liable for special,
indirect, incidental, or consequential damages.
Products may not be returned without prior authorization. The following
contact information is for US and International customers residing in countries
served by Campbell Scientific, Inc. directly. Affiliate companies handle
repairs for customers within their territories. Please visit
www.campbellsci.com to determine which Campbell Scientific company
serves your country. To obtain a Returned Materials Authorization (RMA),
contact CAMPBELL SCIENTIFIC, INC., phone [phone]. After an
applications engineer determines the nature of the problem, an RMA number
will be issued. Please write this number clearly on the outside of the shipping
container. CAMPBELL SCIENTIFIC's shipping address is:
CAMPBELL SCIENTIFIC, INC.
RMA#_____
815 West 1800 North
Logan, Utah 84321-1784
CAMPBELL SCIENTIFIC, INC. does not accept collect calls.

LWS-L Table of Contents
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1. General Description.....................................................1
2. Specifications ..............................................................1
3. Installation....................................................................2
4. Wiring............................................................................3
5. Measurement................................................................3
6. Interpreting Data..........................................................3
7. Operational Principles.................................................4
7.1 Measurement.............................................................................................4
7.2 Leaf Mimicry............................................................................................5
8. Example Programs......................................................5
8.1 CR10X Datalogger Program.....................................................................5
8.2 CR1000 Datalogger Program ...................................................................6
9. Maintenance.................................................................6
Figures
1. LWS-L Dielectric Leaf Wetness Sensor.....................................................2
2. Top View of a Typical LWS-L Installation................................................2
3. Typical LWS-L Response...........................................................................4
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LWS-L Dielectric Leaf Wetness Sensor
1. General Description
Direct measurement of leaf wetness is problematic. Secure long-term
attachment of a sensor to a representative living leaf is difficult. Leaf position,
sun exposure, and health are in constant flux. To avoid these problems, leaf
wetness “sensors” have been developed to estimate by inference the wetness of
nearby leaves. The LWS-L estimates leaf surface wetness by measuring the
dielectric constant of the sensor’s upper surface. The sensor is able to detect
the presence of miniscule amounts of water or ice. Individual sensor
calibration is not normally necessary.
2. Specifications
Measurement Time: 10 ms
Excitation: 2.5 VDC (2 mA) to 5.0 VDC (7 mA)
Minimum Excitation Time: 10 mS
Output: 10% to 50% of excitation
Operating Temperature: -20 to 60 °C
Probe Dimensions: 11.2 cm x 5.8 cm x .075 cm
Datalogger Compatibility: CR10X, CR200, CR800, CR1000, CR3000,
CR5000
Maximum Lead Length: 250 ft
Interchangeability: Interchangeable without painting or individual
calibration
1

LWS-L Dielectric Leaf Wetness Sensor
sensing area
mounting holes
circuitr
y
FIGURE 1. LWS-L Dielectric Leaf Wetness Sensor
3. Installation
The LWS-L is designed to be mounted on a small diameter rod. Deployment
in a plant canopy or on a weather station mast is typical. Two holes in the
sensor body are available for mounting with zip ties or 4-40 bolts.
FIGURE 2. Top View of a Typical LWS-L Installation
2

LWS-L Dielectric Leaf Wetness Sensor
4. Wiring
Color
Description
CR800/850
CR5000
CR3000
CR1000
CR500/510
CR10/10X
21X
CR7
CR23X
CR200
White Excitation Switched
Excitation Switched Excitation Switched Excitation Switched Excitation
Red Analog Out Single-
Ended
Channel
Single-
Ended
Channel
Single-
Ended
Channel
Single-
Ended
Channel
Bare Analog
Ground
AG
5. Measurement
The LWS-L requires excitation voltage between 2.5 and 5 VDC. It produces
an output voltage dependent on the dielectric constant of the medium
surrounding the probe. Output voltage ranges from 10 to 50% of the excitation
voltage.
The LWS-L is intended only for applications wherein the
datalogger provides short excitation, leaving the probe quiescent
most of the time. Continuous excitation may cause the probe to
exceed government specified limits on electromagnetic
emissions.
NOTE
6. Interpreting Data
Many leaf wetness applications, such as phytopathology, require a Boolean
interpretation of leaf wetness data, i.e. whether or not water is present. A
Boolean threshold is determined by analyzing a few days of time series data.
Consider time series data in Fig. 3, which were obtained at 5 VDC excitation.
The sensor yields ≈445 mV when dry, ≈475 mV when frosted, and >>475 mV
when wet. Therefore, a Boolean wetness threshold of 500 mV should serve
well for interpreting these data.
3

LWS-L Dielectric Leaf Wetness Sensor
FIGURE 3. Typical LWS-L Response
Duration of leaf wetness can be determined either by post processing of data,
or by programming the datalogger to accumulate time of wetness based on the
Boolean threshold. Accumulation of dust and debris, such as avian fecal
matter, will change the Boolean threshold. So, while having the datalogger
accumulate time of leaf wetness, or time of frost, may be convenient, assurance
of data quality requires retention of the base mV measurements.
Collect data frequently enough to capture changes in surface
wetness. A sample frequency of 15 minutes or less is usually
necessary to accurately capture leaf wetness duration.
NOTE
7. Operational Principles
7.1 Measurement
The LWS-L measures the dielectric constant of a zone approximately 1 cm
from the upper surface of the sensor. The dielectric constant of water (≈80)
and ice (≈5) are much higher than that of air (≈1), so the measured dielectric
constant is strongly dependent on the presence of moisture or frost on the
sensor surfaces. The sensor outputs a mV signal proportional to the dielectric
of the measurement zone, and therefore proportional to the amount of water or
ice on the sensor surface.
4

LWS-L Dielectric Leaf Wetness Sensor
7.2 Leaf Mimicry
The LWS-L is designed to approximate the thermodynamic properties of most
leaves. If the specific heat of a typical leaf is estimated at 3750 J kg-1 K-1,
density estimated at 0.95 g/cm3, and thickness estimated at 0.4 mm, then the
heat capacity of the leaf is ≈1425 J m-2 K-1. This heat capacity is closely
approximated by the thin (0.65mm) fiberglass construction of the LWS-L,
which has a heat capacity of 1480 J m-2 K-1. By mimicking the thermodynamic
properties of a real leaf, the LWS-L closely matches the wetness state of the
canopy.
The sensor closely matches the radiative properties of real leaves. Healthy
leaves generally absorb solar radiation in much of the visible portion of the
spectrum, but selectively reject much of the energy in the near-infrared. The
surface coating of the LWS-L absorbs well in the near-infra red region, but the
white color reflects most of the visible radiation. Spectroradiometer
measurements indicate that the overall radiation balance of the sensor closely
matches that of a healthy leaf. During normal use, prolonged exposure to
sunlight can cause some yellowing of the coating, which does not affect the
probe’s function. The surface coating is hydrophobic — similar to a leaf with
a hydrophobic cuticle. The sensor should match the wetness state of these
types of leaves well, but may not match the wetness duration of pubescent
leaves or leaves with less waxy cuticles.
8. Example Programs
8.1 CR10X Datalogger Program
Color Description CR10X
White Excitation EX1
Red Analog Out SE1
Bare Analog Ground AG
;{CR10X}
;
*Table 1 Program
01: 60 Execution Interval (seconds)
1: Excite-Delay (SE) (P4)
1: 1 Reps
2: 5 2500 mV Slow Range
3: 1 SE Channel
4: 1 Excite all reps w/Exchan 1
5: 1 Delay (0.01 sec units)
6: 2500 mV Excitation
7: 1 Loc [ LWS_mV ]
8: 1 Multiplier
9: 0 Offset
5

LWS-L Dielectric Leaf Wetness Sensor
8.2 CR1000 Datalogger Program
Color Description CR1000
White Excitation EX1 or VX1
Red Analog Out SE1
Bare Analog Ground
Public LWS_mV
BeginProg
Scan(60,Sec, 3, 0)
‘BRHalf(Dest,Reps,Range,SeChan,ExChan,MeasPEx,ExmV,RevEx,Settling,Integ,Mult,Offset)
BRHalf(LWS_mV, 1, mV2500, 1, VX1, 1, 2500, False, 10000, _60Hz, 2500, 0)
NextScan
EndProg
9. Maintenance
Over time, the accumulation of dust and debris will cause the dry output to
increase and changing the Boolean threshold. Clean the sensing surface with a
moist cloth periodically or when elevated dry output is detected.
LWS-L sensors exposed to high levels of UV radiation develop a chalky
residue on the sensor surface. This causes the surface to lose its “sheen” over
time and a small amount of chalky residue can be rubbed off the board during
aggressive cleaning. Accelerated UV testing, equivalent to five years high UV
exposure, as well as field testing, show there is no apparent change in sensor
function as a result of the chalky residue.
Acknowledgement
Portions of this manual are copyrighted by Decagon Devices, Inc. and are used
by permission.
6

Campbell Scientific Companies
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