Campbell LWS-L Manuel utilisateur

LWS-L Dielectric

Leaf Wetness Sensor

Revision: 3/09

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 (435) 753-2342. 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

This is a blank page.

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

LWS-L Dielectric Leaf Wetness Sensor

sensing area

mounting holes

circuitr

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

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

Red Analog Out Single-

Ended

Channel

Single-

Ended

Channel

Single-

Ended

Channel

Single-

Ended

Channel

Bare Analog

Ground

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.

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.

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

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.

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