Campbell 237-LC Instruction Manual

INSTRUCTION MANUAL

Model 237-LC Leaf Wetness

Sensor for MetData1

4/97

Copyright (c) 1988-1997

Campbell Scientific, Inc.

Warranty and Assistance

The 237-LC LEAF WETNESS SENSOR FOR METDATA1 is warranted by
CAMPBELL SCIENTIFIC, INC. to be free from defects in materials and
workmanship under nor mal use and service for twelve (12) months from date of
shipment unless specifi ed otherwise. Batteries have no warranty. CAMPBELL
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defective products to CAMPBELL SCIENTIFIC, INC. CAMPBELL
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warranty shall not apply to any CAMPBELL SCIENTIFIC, INC. products
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237-LC Table of Contents

PDF viewers note: These page numbers refer to the printed version of this document. Use
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1. General........................................................................1

2. Installation...................................................................1

3. Programming ..............................................................1

3.1 Measurement.............................................................................................1

3.2 Calculating Sensor Resistance..................................................................1

3.3 Output - Recording Fraction of Time Wet and Dry..................................1

3.4 MetData1 Program Example.....................................................................2

4. Calibration...................................................................3

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MODEL 237-LC LEAF WETNESS SENSOR

FOR METDATA1

1. GENERAL

Leaf wetness sensors are classified into three
types: surface contact types that measure the
electrical resistance of a water film on the leaf
surface, artificial leaf electrical resistance types,
and mechanical types that detect a change in
sensor length or weight. The 237 is an artificial
leaf type for use with the CR7, CR10, and 21X
dataloggers.

The sensor consists of a circuit board with
interlacing gold-plated copper fingers.
Condensation on the sensor lowers the
resistance between the fingers which is measured
by the datalogger. Droplets small enough to not
touch two fingers simultaneously do not change
the sensor resistance. For this reason, this type
of sensor is often coated with flat latex paint to
spread the water droplets. The color and type of
paint affect sensor performance. Campbell
Scientific supplies only the raw sensor since
individual modifications vary depending on the
application. The following paper details the effect
of paint color and sensor angle on the response
of the leaf wetness sensor:

Gillespie, T.J. and Kidd, G.E. 1978. Sensing
duration of leaf moisture retention using electrical
impedance grids. Can. J. Plant Sci. 58:179-187.

2. INSTALLATION

The 237 cable is attached to the enclosure
connector labeled #4.

3. PROGRAMMING

NOTE: Information in this section is not

necessary when programming the
MetData1 with the Short Cut Program
Builder software.

There are three steps involved in programming
the datalogger to output results of leaf wetness
with the 237 Sensor. There is the excitation
and measurement of the sensor, the calculation
of sensor resistance, and programming for the
desired output.

3.1 MEASUREMENT

The AC Half Bridge Instruction 5 is used to read
the 237 sensor. Instruction 5 returns the ratio

of the measured voltage to the excitation
voltage (V

), which is equal to a ratio of

s/Vx

sensor resistances as shown below (refer also
to Figure 1).

= R2/(R2+R1+Rs)

V

s/Vx

3.2 CALCULATING SENSOR RESISTANCE

An alternate expression of leaf wetness is
sensor resistance (R
(kohms). R

is calculated as follows:

s

= R2/(Vs/Vx) - R2 - R

R

s

) in thousands of ohms

s

1

where R2 is 1 kohm and R1 is 100 kohms,
substituting these values yields:

= 1/(Vs/Vx) - 101.

R

s

After Instruction 5, use Instruction 42 to invert

. Instruction 34 is then used to add -101

V

s/Vx

kohms. The result is the resistance of the
sensor in kohms. See Section 5 "Program
Example For The CR10."

The sensor resistance varies from above
3,000,000 ohms when dry to around 1,000
ohms when wet. If outputting resistance
measurements, the reading will be 99999 if the
sensor is completely dry. A completely dry
sensor is an open circuit, and therefore has
infinite resistance.

3.3 OUTPUT - RECORDING FRACTION OF TIME WET AND DRY

Outputting the fraction of time wet or dry cannot
be done until the resistance at the wet/dry
transition point is determined. See Section 4
"Calibration."

The fraction of time the sensor is wet or dry can
be obtained using the Histogram Instruction 75
with a single bin and closed form. The bin
select value for the histogram is the Input
Location containing sensor resistance. The
lower limit of the histogram is zero and the
upper limit is the calibrated wet/dry transition
point. This will give the fraction of the output
interval the sensor is wet. Subtract this fraction
from 1 to get the fraction of the output interval
the sensor is dry. Multiply both fractions by 100
to obtain the percent of time wet and the
percent of time dry.

1

237-LC LEAF WETNESS SENSOR FOR METDATA1

3.4 METDATA1 PROGRAM EXAMPLE

The following CR10 datalogger program is an example only, parts of this program will vary for each
application. This program measures one 237 Leaf Wetness Sensor every 15 minutes, calculates
resistance, and outputs the fraction of time wet on a daily basis.

Table 1 Programs 15 minute

01: 900 Sec. Execution Interval table ex.

01: P5 AC Half Bridge Excite and

01: 1 Rep Measure the
02: 13 25 mV fast Range 237 Sensor
03: 8 IN Chan
04: 3 Excite all reps w/EXchan 1
05: 2500 mV Excitation
*06: 1 Loc : VS/VX Loc resulting in
07: 1.0 Mult VS/VX
08: 0.0000 Offset

02: P42 Z=1/X Invert VS/VX

*01: 1 X Loc
*02: 2 Z Loc : Res Loc

03: P34 Z=X+F and

*01: 2 X Loc Add -101.0
02: -101.0 F to get
*03: 2 Z Loc : Res Loc Res.(kohm)

04: P92 If time is Every 24

*01: 0 minutes into a hours
*02: 1440 minute interval
*03: 10 Set high Flag 0 (output) Output

05: P77 Real Time Julian Day

*01: 220 Day,Hour-Minute and Time

06: P75 Histogram and

*01: 1 Rep the fraction
*02: 1 No. of Bins of time
*03: 1 Closed form wet for
04: 2 Bin Select Value Loc that day
05: 0000 WV Loc Option
06: 0.0000 Low Limit
*07: 150 High Limit

07: P End Table 1

* Parameter entries will vary depending on the datalogger program.

2

4. CALIBRATION

The resistance of the sensor at the wet/dry
transition point should be determined. A sharp
change in resistance occurs in the wet-dry
transition on the uncoated sensor. Coated
sensors have a poorly defined transition. The
resistance of the uncoated sensor at the wet/dry
transition is normally between 50 and 200
kohms. The coated sensor transition normally
occurs from 20 kohms to above 1,000 kohms.

For best results, the leaf wetness sensor should
be field calibrated. The transition point will vary
for different areas and vegetation. Place the
sensor among the vegetation whose wetness is
to be monitored. Observe the vegetation until it
reaches the desired wetness by natural means.
When the vegetation is at the desired "wetness",
note the measured resistance on the datalogger.
This resistance is the transition point.

The sensor's resistance is artificially reduced by
contaminants such as fingerprints and
smudges. Before painting and calibrating the
sensor, wash it with alcohol to remove possible
contaminants.

237-LC LEAF WETNESS SENSOR FOR METDATA1

3

237 LEAF WETNESS SENSOR FOR METDATA1

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