INSTRUCTION MANUAL
Model 257-LC (Watermark 200)
Soil Moisture Sensor
for MetData1
4/97
Copyright (c) 1993-1997
Campbell Scientific, Inc.
Warranty and Assistance
The MODEL 257-LC (WATERMARK 200) SOIL MOISTURE SENSOR
FOR METDATA1 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.
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257-LC Table of Contents
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1. General........................................................................1
2. Installation and Removal............................................1
3. Connection..................................................................2
4. Measurement...............................................................2
4.1 Calculate Sensor Resistance - Instruction 59............................................2
4.2 Calculate Soil Water Potential..................................................................2
5. Programming (Measuring Block Resistance)...........2
5.1 CR10(X) ...................................................................................................2
6. Programming (Calculating Soi l Water Potential)......3
6.1 Linear Resistance and Temperature Relationship (0 to 2 Bars)................3
7. Programming (Comprehensive) ................................3
8. Interpreting Results....................................................4
9. Troubleshooting..........................................................4
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MODEL 257-LC (WATERMARK 200)
SOIL MOISTURE SENSOR
FOR METDATA1
1. GENERAL
The Watermark 200 (CSI sensor Model
257-LC) provides a convenient method of
estimating water potential between 0 and 2 bars
(wetter soils) with the MetData1 system.
The Watermark block estimates water potential.
For applications requiring high accuracy, call a
Campbell Scientific applications engineer for
information on precision soil moisture
measurement systems.
The Watermark consists of two concentric
electrodes embedded in a reference matrix
material. The matrix material is surrounded by
a synthetic membrane for protection against
deterioration. An internal gypsum tablet buffers
against the salinity levels found in irrigated soils.
If cultivation practices allow, the sensor can be
left in the soil all year, eliminating the need to
remove the sensor during the winter months.
NOTE: The black outer jacket of the cable
is Santoprene® rubber. This compound was
chosen for its resistance to temperature
extremes, moisture, and UV degradation.
However, this jacket will support
combustion in air. It is rated as slow
burning when tested according to U.L. 94
H.B. and will pass FMVSS302. Local fire
codes may preclude its use inside buildings.
2. INSTALLATION AND REMOVAL
Placement of the Watermark is important. To
acquire representative measurements, avoid
high spots, slope changes, or depressions
where water puddles. Typically, the sensor
must be located in the root system of the crop.
1. Soak the sensors overnight in irrigation
water. Always install a wet sensor. If time
permits, allow the sensor to dry for 1 to 2
days after soaking, and repeat the soak/dry
cycle twice to improve sensor response.
2. Make a sensor access hole to the depth
required with a 7/8" rod. Fill the hole with
water and push the sensor to the bottom of
the hole. Very coarse or gravely soils may
require an oversized hole (1 to 1-1/4") to
prevent abrasion damage to the sensor
membrane. In this case, you will need to
"grout in" the sensor with a slurry made from
the sample soil to get a snug fit in the soil.
Snug fit in the soil is most important. Lack
of a snug fit is the premier problem in
sensor effectiveness. In gravely soils, and
with deeper sensors, sometimes it is hard
to get the sensor in without damaging the
membrane. The ideal method of making
the access hole is to have a "stepped" tool
that makes an oversized hole for the upper
portion and an exact size hole for the lower
portion. In either case, the hole needs to be
carefully backfilled and tamped down to
prevent air pockets which could allow water
to channel down to the sensor.
A length of 1/2" class 315 PVC pipe fits
snugly over the sensor collar and can be
used to push in the sensor.
You can leave the PVC in place with the
wires threaded through the pipe and the
open end taped shut (duct tape is
adequate). This practice also makes it easy
to remove sensors used in annual crops.
When doing this, solvent weld the PVC pipe
to the sensor collar. Use PVC/ABS cement
on the stainless steel sensors with the
green top. Use clear PVC cement only on
the PVC sensors with the gray top.
3. When removing sensors prior to harvest in
annual crops, do so just after the last
irrigation when the soil is moist. Do not pull
the sensor out by the wires. Careful
removal prevents sensor and membrane
damage.
4. When sensors are removed for winter
storage, clean, dry, and place them in a
plastic bag.
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257-LC SOIL MOISTURE SENSOR FOR METDATA1
=
−
3. CONNECTION
The 257-LC cable is attached to the MetData1
connector #6.
4. MEASUREMENT
NOTE: Information in this section is not
necessary when programming the
MetData1 with the Short Cut Program
Builder software.
Instruction 5, AC Half Bridge, is used to excite
and measure the model 257-LC.
4.1 CALCULATE SENSOR RESISTANCE INSTRUCTION 59
Instruction 59, Bridge Transform, is used to
output sensor resistance (Rs). The instruction
takes the AC Half Bridge output (Vs/Vx) and
computes the sensor resistance as follows.
Rs = R1(X/(1-X))
where, X = Vs/Vx (Output from Instruction 5)
A multiplier of 1 should be used to output
sensor resistance (Rs) in terms of kΩ.
4.2 CALCULATE SOIL WATER POTENTIAL
The datalogger can calculate soil water
potential (bars) from the sensor resistance (Rs)
and soil temperature (Ts). See Table 2.
The need for a precise soil temperature
measurement should not be over emphasized.
Soil temperatures vary widely where placement
is shallow and solar radiation impinges on the
soil surface. A soil temperature measurement
may be needed in such situations, particularly in
research applications. Many applications,
however, require deep placement (5 to 10
inches) in soils shaded by a crop canopy. A
common practice is to assume the air
temperature at sunrise will be close to what the
soil temperature will be for the day.
The following equation normalizes the
resistance measurement to 21°C.
R
R
21
=
1 0 018
s
− (. * )
dT
where
R
= resistance at 21°C
21
R
= the measured resistance
s
dT = (T
T
s
-21)
s
= soil temperature
Water potential is then calculated from R
the relationship.
SWP R
0 07407 0 03704
.* . [2]
21
SWP = Soil Water Potential (bars)
5. PROGRAMMING (MEASURING BLOCK RESISTANCE)
NOTE: Information in this section is not
necessary when programming the
MetData1 with the Short Cut Program
Builder software.
The following examples demonstrate the
programming used to measure the resistance
(kohms) of one soil moisture block with the
MetData1 CR10(X) datalogger.
5.1 CR10(X)
XX: P5 AC Half Bridge (Measure
AC Conductivity)
01: 1 Rep
02: 14 250 mV Fast Range
03: 4 In Channel
04: 3 Excite All Reps w/
Excitation Channel 2
05: 250 mV Excitation
06: 1 Location (Indexed Location
to Store) [:kOhms#1]
07: 1 Multiplier
08: 0 Offset
21
[1]
with
4.2.1 Linear Relationship
For applications in the range of 0 to 2 bars, the
water potential and temperature responses of
the Watermark can be assumed to be linear
(measurements beyond 1.25 bars have not
XX: P59 BR Transform Rf[X/(1-X)]
(Compute Resistances)
01: 1 Reps
02: 1 Location [:kOhms#1]
03: 1 Multiplier (Rf/1000)
been verified, but work in practice).
2
257-LC SOIL MOISTURE SENSOR FOR METDATA1
6. PROGRAMMING (CALCULATING SOIL WATER POTENTIAL)
NOTE: Information in this section is not
necessary when programming the
MetData1 with the Short Cut Program
Builder software.
6.1 LINEAR RESISTANCE AND TEMPERATURE RELATIONSHIP (0 TO 2 BARS)
Calculate Temperature Correction Factor. See
Equation [1] in Section 4.2.1...
...Calculate dT = T - 21
XX: P34 Z=X+F
01: 34 X Loc TmpDegC
02: -21 F
03: 36 Z Loc [:CorrFactr]
...Calculate (0.018 * dT)
XX: P37 Z=X*F
01: 36 X Loc CorrFactr
02: .018 F
03: 36 Z Loc [:CorrFactr]
...Calculate (1 - (0.018 * dT))
03: 41 Z Loc [:Bars#1 ]
XX: P34 Z=X+F
01: 41 X Loc Bars#1
02: -.03704 F
03: 41 Z Loc [:Bars#1 ]
7. PROGRAMMING (COMPREHENSIVE)
NOTE: Information in this section is not
necessary when programming the
MetData1 with the Short Cut Program
Builder software.
Follow these steps to create a complete
program:
Step 1. Set the execution interval according to
need:
* 1 Table 1 Programs
01: 3600 Sec. Execution Interval
Step 2. Make a temperature measurement to
correct for temperature effects. Select
from options 1 or 2.
Option 1. If you have a 107-LC probe for
measuring soil temperature:
XX: P34 Z=X+F
01: 36 X Loc CorrFactr
02: -1 F
03: 36 Z Loc [:CorrFactr]
XX: P37 Z=X*F
01: 36 X Loc CorrFactr
02: -1 F
03: 36 Z Loc [:CorrFactr]
Apply Temperature Correction and
Sensor Calibration to Ohm Measurements. See
Equation [2] in Section 4.2.1...
...Temperature Correct Ohms:
XX: P38 Z=X/Y
01: 1 X Loc kOhms#1
02: 36 Y Loc CorrFactr
03: 41 Z Loc [:Bars#1 ]
...Apply Calibration Slope and Offset
XX: P37 Z=X*F
01: 41 X Loc Bars#1
02: .07407 F
XX: P11 Temp 107 Probe
01: 1 Rep
02: 8 or 9* IN Chan
03: 3 Excite all reps w/EXchan 3
04: 34 Loc [:TempDegC ]
05: 1 Mult
06: 0 Offset
* Specify input channel #8 for connector #4.
Specify input channel #9 for connector #7.
Option 2. If you only have an air
temperature measurement,
measure air temperature in the
early morning (6:00 A.M.) and
assume that will be the soil
temperature for the day:
XX: P92 If t ime is
01: 360 minutes (seconds--) into a
02: 1440 minute or second interval
03: 30 Then Do
3
257-LC SOIL MOISTURE SENSOR FOR METDATA1
XX: P11 Temp 107 Probe
01: 1 Rep
02: 8 or 9 IN Chan
03: 3 Excite all reps w/EXchan 3
04: 34 Loc [:TempDegC ]
05: 1 Mult
06: 0 Offset
XX: P95 End
Step 3. Make the resistance measurements. It
may be appropriate to make
measurements only once or twice a day.
This example makes measurements
twice a day at 6:00 AM and 6:00 PM:
XX: P92 If t ime is
01: 360 minutes (seconds--) into a
02: 720 minute or second interval
03: 30 Then Do
Insert example from Section 5 here.
Step 4. Calculate soil water potential using
resistance and temperature:
Insert example from Section 6 here.
Step 5. Output data to final storage after each
measurement and calculation:
XX: P86 Do
01: 10 Set high Flag 0 (output)
XX: P77 Real Time
01: 0220 Day,Hour-Minute
XX: P70 Sample kOhm Resistances
01: 1 Reps
02: 1 Loc
XX: P70 Sample Deg C Temperature
01: 1 Reps
02: 34 Loc
XX: P70 Sample Bar Potential
01: 1 Reps
02: 41 Loc
XX: P95 End 6 am and 6 pm loop
8. INTERPRETING RESULTS
As a general guide, Watermark 200
measurements indicate soil moisture as follows:
0 to 10 centibars = Saturated soil.
10 to 20 centibars = Soil is adequately wet
(except coarse sands,
which are beginning to
lose water).
30 to 60 centibars = Usual range for
irrigation (except heavy
clay).
60 to 100 centibars = Usual range for irrigation
for heavy clay soils.
100 to 200 centibars = Soil is becoming
dangerously dry for
maximum production.
9. TROUBLESHOOTING
To test the sensor, submerge it in water.
Measurements should be from -.03 to .03 bars.
Let the sensor dry for 30 to 48 hours. You
should see the reading increase from 0 to 150+.
Put the sensor back in the water. The reading
should run right back down to zero in 1 to 2
minutes. If the sensor passes these tests,
consider the following.
1. Sensor may not have a snug fit in the soil.
This usually happens when an oversized
access hole has been used and the
backfilling of the area around the sensor is
not complete.
2. Sensor is not in an active portion of the root
system, or the irrigation is not reaching the
sensor area. This can happen if the sensor
is sitting on top of a rock or below a hard pan
which may impede water movement. Reinstalling the sensor usually solves this
problem.
3. When the soil dries out to the point where
you are seeing readings higher than 80
centibars, the contact between soil and
sensor can be lost because the soil may
start to shrink away from the sensor. An
irrigation which only results in a partial
rewetting of the soil will not fully rewet the
sensor, which can result in continued high
4
readings from the Watermark. Full
rewetting of the soil and sensor usually
restores soil/sensor contact. This is most
often seen in the heavier soils and during
peak crop water demand when irrigation
may not be fully adequate. The plotting of
readings on a chart is most useful in getting
a good picture of this sort of behavior.
Reference
Thompson, S.J. and C.F. Armstrong, Calibration
of the Watermark Model 200 Soil Moisture
Sensor, Applied Engineering in Agriculture,
Vol. 3, No. 2, pp. 186-189, 1987.
Parts of this manual were contributed by
Irrometer Company, Inc., manufacturer of the
Watermark 200.
257-LC SOIL MOISTURE SENSOR FOR METDATA1
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257-LC SOIL MOISTURE SENSOR FOR METDATA1
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