Campbell 253 User manual

INSTRUCTION MANUAL
Model 253 and 253-L
(Watermark 200)
Soil Moisture Sensor
Revision: 6/96
Copyright (c) 1993-1996
Campbell Scientific, Inc.

WARRANTY AND ASSISTANCE
This equipment is warranted by CAMPBELL SCIENTIFIC (CANADA) CORP. (“CSC”) 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
are not warranted. ***** CSC's obligation under this warranty is limited to repairing or
replacing (at CSC's option) defective products. The customer shall assume all costs of
removing, reinstalling, and shipping defective products to CSC. CSC will return such
products by surface carrier prepaid. This warranty shall not apply to any CSC 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. CSC is not
liable for special, indirect, incidental, or consequential damages.
Products may not be returned without prior authorization. To obtain a Return
Merchandise Authorization (RMA), contact CAMPBELL SCIENTIFIC (CANADA) CORP.,
at [phone]. An RMA number will be issued in order to facilitate Repair Personnel
in identifying an instrument upon arrival. Please write this number clearly on the outside
of the shipping container. Include description of symptoms and all pertinent details.
CAMPBELL SCIENTIFIC (CANADA) CORP. does not accept collect calls.
Non-warranty products returned for repair should be accompanied by a purchase order to
cover repair costs.

i
253 Table of Contents
PDF viewers note: These page numbers refer to the printed version of this document. Use
the Adobe Acrobat® bookmarks tab for links to specific sections.
1. General ........................................................................1
2. Installation and Removal............................................1
3. Wiring ..........................................................................2
4. Measurement...............................................................2
4.1 Calculate Sensor Resistance - Instruction 59 ............................................2
4.2 Calculate Soil Water Potential ..................................................................2
5. Programming (Measuring Block Resistance) ...........3
5.1 AM32 and 21X .........................................................................................3
5.2 AM32 and CR10.......................................................................................4
5.3 AM416 and 21X .......................................................................................4
5.4 AM416 and CR10.....................................................................................5
6. Programming (Calculating Soil Water Potential)......5
6.1 Linear Resistance and Temperature Relationship (0 to 2 Bars)................5
6.2 Non-Linear Resistance and Temperature Relationship (0.1 to 1 Bar) ......6
7. Programming (Comprehensive) ................................7
8. Interpreting Results ....................................................8
9. Troubleshooting..........................................................8

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1
MODEL 253 AND 253-L (WATERMARK 200)
SOIL MOISTURE SENSOR
1. GENERAL
The Watermark 200 (CSI sensor Models 253,
253-L, 257, and 257-L) provides a convenient
method of estimating water potential between 0
and 2 bars (wetter soils) with a Campbell
Scientific CR10, 21X, or CR7 datalogger. CSI
Models 253 and 253-L are for connection to the
AM32 or AM416 Analog Multiplexers. Models
257 and 257-L connect directly to a datalogger.
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.
FIGURE 1.1 253 Soil Moisture Sensor
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 gravelly 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 gravelly 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.

253 AND 253-L SOIL MOISTURE SENSOR
2
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.
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.
3. WIRING
The model 253 sensor is supplied with two green
leads from Watermark. The leads from the
Watermark electrode are connected directly to
the H and L inputs on the AM32 or AM416. The
lead coming from the center of the sensor is
connected to H and the lead from the outer
portion of the sensor to L. The wires can be
differentiated by the grooved strip in one of the
leads of the green wires. On the 253-L, Campbell
Scientific splices a two conductor shielded cable
to the two conductor green cable supplied from
Watermark. The black conductor is connected to
H, the white conductor to L, and the shield wire to
G or . A 1k:resistor at the datalogger is used
to complete the half bridge measurement.
4. MEASUREMENT
Instruction 5, AC Half Bridge, is used to excite
and measure the model 253. Recommended
excitation voltages and input ranges are listed in
Table 1.
TABLE 1. Excitation and Voltage Range
DATALOGGER mV EX RANGE FSR
CODE
21X 500 14 ± 500 mV
CR10 250 14 ± 250 mV
NOTE: Do not use a slow integration time as
sensor polarization errors will occur.
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 k:.
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.
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
been verified, but work in practice).
The following equation normalizes the
resistance measurement to 21qC.
RR
dT
s
21 1 0 018
(. * ) [1]
where
R21 = resistance at 21qC
Rs= the measured resistance
dT = (Ts-21)
Ts= soil temperature
Water potential is then calculated from R21 with
the relationship.
SWP R 0 07407 0 03704
21
.* . [2]
SWP = Soil Water Potential (bars)

253 AND 253-L SOIL MOISTURE SENSOR
3
4.2.2 Non-Linear Relationship
For more precise work, calibration and temperature
compensation in the range of 0.1 to 1.00 bar has
been refined by Thompson and Armstrong (1987),
as defined in the non-linear equation,
SWP R
TTR
s
sss
 0 01306 1062 34 21 0 01060 01
2
.[.(. . )]
*. [3]
Table 2. Comparison of Estimated Soil
Water Potential and Rs at 21°C
Bars
(Non- Bars
Linear (Linear
Equation) Equations) (Rs)kOhms
.037 1.00
.09 .11 2.00
.14 .18 3.00
.20 .26 4.00
.27 .33 5.00
.35 .41 6.00
.45 .48 7.00
.56 .56 8.00
.69 .63 9.00
.85 .70 10.00
1.05 .78 11.00
.85 12.00
.92 13.00
.99 14.00
1.07 15.00
1.15 16.00
1.22 17.00
1.29 18.00
1.44 20.00
1.59 22.00
1.74 24.00
1.88 26.00
1.99 27.50
5. PROGRAMMING (MEASURING
BLOCK RESISTANCE)
The following examples demonstrate the
connections and programming used to measure
the resistances (kohms) of 32 soil moisture
blocks.
5.1 AM32 AND 21X
See Figure 5.1 for wiring diagram.
01: P20 Set Port (Enable AM32)
01: 1 Set High
02: 1 Port One
02: P87 Beginning of Loop
01: 0 Delay
02: 32 Loop Count
03: P22 Excitation with delay (clock)
01: 1 Excitation Channel #1
02: 1 Excite for 0.01 seconds
03: 0 0 second delay after excitation
04: 5000 Excitation = 5000 mV
04: P5 AC Half Bridge (Measure
AC Conductivity)
01: 1 Rep
02: 14 500 mV Fast Range
03: 1 In Channel
04: 2 Excite All Reps w/Excite
Channel 2
05: 500 mV excitation
06: 1-- Location (Indexed Location
to Store) [:kOhms#1]
07: 1 Multiplier
08: 0 Offset
05: P95 End
06: P20 Set Port (Reset AM32)
01: 0 Set Low
02: 1 Port Number
07: P59 BR Transform Rf[X/(1-X)]
(Compute Resistances)
01: 32 Reps
02: 1 Location [:kOhms#1]
03: 1 Multiplier (Rf/1000)
FIGURE 5.1

253 AND 253-L SOIL MOISTURE SENSOR
4
5.2 AM32 AND CR10
See Figure 5.2 for wiring diagram. This
program can also be used with 21Xs with OSX
PROMs although the clock pulse delay is 0.1
seconds (Figure 5.3).
01: P86 Do
01: 45 Set Port 5 high
02: P87 Beginning of Loop
01: 0 Delay
02: 32 Loop Count
03: P86 Do (Clock Pulse, 10 ms)
01: 76 Pulse Port 6
04: P5 AC Half Bridge (Measure
AC Conductivity)
01: 1 Rep
02: 14 250 mV Fast Range
03: 1 In Channel
04: 2 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
05: P95 End
06: P86 Do (Reset AM32)
01: 55 Set Port 5 low
07: P59 BR Transform Rf[X/(1-X)]
(Compute Resistances)
01: 32 Reps
02: 1 Location [:kOhms#1]
03: 1 Multiplier (Rf/1000)
FIGURE 5.2
FIGURE 5.3
5.3 AM416 AND 21X
See Figure 5.4 for wiring diagram.
01: P20 Port Set (Enable AM416)
01: 1 Set High
02: 1 Port Number
02: P87 Beginning of Loop
01: 0 Delay
02: 16 Loop Count
03: P22 Excitation with delay (clock)
01: 1 Excitation Channel #1
02: 1 Excite for 0.01 seconds
03: 0 0 second delay after excitation
04: 5000 Excitation = 5000 mV
04: P90 Step Loop Index
01: 2 Step
05: P5 AC Half Bridge (Measure
AC Conductivity)
01: 2 Rep
02: 14 500 mV Fast Range
03: 1 In Channel
04: 2 Excite All Reps w/
Excitation Channel 2
05: 500 mV Excitation
06: 1-- Location (Indexed Location
to Store) [:kOhms#1]
07: 1 Multiplier
08: 0 Offset
06: P95 End
07: P20 Set Port (Reset AM416)
01: 0 Set Low
02: 1 Port Number
08: P59 BR Transform Rf[X/1(1-X)]
Compute Resistances
01: 32 Reps
02: 1 Location [:kOhms#1]
03: 1 Multiplier (Rf/1000)

253 AND 253-L SOIL MOISTURE SENSOR
5
FIGURE 5.4
5.4 AM416 AND CR10
See Figure 5.5 for wiring diagram. This
program can also be used with 21X with OSX
PROMs although the clock pulse delay is 0.1
seconds (Figure 5.6).
01: P86 Do Set Port (Enable AM416)
01: 41 Set High Port 1
02: P87 Beginning of Loop
01: 0 Delay
02: 16 Loop Count
03: P86 Do (Clock Pulse, 10 ms)
01: 72 Pulse Port 2
04: P90 Step Loop Index
01: 2 Step
05: P5 AC Half Bridge (Measure
AC Conductivity)
01: 2 Rep
02: 14 250 mV Fast Range
03: 1 In Channel
04: 2 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
06: P95 End
07: P86 Do (Reset AM416)
01: 51 Set Low Port 1
08: P59 BR Transform Rf[X/(1-X)]
01: 32 Reps
02: 1 Location [:kOhms#1]
03: 1 Multiplier (Rf/1000)
Figure 5.5
Figure 5.6
6. PROGRAMMING (CALCULATING
SOIL WATER POTENTIAL)
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
04: P34 Z=X+F
01: 34 X Loc TmpDegC
02: -21 F
03: 36 Z Loc [:CorrFactr]
...Calculate (0.018 * dT)
05: P37 Z=X*F
01: 36 X Loc CorrFactr
02: .018 F
03: 36 Z Loc [:CorrFactr]
...Calculate (1 - (0.018 * dT))

253 AND 253-L SOIL MOISTURE SENSOR
6
06: P34 Z=X+F
01: 36 X Loc CorrFactr
02: -1 F
03: 36 Z Loc [:CorrFactr]
07: 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...
08: P87 Beginning of Loop
01: 0 Delay
02: 32 Loop Count
...Temperature Correct Ohms:
09: P38 Z=X/Y
01: 1-- X Loc kOhms#1
02: 36 Y Loc CorrFactr
03: 41-- Z Loc [:Bar#1 ]
...Apply Calibration Slope and Offset
10: P37 Z=X*F
01: 41-- X Loc Bar#1
02: .07407 F
03: 41-- Z Loc [:Bar#1 ]
11: P34 Z=X+F
01: 41-- X Loc Bar#1
02: -.03704 F
03: 41-- Z Loc [:Bar#1 ]
12: P95 End
6.2 NON-LINEAR RESISTANCE AND
TEMPERATURE RELATIONSHIP (0.1 TO 1
BAR)
The following instructions convert Rsto Soil
Water Potential in bars. See Equation [3] in
Section 4.2.2.
08: P87 Beginning of Loop
01: 0 Delay
02: 32 Loop Count
05: P36 Z=X*Y SWP = Tsoil^2
01: 34 X Loc Tsoil C
02: 34 Y Loc Tsoil C
03: 41-- Z Loc [:Bars#1]
SWP = Tsoil^2 * 0.0106
06: P37 Z=X*F
01: 41-- X Loc Bars#1
02: 0.0106 F
03: 41-- Z Loc [:Bars#1]
07: P30 Z=F
01: 34.21 F
02: 35 Z Loc [:Constant ]
SWP = 34.21-Ts
08: P35 Z=X-Y
01: 35 X Loc Constant
02: 34 Y Loc Tsoil C
03: 35 Z Loc [:Constant ]
SWPcalc.=[(34.21-Ts)+(Ts^2*0.01060)]
09: P33 Z=X+Y
01: 41-- X Loc Bars#1
02: 3 Y Loc Constant
03: 41-- Z Loc [:Bars#1]
SWP = 1.062[SWPcalc.]
10: P37 Z=X*F
01: 41-- X Loc Bars#1
02: 1.062 F
03: 41-- Z Loc [:Bars#1]
11: P 35 Z=X-Y
SWP = SWPcalc.-Rs
01: 41-- X Loc Bars#1
02: 1-- Y Loc kOhms#1
03: 41-- Z Loc [:Bars#1]
SWP = 0.01306*SWPcalc.
12: P37 Z=X*F
01: 41-- X Loc Bars#1
02: 0.0130 F
03: 41-- Z Loc [:Bars#1]
SWP = Rs/SWPcalc.
13: P38 Z=X/Y
01: 1-- X Loc kOhms#1
02: 41-- Y Loc Bars#1
03: 41-- Z Loc [:Bars#1]
SWPbars = SWP(kPa) * 0.01
14: P37 Z=X*F
01: 41-- X Loc Bars#1
02: 0.01 F
03: 41-- Z Loc [:Bars#1]
15: P95 End Calculation Loop

253 AND 253-L SOIL MOISTURE SENSOR
7
7. PROGRAMMING (COMPREHENSIVE)
Follow these steps to create a complete
program:
Step 1. Allocate at least 75 input locations in
EDLOG.
Step 2. Set the execution interval according to
need:
* 1 Table 1 Programs
01: 3600 Sec. Execution Interval
Step 3. Make a temperature measurement to
correct for temperature effects. Select
from options 1 or 2.
Option 1. If a 107B Probe is part of your
system, measure soil
temperature:
01: P11 Temp 107 Probe
01: 1 Rep
02: 3 IN Chan
03: 3 Excite all reps w/EXchan 3
04: 34 Loc [:TempDegC ]
05: 1 Mult
06: 0 Offset
Option 2. If a 107B Probe is not available
but a 107 Probe is part of your
system, measure air
temperature in the early morning
(6:00 A.M.) and assume that will
be the soil temperature for the
day:
02: P92 If time is
01: 360 minutes (seconds--) into a
02: 1440 minute or second interval
03: 30 Then Do
03: P11 Temp 107 Probe
01: 1 Rep
02: 3 IN Chan
03: 3 Excite all reps w/EXchan 3
04: 34 Loc [:TempDegC ]
05: 1 Mult
06: 0 Offset
04: P95 End
Step 4. 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:
05: P92 If time is
01: 360 minutes (seconds--) into a
02: 720 minute or second interval
03: 30 Then Do
Insert one of the examples from Section 5
here.
Step 5. Calculate soil water potential using
resistance and temperature:
Insert one of the examples from Section 6
here.
Step 6. Output data to final storage after each
measurement and calculation:
06: P86 Do
01: 10 Set high Flag 0 (output)
07: P77 Real Time
01: 0220 Day,Hour-Minute
08: P70 Sample kOhm Resistances
01: 32 Reps
02: 1 Loc
09: P70 Sample Deg C Temperature
01: 1 Reps
02: 34 Loc
10: P70 Sample Bar Potential
01: 32 Reps
02: 41 Loc
11: P95 End 6 am and 6 pm loop

253 AND 253-L SOIL MOISTURE SENSOR
8
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.
Re-installing 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
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.

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