Model 227 Delmhorst
Cylindrical Soil Moisture Block
Revision: 11/08
Copyright © 1983-2008
Campbell Scientific, Inc.
Warranty and Assistance
The 227 DELMHORST CYLINDRICAL SOIL MOISTURE BLOCK 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
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227 Table of Contents
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1. General Description.....................................................1
2. Specifications ..............................................................1
3. Installation....................................................................2
4. Wiring............................................................................2
5. Programming ...............................................................3
5.1 Excite and Measure the 227......................................................................3
5.2 Calculate Sensor Resistance .....................................................................4
5.3 Calculate Soil Water Potential..................................................................6
5.4 Programming Examples............................................................................8
5.4.1 CRBasic ..........................................................................................8
5.4.2 Edlog...............................................................................................9
List of Figures
1. 227 Schematic.............................................................................................2
2. Polynomial Fit to Typical Block Resistance vs. Water Potential ...............4
List of Tables
1. 227 Wiring..................................................................................................3
2. Excitation and Voltage Ranges...................................................................5
3. Typical Soil Water Potential, R
4. Polynomial Coefficients for Converting Sensor Resistance to Bars...........6
5. Polynomial Error - 10 Bar Range...............................................................7
6. Wiring for CR1000 Example Program.......................................................8
7. Wiring for CR10X Example Program........................................................9
and Vs/Vx................................................6
s
i
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Model 227 Delmhorst Cylindrical Soil Moisture Block
1. General Description
The 227 gypsum soil moisture block connects directly with a Campbell
Scientific datalogger; it is not compatible with our CR200-series.
The -L option on the Model 227-L indicates that the cable length is user
specified. This manual refers to the sensor as the 227.
The Delmhorst cylindrical block is composed of gypsum cast around two
concentric electrodes which confine current flow to the interior of the block,
greatly reducing potential ground loops. Gypsum located between the outer
electrode and the soil creates a buffer against salts which may affect the
electrical conductivity. Individual calibrations are required for accurate
readings of soil water potential.
The 227 circuit has capacitors in the cable that block direct current flow from
the 227 to datalogger ground. This is done to block electrolysis from
prematurely destroying the sensor.
Gypsum blocks typically last for one to two years. Saline or acidic soils tend
to degrade the block, reducing longevity. To maximize longevity, it is
recommended that gypsum blocks not used during the winter be removed from
the field. Shallow blocks may become frozen and crack, while blocks located
below the frost line may not maintain full contact with the soil. Regardless of
depth, blocks left in the field over winter are subject to the corrosive chemistry
of the soil.
2. Specifications
Approximate Cylinder Dimensions
Diameter 2.25 cm (0.88”)
Length 2.86 cm (1.25”)
Material Gypsum
Electrode Configuration Concentric cylinders
Center electrode Excitation
Outer electrode Ground
Calibration: Measurements are affected by soil salinity,
including fertilizer salts. Individual calibrations are
required for accurate measurement of soil water
potential. The soil water potential versus resistance
values in Table 2 are “typical” values supplied by
Delmhorst Corporation. Neither Delmhorst nor
Campbell Scientific make any claim as to the
accuracy of these values. The calibration equations
in Section 4.5 were fit to the values in Table 2 to
allow output of an estimated water potential.
1
Model 227 Delmhorst Cylindrical Soil Moisture Block
3. Installation
4. Wiring
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.
Delmhorst recommends the blocks go through two wetting-drying cycles
before installation to improve block uniformity. For each cycle, the blocks
should be soaked in water for one hour and allowed to dry.
Soil moisture blocks measure only the moisture they "see", therefore placement
is important. Avoid depressions where the water will puddle after a rain.
Likewise, don't place the blocks in high spots or near changes in slope unless
you are trying to measure the variability created by such differences.
Prior to installation, soak the blocks for two to three minutes. Mix a slurry of
soil and water to a creamy consistency and place one or two tablespoons into
the installation hole. Insert the block, forcing the slurry to envelope the block.
This will insure uniform soil contact. Back fill the hole, tamping lightly at
frequent intervals.
Voltage
Excitation
Signal
Ground
Shield
The 227 schematic is shown in Figure 1. The capacitors block galvanic action
due to the differences in potential between the datalogger earth ground and the
electrodes in the block. Such current flow would cause rapid block
deterioration.
The 227 uses a single-ended analog channel. Table 1 shows the datalogger
wiring.
2
FIGURE 1. 227 Schematic
Model 227 Delmhorst Cylindrical Soil Moisture Block
TABLE 1. 227 Wiring
Color Function
Black Excitation Switched Voltage
Red Signal Single-ended Channel Single-ended Channel Single-ended Channel
White Signal
Ground
Clear Shield G
CR10(X), CR510
Excitation
AG
21X, CR7, CR23X
Switched Voltage
Excitation
CR800, CR850, CR1000,
CR3000, CR5000
Switched Voltage
Excitation
5. Programming
NOTE
This section is for users who write their own datalogger
programs. A datalogger program to measure this sensor can be
generated using Campbell Scientific’s Short Cut Program
Builder software. You do not need to read this section to use
Short Cut.
The datalogger is programmed using either CRBasic or Edlog. Dataloggers
that use CRBasic include our CR800, CR850, CR1000, CR3000, CR5000, and
CR9000(X). Dataloggers that use Edlog include our CR510, CR10(X), 21X,
CR23X, and CR7. CRBasic and Edlog are included with LoggerNet, PC400,
and RTDAQ software.
The datalogger program needs to measure the sensor, calculate the sensor
resistance, and convert the transform resistance to potential in bars.
5.1 Excite and Measure the 227
The sensor is excited and measured using the BrHalf instruction in CRBasic or
Instruction 5 (AC Half Bridge) in Edlog. Recommended excitation voltages
and input ranges are given in Table 2.
3
Model 227 Delmhorst Cylindrical Soil Moisture Block
18
16
14
12
10
8
6
4
2
0
Soil Water Potential (-Bars)
0 5 10 15 20 25 30 35
Typical Values from Table 2
0.1 to 10 Bar Polynomial Fit
Block Resistance (kOhms)
FIGURE 2. Polynomial Fit to Typical Block Resistance vs. Water Potential
5.2 Calculate Sensor Resistance
The sensor resistance is calculated using an expression in CRBasic or
Instruction 59 (Bridge Transform) in Edlog. The expression or Instruction 59
takes the Half Bridge output (Vs/Vx) and computes sensor resistance as
follows:
R
where, X = Vs/Vx
The bridge transform multiplier would normally be 1000, representing the
fixed resistor (R1) shown in Figure 1. A bridge multiplier of 1000 produces
values of Rs larger than 6999 Ohms causing the datalogger to overrange when
using low resolution. To avoid overranging, a bridge multiplier of 1 should be
used to output sensor resistance (Rs) in terms of kohms.
= R1(X/(1-X))
s
4
Model 227 Delmhorst Cylindrical Soil Moisture Block
TABLE 2. Excitation and Voltage Ranges
Datalogger mV Excitation Full Scale Range
CR800/CR850 250 ±250 mV
CR1000 250 ±250 mV
CR3000 200 ±200 mV
CR5000 200 ±200 mV
CR9000(X) 200 ±200 mV
21X 500 ±500 mV
CR7 500 ±500 mV
CR10(X) 250 ±250 mV
CR23X 200 ±200 mV
NOTE: Do not use a slow integration time as sensor
polarization errors will occur.
The output from the BrHalf instruction or Instruction 5 is
the ratio of signal voltage to excitation voltage :
V
where, V
V
R
and, R
= Rs/(Rs+R1)
s/Vx
= Signal Voltage
s
= Excitation Voltage
x
= Sensor Resistance
s
= Fixed Bridge Resistor.
1
Table 4 lists typical block resistance at different soil water
potentials and the resulting Vs/Vx. Figure 2 is a plot of
Vs/Vx versus bars. The non-linear relationship of Vs/Vx
to bars precludes computing bars from an average of
Vs/Vx.
5
Model 227 Delmhorst Cylindrical Soil Moisture Block
TABLE 3. Typical Soil Water
Potential, Rs and Vs/Vx
BARS Rs(kohms) Vs/Vx
0.1 0.060 0.0566
0.2 0.130 0.1150
0.3 0.260 0.2063
0.4 0.370 0.2701
0.5 0.540 0.3506
0.6 0.750 0.4286
0.7 0.860 0.4624
0.8 1.100 0.5238
0.9 1.400 0.5833
1.0 1.700 0.6296
1.5 3.400 0.7727
1.8 4.000 0.8000
2.0 5.000 0.8333
3.0 7.200 0.8780
6.0 12.500 0.9259
10.0 17.000 0.9444
11.0 22.200 0.9569
12.0 22.400 0.9573
13.0 30.000 0.9677
14.0 32.500 0.9701
15.0 35.000 0.9722
TABLE 4. Polynomial Coefficients for Converting Sensor Resistance to Bars
BARS = C0 + C1(Rs) +C2(Rs)2+C3(Rs)3+C4(Rs)4+C5(Rs)5
BARS) MULT. (R1)
0.1-10 0.1 .15836 6.1445 -8.4189 9.2493 -3.1685 .33392
C
0
C1 C
C
2
C
3
C
4
5
5.3 Calculate Soil Water Potential
The datalogger program can be written to store block resistance or can
calculate water potential from a block calibration.
For the typical resistance values listed in Table 2, soil water potential (bars) is
calculated from sensor resistance (Rs) using the 5th order Polynomial
Instruction. The non linear relationship of Rs to bars rules out averaging Rs
directly.
The polynomial is entered as an expression in CRBasic or by using In struction
55 in Edlog. The polynomial to calculate soil water potential is fit to the 0.1 to
10 bar range using a least square fit. Table 4 lists the coefficients and equation
for the 0.1 to 10 bar polynomial.
6
Model 227 Delmhorst Cylindrical Soil Moisture Block
NOTE
NOTE
The coefficients used for the 10 bar range require Rs to be scaled
down by a factor of 0.1. In Edlog, this multiplier can be en tered
in the Bridge Transform Instruction or in Processing Instruction
37.
Table 5 shows errors between from the least-squares polynomial
approximation and the typical water potential values.
Our manuals used to show a separate polynomial for the 0.1 to 2
bar range that had slightly smaller deviations from the typical
values over the narrower range. However, the variability
between blocks is much greater than the improved fit and does
not warrant the more complicated program.
TABLE 5. Polynomial Error - 10 Bar Range
BARS Vs/Vx
Rs
BARS
COMPUTED ERROR
0.1 0.0566 0.006 0.1949 0.0949
0.2 0.115 0.013 0.2368 0.0368
0.3 0.2063 0.026 0.3126 0.0126
0.4 0.2701 0.037 0.3746 -0.0254
0.5 0.3506 0.054 0.4670 -0.0330
0.6 0.4286 0.075 0.5756 -0.0244
0.7 0.4624 0.086 0.6302 -0.0698
0.8 0.5238 0.11 0.7442 -0.0558
0.9 0.5833 0.14 0.8778 -0.0222
1.0 0.6296 0.17 1.0025 0.0025
1.5 0.7727 0.34 1.5970 0.0970
1.8 0.8000 0.40 1.7834 -0.0166
2 0.8333 0.50 2.0945 0.0945
3 0.8780 0.72 2.8834 -0.1166
6 0.9259 1.25 6.0329 0.0329
10 0.9444 1.70 9.9928 -0.0072
NOTE: ERROR (BARS) = TABLE 3 VALUES COMPUTED
7
Model 227 Delmhorst Cylindrical Soil Moisture Block
5.4 Programming Examples
5.4.1 CRBasic
This example program is written for a CR1000. Programming for other
CRBasic dataloggers is similar. The 227 sensor is measured with the BrHalf
instruction. An expression uses the result of the BrHalf instruction (Vs/Vx) to
generate Rs in kohms. If Rs is less than 17 kohms, soil water potential is
generated using the polynomial. If Rs is greater than 17 kohms, 1000 is stored
in the variable.
TABLE 6. Wiring for CR1000 Example
Color Function CR1000
Black Voltage Excitation VX1 or EX1
Red Signal SE1
White Signal Ground
Clear Shield
Program
'CR1000
'Declare Variables and Units
Public Batt_Volt
Public Rs_kOhm
Public WP_kPa
Units Batt_Volt=Volts
Units Rs_kOhm=kOhms
Units WP_kPa=kPa
'Define Data Tables
DataTable(Table1,True,-1)
DataInterval(0,60,Min,10)
Sample(1,Rs_kOhm,FP2)
EndTable
DataTable(Table2,True,-1)
DataInterval(0,1440,Min,10)
Minimum(1,Batt_Volt,FP2,False,False)
EndTable
'Main Program
BeginProg
Scan(5,Sec,1,0)
'Default Datalogger Battery Voltage measurement Batt_Volt:
Battery(Batt_Volt)
'227 Soil Moisture Block measurements Rs_kOhm and WP_kPa:
BrHalf(Rs_kOhm,1,mV250,1,Vx1,1,250,True,0,250,1,0)
Rs_kOhm=Rs_kOhm/(1-Rs_kOhm)
If Rs_kOhm<17 Then
8
Model 227 Delmhorst Cylindrical Soil Moisture Block
WP_kPa=Rs_kOhm*0.1
WP_kPa=0.15836+(6.1445*WP_kPa)+(-8.4189*WP_kPa^2)+(9.2493*WP_kPa^3)+
(-3.1685*WP_kPa^4)+(0.33392*WP_kPa^5)
WP_kPa=WP_kPa*100
Else
WP_kPa=1000
EndIf
'Call Data Tables and Store Data
CallTable(Table1)
CallTable(Table2)
NextScan
EndProg
5.4.2 Edlog
This program example is intended to be a portion of a larger program with
instructions that are executed at a 10 second interval. It is a CR10X program
but other Edlog dataloggers are programmed similarly.
The 227 sensor is measured with Measurement Instruction (5). The Bridge
Transform Instruction (59) uses the result of Instruction 5 (Vs/Vx) to generate
Rs in kohms. If Rs is less than 17 kohms, soil water potential is generated
using the polynomial. If Rs is greater than 17 kohms, the overrange indicator 99999 is loaded into the water potential location.
Every 6 hours the time (day, hour, minute), sensor resistance, and calculated
water potential are output.
TABLE 7. Wiring for CR10X Example
Program
Color Function CR10(X)
Black Excitation E1
Red Signal SE1
White Signal Ground AG
Clear Shield G
*Table 1 Program
01: 10.0000 Execution Interval (seconds)
01: AC Half Bridge (P5) ;Measure and store Vs/Vx
1: 1 Reps
2: 14 250 mV Fast Range
3: 1 SE Channel
4: 1 Excite all reps w/Exchan 1
5: 250 mV Excitation
6: 1 Loc [ Rs ]
7: 1 Mult
8: 0 Offset
9
Model 227 Delmhorst Cylindrical Soil Moisture Block
02: BR Transform Rf[X/(1-X)] (P59) ;Convert Vs/Vx to Rs
1: 1 Reps
2: 1 Loc [ Rs ]
3: 1 Multiplier (Rf)
03: If (X<=>F) (P89) ;If Rs < 17, Use 10 bar polynomial
1: 1 X Loc [ Rs ]
2: 4 <
3: 17 F
4: 30 Then Do
04: Z=X*F (P37) ;Scale Rs for polynomial
1: 1 X Loc [ Rs ]
2: .1 F
3: 2 Z Loc [ WatPoten ]
05: Polynomial (P55) ;Convert Rs to bars with 10 bar polynomial
1: 1 Reps
2: 2 X Loc [ WatPoten ]
3: 2 F(X) Loc [ WatPoten ]
4: .15836 C0
5: 6.1445 C1
6: -8.4198 C2
7: 9.2493 C3
8: -3.1685 C4
9: .33392 C5
06: Else (P94) ;If Rs > 17 load overrange value for potential
07: Z=F (P30)
1: -99999 F
2: 0 Exponent of 10
3: 2 Z Loc [ WatPoten ]
08: End (P95) ;End then do
09: If time is (P92) ;Output every six hours
1: 0 Minutes (Seconds --) into a
2: 360 Interval (same units as above)
3: 10 Set Output Flag High
10: Real Time (P77) ;Output time
1: 220 Day,Hour/Minute (midnight = 2400)
11: Sample (P70) ;Output Rs and Water potential
1: 2 Reps
2: 1 Loc [ Rs ]
10
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