Campbell | 229 | Specifications | Campbell 229 Specifications

Campbell 229 Specifications
229 Heat Dissipation Matric
Water Potential Sensor
Revision: 5/09
C o p y r i g h t © 2 0 0 6 - 2 0 0 9
C a m p b e l l S c i e n t i f i c , I n c .
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The 229 HEAT DISSIPATION MATRIC WATER POTENTIAL
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229 Sensor 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 Description.....................................................1
1.1 Compatibility ............................................................................................2
1.2 Measurement Principle .............................................................................2
2. Specifications ..............................................................4
3. Installation....................................................................4
3.1 Orientation ................................................................................................4
3.2 Contact......................................................................................................4
3.3 Equilibration and Saturation of the Sensor Before Installation ................5
4. Wiring............................................................................5
5. Example Programs ......................................................6
5.1
5.2
5.3
5.4
5.5
5.6
Choosing a Reference for the Thermocouple Readings ...........................6
Adjusting for Thermal Properties of Sensor During Early Heating Times7
Datalogger Program Structure and Multiplexers ......................................7
Temperature Correction............................................................................8
Example #1 — CR1000 with CE4 and Four 229s....................................9
Example #2 — CR1000 with AM16/32-series Multiplexer,
CE4 and Sixteen 229 Sensors with Temperature Correction .............11
5.7 Example #3 — CR10X with 229 Sensor ................................................13
5.8 Example #4 — CR10X with AM16/32-series, CE4, and
Sixteen 229 Sensors ...........................................................................16
6. Calibration ..................................................................19
6.1 General....................................................................................................19
6.2 Normalized Temperature Change and Correction for Soil Temperature......20
6.2.1 Normalized Temperature Change .................................................20
6.2.2 Correction for Soil Temperature ...................................................21
6.3 Using Pressurized Extraction Methods...................................................24
6.4 General Description of Calibration/Measurement Process using
Pressure Plate Extractor .....................................................................24
6.4.1 Wiring for Calibration using Pressure Plate Extractor..................25
7. Maintenance ...............................................................26
8. Troubleshooting ........................................................27
i
229 Sensor Table of Contents
9. References..................................................................28
List of Figures
1-1. 229 Heat Dissipation Matric Water Potential Sensor and Hypodermic
Assembly ............................................................................................. 1
1-2. CE4 and CE8 Current Excitation Modules............................................. 2
1-3. Typical Temperature Response of 229 Sensor in Silt Loam Soil ........... 3
4-1. Schematic of Connections for Measurement of a 229 Sensor ................ 6
6-1. Data Points and Regression for Typical Calibration ............................ 20
6-2. Measurement error for range of soil temperatures and wide range of
matric potential.................................................................................. 22
6-3. Measurement error for range of soil temperatures and wetter range of
matric potential.................................................................................. 23
6-4. Datalogger and Peripheral Connections for 229 Calibration................ 26
List of Tables
4-1.
5-1.
5-2.
5-3.
5-4.
229 Sensor and CE4/CE8 Wiring........................................................... 5
Wiring for Four 229s with CR1000 and CE4......................................... 9
229 Sensor and CE4 Wiring with CR1000 and AM16/32-series ......... 11
229 Sensor and CE4/CE8 Wiring with CR10XTCR ............................ 13
229 Sensor and CE4/CE8 Wiring with AM16/32 Multiplexer............. 16
ii
229 Heat Dissipation Matric Water
Potential Sensor
1. General Description
The 229 Heat Dissipation Matric Water Potential Sensor uses a heat dissipation
method to indirectly measure soil water matric potential. The active part of the
229 Soil Water Potential Sensor is a cylindrically-shaped porous ceramic body.
A heating element which has the same length as the ceramic body is positioned
at the center of the cylinder. A thermocouple is located at mid-length of the
ceramic and heating element. The position of the heating element and the
thermocouple is maintained by placing both inside a hypodermic needle. This
also protects the delicate wires. The volume inside the needle which is not
occupied by wiring is filled with epoxy.
FIGURE 1-1. A 229 Heat Dissipation Matric Water Potential Sensor is shown at the top. The
hypodermic assembly (without epoxy and ceramic) is shown just below. Cutaway view shows
longitudinal section of the needle with heater and thermocouple junction.
The ceramic cylinder has a diameter of 1.5 cm and a length of 3.2 cm. Three
copper wires and one constantan wire, contained in a shielded, burial-grade
sheath provide a path for connection to measuring instrumentation. An epoxy
section which is the same diameter as the ceramic matrix gives strain relief to
the cable.
The 229 is used to measure soil water matric potential in the range -10 kPa to
-2500 kPa. The method relies on hydraulic continuity between the soil and the
sensor ceramic for water exchange. The variability in heat transfer properties
among sensors makes individual calibration by the user a requirement. See
Section 6 for calibration information.
1
229 Heat Dissipation Matric Water Potential Sensor
Use of the 229 sensor requires a constant current source. Campbell Scientific
offers the CE4 and CE8 current excitation modules (Figure 1-2), which have
respectively four and eight regulated outputs of 50 milliamp ±0.25 milliamp.
All of the outputs of the excitation module are switched on or off
simultaneously by setting a single datalogger control port to its high or low
state.
The –L option on the model 229 Heat Dissipation Matric Water Potential
Sensor (229-L) indicates that the cable length is user specified. This manual
refers to the sensor as the 229.
FIGURE 1-2. CE4 and CE8 Current Excitation Modules
1.1 Compatibility
Compatible dataloggers include our 21X, CR7, CR10(X), CR23X, CR800,
CR850, CR1000, and CR3000. The 229 is not compatible with our CR200series, CR500, or CR510 dataloggers. The 229 can be connected with a
multiplexer. Compatible multiplexers include our AM16/32, AM16/32A, and
AM16/32B.
NOTE
When using multiplexers, the user should be aware that
switching currents of greater than 30 mA will degrade the
contact surfaces of the mechanical relays. This degradation will
adversely affect the suitability of these relays to multiplex low
voltage signals. Although a relay used in this manner no longer
qualifies for low voltage measurements, it continues to be useful
for switching currents in excess of 30 mA. Therefore, the user is
advised to record which multiplexer channels are used to
multiplex the 50 mA excitation for the 229-L sensors in order to
avoid using those channels for low voltage measurements in
future applications.
1.2 Measurement Principle
Movement of water between the 229 ceramic matrix and the surrounding soil
occurs when a water potential gradient exists. When the water potential of the
soil surrounding a 229 sensor changes, a water flux with the ceramic matrix
will occur. The time required for hydraulic equilibration of the water in the
soil and ceramic depends on both the magnitude of the water potential gradient
and the hydraulic conductivity. Typically this equilibration time is on the order
of minutes or tens of minutes.
2
229 Heat Dissipation Matric Water Potential Sensor
A change in the water potential and water content of the ceramic matrix causes
a corresponding change in the thermal conductivity of the ceramic/water
complex. As the water content in the ceramic increases, the thermal
conductivity of the complex also increases. At very low water contents, the
ceramic material controls the thermal conductivity. As water content in the
ceramic increases, water films are established between the solid particles,
resulting in a rapid increase in thermal conductivity. As the pores in the
ceramic continue to fill, the thermal conductivity becomes increasingly
controlled by the continuous water and the increase in thermal conductivity of
the ceramic/water complex approaches a constant value.
When a constant power is dissipated from the line heat source, the temperature
increase near the heat source will depend on the thermal conductivity of the
ceramic/water complex surrounding the heater. A temperature increase is
caused by heat that is not dissipated. As the water content and thermal
conductivity of the ceramic increases, the temperature increase as measured by
the thermocouple will be reduced because conduction of the thermal energy
from the heat source is greater. A drier sensor will have a lower thermal
conductivity, so the thermal energy will not dissipate as quickly and the
temperature rise will be greater. When 50 milliamps is passed through the
heating element for 30 seconds, the temperature increase ranges from
approximately 0.7ºC under wet conditions to 3.0ºC when dry. Figure 1-3
presents a typical temperature response in a silt loam.
3
200 kPa
100 kPa
50 kPa
temperature increase (C)
2.5
10 kPa
2
1.5
1
0.5
0
5
10
15
heating time (s)
20
25
30
FIGURE 1-3. Typical Temperature Response of 229 Sensor in Silt Loam Soil
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229 Heat Dissipation Matric Water Potential Sensor
2. Specifications
229
Measurement range:
-10 to -2500 kPa
Measurement time:
30 seconds typical
Thermocouple type:
copper / constantan (type T)
Dimensions:
1.5 cm (0.6”) diameter
3.2 cm (1.3”) length of ceramic cylinder
6.0 cm (2.4”) length of entire sensor
Weight:
10 g (0.35 oz) plus 23 g/m (0.25 oz/ft) of cable
Heater resistance:
34 ohms plus cable resistance
Resolution:
~1 kPa at matric potentials greater than -100 kPa
CE4/CE8
Output:
50 mA ±0.25 mA per channel, regulated
Output channels:
CE4: 4
CE8: 8
Current drain(while active): 25 mA + 50 mA * no. of 229’s connected to the
CE4 or CE8 output channels.
Dimensions:
CE4: 11.5 cm (4.5”) x 5.4 cm (2.1”) x 2.7 cm (1.1”)
CE8: 16.5 cm (6.5”) x 5.4 cm (2.1”) x 2.7 cm (1.1”)
Weight:
CE4: 131 g (4.6 oz)
CE8: 184 g (6.5 oz)
3. Installation
3.1 Orientation
For best measurement results, the 229 should be installed horizontally at the
desired depth of the soil. This will reduce distortion of typical vertical water
flux.
3.2 Contact
Good contact must exist between the ceramic matrix and the soil since the
measurement relies on water flux between the two. Adequate contact will
result if the sensor is ‘planted’ in a manner similar to that used for seedlings.
Sufficient contact in coarse texture soils such as medium and coarse sand can
be obtained by surrounding the ceramic portion with a slurry of fine silica
(sometimes referred to as silica flour).
4
229 Heat Dissipation Matric Water Potential Sensor
3.3 Equilibration and Saturation of the Sensor Before
Installation
The smaller the difference in water potential between the 229 ceramic and the
surrounding soil, the sooner equilibrium will be reached. Filling the ceramic
pores with liquid water will optimize the hydraulic conductivity between the
ceramic and soil.
Simple immersion of the sensors in water can leave some entrapped air in the
pores. Complete saturation can be closely approached if (1) deaerated water is
used, and (2) saturation occurs in a vacuum. Soaking the ceramic in free water
for 12 hours followed by soaking under a vacuum of ≥71 kPa (0.7 atm)
atmosphere for 1 hour results in complete saturation of the sensor.
4. Wiring
Table 4-1 shows wiring information for the 229 sensor and CE4 or CE8
excitation module when connecting sensors directly to the datalogger without
the use of a multiplexer. Figure 4-1 shows a simple schematic of these
connections.
See the multiplexer program in Section 5 for details on multiplexer wiring.
TABLE 4-1. 229 Sensor and CE4/CE8 Wiring
229 Wire Color
Function
CR10(X), CR23X, CR800, CR850,
CR1000, CR3000
Blue
Thermocouple High
High side of differential channel
Red
Thermocouple Low
Low side of differential channel
Green
Heater High
Black
Heater Low
Clear
Shield
G
CE4/CE8 Power
12V
CE4/CE8 Ground
G
CE4/CE8 Enable
Control Port
CE4/CE8
Current excitation channel
+12V
CTRL
5
229 Heat Dissipation Matric Water Potential Sensor
FIGURE 4-1. Schematic of Connections for Measurement of
a 229 Sensor
5. Example Programs
5.1 Choosing a Reference for the Thermocouple Readings
A fundamental thermocouple circuit uses two thermocouple junctions with one
pair of common-alloy leads tied together and the other pair connected to a
voltage readout device. One of the junctions is the reference junction and is
generally held at a known temperature. The temperature at the other junction
can be determined by knowing the voltage potential difference between the
junctions and the reference temperature.
A Campbell Scientific datalogger can read a single thermocouple junction
directly because the temperature at the wiring panel is measured with a
thermistor and this temperature is converted to a voltage which is then used as
a thermocouple reference. A thermocouple circuit voltage potential is affected
by the temperature of all dissimilar metal junctions.
When using a multiplexer with the 229 sensor, the temperature of the
multiplexer can be used as the reference temperature if a thermistor probe such
as the 107 is taped to the multiplexer panel near the 229 wires. Alternately, a
CR10XTCR can be used to get an accurate reading of the CR10X wiring panel
temperature and type T thermocouple wire (copper-constantan) can be used for
the signal wires between the differential voltage channel on the datalogger and
the appropriate common channels on the multiplexer (see program example #2
below). The CR23X, CR800, CR850, CR1000, and CR3000 can use their own
internal panel temperature measurement instead of the CR10XTCR and type T
thermocouple wire to the multiplexer common channels as previously noted.
The use of insulation or an enclosure to keep the multiplexer and temperature
sensor at the same temperature will improve measurement quality.
6
229 Heat Dissipation Matric Water Potential Sensor
5.2 Adjusting for Thermal Properties of Sensor During Early
Heating Times
The discussion presented at the beginning of the calibration section (Section 6)
describes how thermal properties can vary from sensor-to-sensor. The thermal
properties of the needle casing, wiring, and the amount of contact area between
the needle and the ceramic have a slight effect on the temperature response.
Most of the nonideal behavior of the sensor is manifest in the first second of
heating. The measurement is improved if the temperature after 1 s is
subtracted from some final temperature. A typical ΔT would be T(30 s) T(1 s).
5.3 Datalogger Program Structure and Multiplexers
The sequence of datalogger instructions for a 229 measurement is as follows:
1) Measure sensor temperature prior to heating.
2) Set a control port high to enable constant current excitation module and
being heating.
3) Wait for one second of heating and measure sensor temperature.
4) Wait for 29 more seconds of heating and measure sensor temperature.
5) Set control port low to disable the constant current excitation module and
end heating.
6) Calculate temperature rise by subtracting T(1 s) from T(30 s).
Since all of the output channels of the CE4 or CE8 are activated when the
control terminal is set high, power will be applied to all of the 229 sensors
connected to the current source. Inaccurate measurements can result if the
temperature of multiple sensors is simply read sequentially. The inaccuracy
can occur because a finite amount of time is required to execute each of the
temperature measurement instructions.
For example, a CR10X making multiple differential thermocouple readings
with 60 Hz rejection takes 34.9 ms to read one thermocouple, and 30.9 ms
more for each additional thermocouple. In a configuration where six 229
sensors are connected to a CE8 with their thermocouple wires connected
sequentially to the CR10X wiring panel, the sixth 229 sensor will heat for
154.5 ms longer than the first sensor each time its temperature is measured.
The amount of time between temperature measurement of the first sensor and
the last sensor can be as long as 0.5 seconds under some measurement
configurations.
The error caused by this difference in heating times can be minimized if the
sensors are connected to the constant current excitation module and datalogger
during calibration in exactly the same order they will be wired during field
deployment. The difference in heating times can be eliminated altogether by
heating the sensors one at a time through a multiplexer such as the AM16/32B.
7
229 Heat Dissipation Matric Water Potential Sensor
The AM16/32B multiplexer in 4x16 mode provides a convenient method to
measure up to sixteen 229 sensors. Since four lines are switched at once, both
the thermocouple and the heating element leads for each sensor can be
connected to a multiplexer channel. A measurement sequence is executed on
each sensor. See program example #2 below for instructions on wiring and
programming multiple 229 sensors on a multiplexer.
NOTE
When using multiplexers, the user should be aware that
switching currents of greater than 30 mA will degrade the
contact surfaces of the mechanical relays. This degradation will
adversely affect the suitability of these relays to multiplex low
voltage signals. Although a relay used in this manner no longer
qualifies for low voltage measurements, it continues to be useful
for switching currents in excess of 30 mA. Therefore, the user is
advised to record which multiplexer channels are used to
multiplex the 50 mA excitation for the 229-L sensors in order to
avoid using those channels for low voltage measurements in
future applications.
5.4 Temperature Correction
The rise in temperature over the 30 second heating time will be affected by the
initial temperature of the 229 sensor. The measurement can be corrected for
temperature if it can first be normalized. See Section 6.2.1 for more
information on normalizing the sensor output.
Correct for temperature in the datalogger program following these steps
outlined in Flint, et al. (2002):
1.
Determine normalized dimensionless temperature rise, Tnorm as described
in Section 6.2.1 equation 3
Tnorm =
2.
ΔTdry − ΔT
ΔTdry − ΔTwet
Determine where Tnorm falls on the dimensionless slope function, s*
5
4
3
2
s* = −0.0133Tnorm + 0.0559 Tnorm − 0.0747 Tnorm + 0.0203Tnorm + 0.011Tnorm + 0.0013
3.
Determine corrected dimensionless temperature rise, Tnormcorr
Tnormcorr = Tnorm − s * (Tsoil − Tcal )
4.
Determine corrected temperature rise, ΔTcorr
ΔTcorr = ΔTd − Tnormcorr (ΔTdry − ΔTwet )
The corrected temperature rise can now be used to calculate soil water matric
potential. Example 4 below shows how to make the temperature correction
with a CR1000.
8
229 Heat Dissipation Matric Water Potential Sensor
5.5 Example #1 — CR1000 with CE4 and Four 229s
Table 5-1 shows wiring information for reading four 229 sensors with a
CR1000 datalogger and CE4 current excitation module.
TABLE 5-1. Wiring for Four 229s with CR1000 and CE4
229
CR1000
229
CE4
229 #1Blue
1H
229 #1 Green
Channel 1
229 #1Red
1L
229 #1 Black
229 #2Blue
2H
229 #2 Green
229 #2Red
2L
229 #2 Black
229 #3Blue
3H
229 #4 Green
229 #3Red
3L
229 #3 Black
229 #4Blue
4H
229 #4 Green
229 #4Red
4L
229 #4 Black
Channel 2
Channel 3
Channel 4
229 #1 Clear
229 #2 Clear
229 #3 Clear
229 #4 Clear
12V
+12V
G
C1
CTRL
9
229 Heat Dissipation Matric Water Potential Sensor
'CR1000
SequentialMode
Const Num229 = 4
'Enter number of 229 sensors to measure
Dim LoopCount
Public RefTemp_C, StartTemp_C(Num229), Temp_1sec_C(Num229)
Public Temp_30sec_C(Num229), DeltaT_C(Num229)
Public Flag(1) as Boolean
Units StartTemp_C()=Deg C
Units DeltaT_C()=Deg C
DataTable(Matric,Flag(1),-1)
Sample(Num229,StartTemp_C(),FP2)
Sample(Num229,DeltaT_C(),FP2)
EndTable
BeginProg
Scan(30,Sec,1,0)
PanelTemp (RefTemp_C,250)
If IfTime (0,240,Min) Then Flag(1)=True 'Every 4 hours set Flag(1) high
If Flag(1) = True Then
'Flag(1) true triggers 229 readings
'Measure starting temperature before heating
TCDiff(StartTemp_C(),Num229,mV2_5C,1,TypeT,RefTemp_C,True,0,_60Hz,1,0)
PortSet (1,1 )
'Set C1 high to activate CE4
Delay (0,1,Sec)
'Wait 1 second
'Measure temperature after 1 second of heating
TCDiff(Temp_1sec_C(),Num229,mV2_5C,1,TypeT,RefTemp_C,True,0,_60Hz,1,0)
'Measure temperature after 30 second of heating
Delay (0,29,Sec)
'Wait 29 seconds more for total of 30 seconds heating
TCDiff(Temp_30sec_C(),Num229,mV2_5C,1,TypeT,RefTemp_C,True,0,_60Hz,1,0)
PortSet (1,0 )
'Set C1 low to deactivate CE4
For LoopCount=1 to Num229
'Calculate temperature rise
DeltaT_C(LoopCount)=Temp_30sec_C(LoopCount)-Temp_1sec_C(LoopCount)
'LoopCount=LoopCount+1
Next LoopCount
EndIf
'Ends Flag(1) true condition
CallTable(Matric)
'Call Data Tables and Store Data
Flag(1)=False
'Set Flag 1 false to disable 229 measurements
NextScan
EndProg
10
229 Heat Dissipation Matric Water Potential Sensor
5.6 Example #2 — CR1000 with AM16/32-series Multiplexer,
CE4 and Sixteen 229 Sensors with Temperature
Correction
Table 5-2 shows wiring information for connecting multiple 229 sensors and
CE4 excitation module to an AM16/32 multiplexer and CR1000 datalogger.
See Figure 6-4 for a schematic of this wiring configuration.
TABLE 5-2. 229 Sensor and CE4 Wiring with CR1000 and AM16/32-series
229
107
Function
CR1000
CE4
Multiplexer
(4x16 mode)
Blue
229 Thermocouple High
ODD H
(1H, 3H, etc)
Red
229 Thermocouple Low
ODD L
(1L, 3L, etc)
Green
229 Heater High
EVEN H
(2H, 4H, etc)
Black
229 Heater Low
EVEN L
(2L, 4L, etc)
Clear
229 Shield
G
CE4 Power
12V
CE4 Ground
G
CE4 Enable
C3
+12V
CTRL
1H
COM ODD H
1L
COM ODD L
CE4 current excitation channel
Channel 1
COM EVEN H
COM EVEN L
CE4 Ground
AM16 Power
12V
12V
AM16/32 Ground
G
GND
AM16/32 Enable
C1
RES
AM16/32 Advance
C2
CLK
Red
107 Signal
SE 3
Black
107 Excitation
EX1
Purple
107 Signal Ground
Clear
107 Shield
G
11
229 Heat Dissipation Matric Water Potential Sensor
'CR1000
SequentialMode
Const Num229 = 16
'Enter number of 229 sensors to measure
Const read229 = 60
'Enter Number of minutes between 229-L readings
Const CalTemp = 20
'Enter calibration temperature (deg C)
Dim i, dTdry(Num229), dTwet(Num229)
Dim Tstar, Tstarcorr, DeltaTcorr, s
Public RefTemp_C, StartTemp_C(Num229), Temp_1sec_C(Num229)
Public Temp_30sec_C(Num229), DeltaT_C(Num229), dTcorr(Num229)
Public Flag(1) as Boolean
Units StartTemp_C()=Deg C
Units DeltaT_C()=Deg C
Units dTcorr() = Deg C
DataTable(Matric,Flag(1),-1)
Sample(Num229,StartTemp_C(),FP2)
Sample(Num229,DeltaT_C(),FP2)
Sample(Num229,dTcorr(),FP2)
EndTable
Sub TempCorr
'Subroutine to temperature correct DeltaT_C
Tstar=(dTdry(i)-DeltaT_C(i))/(dTdry(i)-dTwet(i))
s=-0.0133*Tstar^5+0.0559*Tstar^4-0.0747*Tstar^3+0.0203*Tstar^2+0.011*Tstar+0.0013
Tstarcorr=Tstar-s*(StartTemp_C(i)-CalTemp)
DeltaTcorr=dTdry(i)-Tstarcorr*(dTdry(i)-dTwet(i))
EndSub
BeginProg
'Enter dTdry and dTwet values obtained for each probe (these are examples):
dTdry(1)= 3.421: dTdry(2)=3.417: dTdry(3)=3.433: dTdry(4)=3.418
dTdry(5)= 3.412: dTdry(6)=3.407: dTdry(7)=3.422: dTdry(8)=3.428
dTdry(9)= 3.399: dTdry(10)=3.377: dTdry(11)=3.405: dTdry(12)=3.406
dTdry(13)=3.422: dTdry(14)=3.431: dTdry(15)=3.423: dTdry(16)=3.408
dTwet(1)= 0.752: dTwet(2)=0.695: dTwet(3)=0.731: dTwet(4)=0.724
dTwet(5)= 0.709: dTwet(6)=0.752: dTwet(7)=0.739: dTwet(8)=0.737
dTwet(9)= 0.723: dTwet(10)=0.754: dTwet(11)=0.691: dTwet(12)=0.760
dTwet(13)=0.722: dTwet(14)=0.745: dTwet(15)=0.739: dTwet(16)=0.748
Scan(30,Sec,1,0)
Therm107(RefTemp_C,1,3,1,0,_60Hz,1.0,0.0)
'Reference temperature measurement
If IfTime (0,read229,Min) Then Flag(1)=True
'Set Flag(1) high based on time
If Flag(1) = True Then
'Flag(1) triggers 229 readings
PortSet(1,1)
'Set C1 High to turn on multiplexer
For i=1 to Num229
PulsePort(2,10000)
'Switch to next multiplexer channel
'Measure starting temperature before heating
TCDiff(StartTemp_C(i),1,mV2_5C,1,TypeT,RefTemp_C,True,0,_60Hz,1,0)
PortSet (3,1 )
'Set C3 high to activate CE4
Delay (0,1,Sec)
'Wait 1 second
'Measure temperature after 1 second of heating
TCDiff(Temp_1sec_C(i),1,mV2_5C,1,TypeT,RefTemp_C,True,0,_60Hz,1,0)
Delay (0,29,Sec)
'Wait 29 seconds more
12
229 Heat Dissipation Matric Water Potential Sensor
'Measure temperature after 30 second of heating
TCDiff(Temp_30sec_C(i),1,mV2_5C,1,TypeT,RefTemp_C,True,0,_60Hz,1,0)
PortSet (3,0 )
'Set C3 low to deactivate CE4
DeltaT_C(i)=Temp_30sec_C(i)-Temp_1sec_C(i)'Calculate temperature rise
Call TempCorr
'Call temperature correction subroutine
dTcorr(i)=DeltaTcorr
Next i
EndIf
'Ends Flag(1) high condition
PortSet(1,0)
'Turn multiplexer Off
CallTable(Matric)
'Call Data Tables and Store Data
Flag(1)=False
'Disable 229 measurements
NextScan
EndProg
5.7 Example #3 — CR10X with 229 Sensor
Table 5-3 shows wiring information for reading a single sensor with a CR10X
datalogger, CE4 current excitation module, and CR10XTCR thermocouple
reference temperature sensor.
TABLE 5-3. 229 Sensor and CE4/CE8 Wiring with CR10XTCR
229
Function
CR10(X)
CE4
CR10XTCR
Blue
229 Thermocouple High
1H
Red
229 Thermocouple Low
1L
Green
229 Heater High
Black
229 Heater Low
Clear
229 Shield
G
CE4/CE8 Power
12V
CE4/CE8 Ground
G
CE4/CE8 Enable
C1
CR10XTCR Signal
2H
Red
CR10XTCR Signal Return
AG
Clear
CR10XTCR Excitation
E3
Black
Channel 1
+12V
CTRL
13
229 Heat Dissipation Matric Water Potential Sensor
;{CR10X}
;Program to read 1 229-L sensor
;Reading 1 sensor takes 30 seconds
*Table 1 Program
01: 60
Execution Interval (seconds)
1: If time is (P92)
1: 0
Minutes (Seconds --) into a
2: 60
Interval (same units as above)
3: 11
Set Flag 1 High
2: If Flag/Port (P91)
1: 11
Do if Flag 1 is High
2: 30
Then Do
3: Temp (107) (P11)
1: 1
Reps
2: 3
SE Channel
3: 3
Excite all reps w/E3
4: 1
Loc [ Ref_Temp ]
5: 1.0
Multiplier
6: 0.0
Offset
;Measure reference temperature
4: Thermocouple Temp (DIFF) (P14)
;Measure initial sensor temperature
1: 1
Reps
2: 1
2.5 mV Slow Range
3: 1
DIFF Channel
4: 1
Type T (Copper-Constantan)
5: 1
Ref Temp (Deg. C) Loc [ Ref_Temp ]
6: 2
Loc [ Tinit_1 ]
7: 1.0
Mult
8: 0.0
Offset
5: Do (P86)
1: 41
;Turn on CE8
Set Port 1 High
6: Excitation with Delay (P22)
;Wait one second before taking first reading
1: 1
Ex Channel
2: 0000
Delay W/Ex (0.01 sec units)
3: 100
Delay After Ex (0.01 sec units)
4: 0
mV Excitation
7: Thermocouple Temp (DIFF) (P14)
;Take 1 second temperature reading
1: 1
Reps
2: 1
2.5 mV Slow Range
3: 4
DIFF Channel
4: 1
Type T (Copper-Constantan)
5: 1
Ref Temp (Deg. C) Loc [ ref_temp ]
6: 3
Loc [ T1sec_1 ]
7: 1.0
Mult
8: 0.0
Offset
14
229 Heat Dissipation Matric Water Potential Sensor
8: Excitation with Delay (P22)
;Wait 29 more seconds for next reading
1: 1
Ex Channel
2: 0
Delay W/Ex (0.01 sec units)
3: 2900
Delay After Ex (0.01 sec units)
4: 0000
mV Excitation
9: Thermocouple Temp (DIFF) (P14)
;Take 30 second temperature reading
1: 1
Reps
2: 1
2.5 mV Slow Range
3: 4
DIFF Channel
4: 1
Type T (Copper-Constantan)
5: 1
Ref Temp (Deg. C) Loc [ ref_temp ]
6: 4
Loc [ T30sec_1 ]
7: 1.0
Mult
8: 0.0
Offset
10: Do (P86)
1: 52
11: Z=X-Y (P35)
1: 4
2: 3
3: 5
12: Do (P86)
1: 10
;turn off CE8
Set Port 2 Low
;Calculate delta T
X Loc [ T30sec_1 ]
Y Loc [ T1sec_1 ]
Z Loc [ deltaT_1 ]
;Save readings to final storage
Set Output Flag High (Flag 0)
13: Set Active Storage Area (P80)
1: 1
Final Storage Area 1
2: 229
Array ID
14: Real Time (P77)
1: 1220
Year,Day,Hour/Minute (midnight = 2400)
15: Sample (P70)
1: 1
Reps
2: 2
Loc [ Tinit_1 ]
16: Sample (P70)
1: 1
Reps
2: 5
Loc [ deltaT_1 ]
17: Do (P86)
1: 21
;Disable flag 1
Set Flag 1 Low
18: End (P95)
15
229 Heat Dissipation Matric Water Potential Sensor
5.8 Example #4 — CR10X with AM16/32-series, CE4, and
Sixteen 229 Sensors
Table 5-4 shows wiring information for connecting multiple 229 sensors and
CE4 or CE8 excitation module to an AM16/32-series multiplexer and CR10X
datalogger. A CR10XTCR or 107 probe should be used for the reference
temperature measurement as described at the beginning of this section. See
Figure 6-4 for a schematic of this wiring configuration.
TABLE 5-4. 229 Sensor and CE4/CE8 Wiring with AM16/32 Multiplexer
229
107/CR10XTCR
Function
CR10(X)
CE4/CE8
AM16/32
(4x16 mode)
Blue
229 Thermocouple High
ODD H
(1H, 3H, etc)
Red
229 Thermocouple Low
ODD L
(1L, 3L, etc)
Green
229 Heater High
EVEN H
(2H, 4H, etc)
Black
229 Heater Low
EVEN L
(2L, 4L, etc)
Clear
229 Shield
G
CE4/CE8 Power
12V
CE4/CE8 Ground
G
CE4/CE8 Enable
C1
+12V
CTRL
1H
COM ODD H*
1L
COM ODD L*
Channel 1
CE4/CE8 current
excitation channel
COM EVEN H
COM EVEN L
CE4/CE8 ground
AM16/32 Power
12V
12V
AM16/32 Ground
G
GND
AM16/32 Enable
C2
RES
AM16/32 Advance
C3
CLK
Red
107 Signal
SE5
Black
107 Excitation
E1
Purple (Clear for
CR10XTCR)
107 Signal Ground
AG
Clear (107 only)
107 Shield
G
* Run copper wire from COM ODD H and COM ODD L to 1H/1L if using a
107 probe taped to the AM16/32 as the reference temperature sensor. If using
a CR10XTCR as the reference temperature sensor, run copper wire from COM
ODD H to 1H and run constantan wire from COM ODD L to 1L.
16
229 Heat Dissipation Matric Water Potential Sensor
;{CR10X}
;Program to read 16 229-L sensors using 1 AM16/32 multiplexer
;and 1 CE4 or CE8 constant current interface
;Manually set Flag 1 high to force readings
*Table 1 Program
01: 30
Execution Interval (seconds)
1: Batt Voltage (P10)
1: 1
Loc [ Batt_Volt ]
2: If time is (P92)
1: 0
Minutes (Seconds --) into a
2: 60
Interval (same units as above)
3: 11
Set Flag 1 High
;Set Flag 1 high each hour
3: If Flag/Port (P91)
1: 11
Do if Flag 1 is High
2: 30
Then Do
4: Do (P86)
1: 4
;Turn on AM16/32
Set Port 2 High
5: Beginning of Loop (P87)
1: 0
Delay
2: 16
Loop Count
;Loop of 16 for 16 sensors on AM16/32
6: Do (P86)
1: 73
;Advance to next multiplexer channel
Pulse Port 3
7: Temp (107) (P11)
1: 1
2: 5
3: 1
4: 2
5: 1.0
6: 0.0
Reps
SE Channel
Excite all reps w/E1
Loc [ Tref_C ]
Mult
Offset
;Measure reference temperature
;This is the same instruction
;for both the CR10X and 107
8: Thermocouple Temp (DIFF) (P14)
;Read initial temperature
1: 1
Reps
2: 21
10 mV, 60 Hz Reject, Slow Range
3: 1
DIFF Channel
4: 1
Type T (Copper-Constantan)
5: 2
Ref Temp (Deg. C) Loc [ Tref_C ]
6: 3
-- Loc [ Tinit_1 ]
7: 1.0
Mult
8: 0.0
Offset
9: Do (P86)
1: 41
;turn on CE8
Set Port 1 High
10: Excitation with Delay (P22)
;Delay of 1 second
1: 1
Ex Channel
2: 0000
Delay W/Ex (0.01 sec units)
3: 100
Delay After Ex (0.01 sec units)
4: 0
mV Excitation
17
229 Heat Dissipation Matric Water Potential Sensor
11: Thermocouple Temp (DIFF) (P14)
;Read thermocouple after 1 second of heating
1: 1
Reps
2: 21
10 mV, 60 Hz Reject, Slow Range
3: 1
DIFF Channel
4: 1
Type T (Copper-Constantan)
5: 2
Ref Temp (Deg. C) Loc [ Tref_C ]
6: 19
-- Loc [ T1s_1 ]
7: 1.0
Mult
8: 0.0
Offset
12: Excitation with Delay (P22)
;delay 29 more seconds
1: 1
Ex Channel
2: 0
Delay W/Ex (0.01 sec units)
3: 2900
Delay After Ex (0.01 sec units)
4: 0000
mV Excitation
13: Thermocouple Temp (DIFF) (P14)
;read temperature after 30 seconds heating
1: 1
Reps
2: 21
10 mV, 60 Hz Reject, Slow Range
3: 1
DIFF Channel
4: 1
Type T (Copper-Constantan)
5: 2
Ref Temp (Deg. C) Loc [ Tref_C ]
6: 35
-- Loc [ T30s_1 ]
7: 1.0
Mult
8: 0.0
Offset
14: Do (P86)
1: 51
;turn off CE8
Set Port 1 Low
15: Z=X-Y (P35)
1: 35
2: 19
3: 5
;Calculate temperature rise
-- X Loc [ T30s_1 ]
-- Y Loc [ T1s_1 ]
-- Z Loc [ dT_1 ]
16: End (P95)
17: Do (P86)
1: 52
;end of loop
;Turn off AM16/32
Set Port 2 Low
18: End (P95)
;End of then do statement
19: If Flag/Port (P91)
1: 11
Do if Flag 1 is High
2: 10
Set Output Flag High (Flag 0)
20: Set Active Storage Area (P80)
1: 1
Final Storage Area 1
2: 229
Array ID
21: Real Time (P77)
1: 1220
Year,Day,Hour/Minute (midnight = 2400)
18
229 Heat Dissipation Matric Water Potential Sensor
22: Sample (P70)
1: 16
Reps
2: 3
Loc [ Tinit_1 ]
;Sample 16 initial soil temperature readings
23: Sample (P70)
1: 16
Reps
2: 51
Loc [ dT_1
;Sample 16 delta T readings
24: Do (P86)
1: 21
]
Set Flag 1 Low
6. Calibration
6.1 General
The heat transfer properties of a 229 sensor depend both on the thermal
properties of the various sensor materials and on the interfaces between the
different materials. Heat transfer between the stainless steel needle containing
the heating element and thermocouple and the ceramic material depends on the
density of points-of-contact between two different materials. Heat transfer also
depends on the arrangement of the wires in the hypodermic needle and the
amount of contact between the needle and the ceramic. The uncontrollable
variability in heat transfer properties warrants individual calibration of the 229
sensors.
The calibration used to relate temperature increase and the soil water potential
is strictly empirical, and the functional expression of the relationship can take
several forms. The most commonly used function is:
ψ = exp(α * ΔT + β )
[1]
with ψ the soil water potential, exp the exponential function, ΔT the
temperature increase during the chosen heating period of time, α the slope and
β the intercept.
The relationship between the natural logarithm of soil water tension and the
temperature increase is linear which simplifies derivation of the calibration
function.
ln(| ψ |) = α * ΔT + β
[2]
(Soil water potential is a negative value and becomes more negative as soil
dries.) Figure 6-1 is a typical calibration and the data set is easily described
with linear regression. A power function works well in applications when
calibration is needed for -500 kPa ≤ matric potential ≤ -10 kPa. A power
function calibration has the form |ψ| = a*(ΔT)b with the multiplier, a, and
exponent, b, as fitted parameters.
A variety of calibration methods are suitable. The sole requirement is that the
water potential of the medium surrounding the sensor must be known. Either
the applied potential can be controlled at a specified value or the water
potential can be independently measured. Hanging water columns and
pressure plate extractors are typically used. Several data values which
19
229 Heat Dissipation Matric Water Potential Sensor
correspond to the water potential expected during sensor use should be
included in the calibration
ln(|matric water potential|) (kPa)
5.5
5
4.5
4
3.5
3
2.5
2
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
Temperature rise,T(30s) - T(1s), deg-C
FIGURE 6-1. Data Points (x) and Regression for Typical Calibration
6.2 Normalized Temperature Change and Correction for Soil
Temperature
6.2.1 Normalized Temperature Change
The effect of sensor-to-sensor variability can be reduced by using normalized
temperature increase, ΔTnorm , described by
ΔTnorm =
where
ΔTdry − ΔT
ΔTdry − ΔTwet
[3]
ΔTdry is the change in temperature during measurement when the 229
ΔTwet is the change in temperature during measurement when
the 229 sensor is fully saturated and ΔT is the change in temperature during
the measurement. The range of ΔTnorm will be 0 to 1 with ΔTnorm equal 0 for
sensor is dry,
dry soil and 1 for saturated soil.
The ΔTwet value requires full saturation of the ceramic. The ceramic portion
of the sensor must be immersed in water while under vacuum to remove all air
from the ceramic. Measurements at the factory show that complete saturation
will occur after 3-4 hours at 0.7 atmospheres (70 kPa) vacuum. However, the
arrangement of the sensors in the water during vacuum extraction may hamper
movement of air from the ceramic. When tapping or jarring the container
shows no release of bubbles, the ceramic is saturated.
20
229 Heat Dissipation Matric Water Potential Sensor
The ΔTdry value requires that the ceramic be as dry as possible. Sensors can
be dried with desiccant or in an oven at temperature no greater than 60 °C.
Temperatures greater than 60 °C may damage the sensor cable.
Reece (1996) suggested that inverse thermal conductivity can also be used as a
normalization technique but work by Campbell Scientific has not shown
significant advantage for this method over normalization as described by
equation [3].
A calibration equation using ΔTnorm and having a form similar to equation [2]
is
ln(| ψ |) = α * ΔTnorm + β .
[4]
The slope of equation [2] will be positive while the slope of equation [4] will
be negative.
6.2.2 Correction for Soil Temperature
The heat dissipation method of matric potential measurement is sensitive to
temperature and correction for the temperature dependence may be necessary
to maintain accuracy of the measurement. If the soil temperature when the
matric potential measurements are made is close to the temperature at the time
of sensor calibration there is no need for correction.
The 229 measurement method uses heat transfer away from a heated line
source and the heat transfer depends on the thermal conductivity of the
ceramic. The thermal conductivity of the ceramic depends on the combination
of the conductivities of water, vapor and solid constituents. The vapor
component has a strong temperature dependence and consequently imparts
sensitivity of the measurement to temperature. The sensitivity is related to the
difference between the temperature of the sensor at time of measurement (soil
temperature) and the temperature of the sensor during calibration. Figures 6-2
and 6-3 show the response of the matric potential measurement for a range of
temperatures when the calibration temperature is 20 C.
21
229 Heat Dissipation Matric Water Potential Sensor
600
400
error (-kPa)
200
0
200
400
600
0
500
1000
matric potential (-kPa)
1500
2000
10 degrees C
16 degrees C
18 degrees C
22 degrees C
24 degrees C
30 degrees C
FIGURE 6-2. Measurement error for range of soil temperatures and wide range of matric potential.
22
229 Heat Dissipation Matric Water Potential Sensor
200
error (-kPa)
100
0
100
200
0
100
200
300
matric potential (-kPa)
400
500
10 degrees C
16 degrees C
18 degrees C
22 degrees C
24 degrees C
30 degrees C
FIGURE 6-3. Measurement error for range of soil temperatures and wetter range of matric potential.
A temperature correction for the difference in temperature at time of calibration
and time of measurement is provided in the work of Flint et al., 2002. To
implement the correction, normalized temperature must be used for the
calibration variable. Normalized temperature is as defined in equation [3].
The correction procedure is an iterative method. Examples of implementing
the iterative routine with dataloggers are given in programming examples 5 and
6. The general sequence for using the temperature correction method is:
1.
Determine ΔTdry and ΔTwet prior to calibration and with sensor
conditions as described in 6.2.1. The recommended ΔT is the sensor
temperature after 30 seconds of heating minus the sensor temperature
after 1 second of heating.
23
229 Heat Dissipation Matric Water Potential Sensor
2.
With the sensor in place, use the ΔT from the in situ measurement
along with the
calculate
ΔTdry , ΔTwet values for the particular sensor to
ΔTnorm .
3.
Implement the iterative temperature correction as presented in
datalogger example program #4 to obtain a corrected ΔTnorm .
4.
Use the corrected
ΔTnorm in the calibration equation, e.g. equation
[4].
6.3 Using Pressurized Extraction Methods
Pressurized extraction methods use a porous material (typically ceramic or
stainless steel) to separate the pressurized soil sample from atmospheric
pressure. One side of the porous material is in contact with the soil sample and
the other side is at atmospheric pressure. A simple configuration is a cylinder
which has the porous material as a bottom and a solid cap at the top which
provides a pressure-tight seal after the soil sample has been placed on the
porous bottom. This allows pressurization of the soil sample which will force
water from the soil. The air entry pressure of the porous material is dependent
on the effective pore diameter and must be greater than the maximum pressure
applied during calibration. When a specified pressure is applied, all soil water
at water potentials greater than -1*(applied pressure) will move through the
soil sample and through the porous bottom. The time required for the soil
water to leave the sample system depends on the pressure gradient and the
hydraulic conductivity of the soil and the porous bottom plate.
Equilibration of water potential throughout the system must be attained or the
accuracy of the water potential measurement using the derived calibration will
be reduced. The simplest way to confirm equilibration is to repeat the
measurements at a given applied pressure until readings do not change.
6.4 General Description of Calibration/Measurement Process
using Pressure Plate Extractor
A pressure plate extractor consists of a high-pressure vessel, a porous plate and
tubing to remove soil water from the soil sample. A ring with diameter slightly
less than the diameter of the ceramic plate can be used to hold the soil in which
the 229 sensors are buried. Smaller rings can also be used. The porous plate,
soil and 229 sensors must be thoroughly saturated at the beginning of the
calibration routine. Complete saturation of the ceramic plate and 229 sensors
is better achieved by applying vacuum. The number of sensors that can be
calibrated in a single pressure vessel will depend on the 229 cable lengths
because of the limited space in the vessel.
The general calibration sequence is:
24
1.
The extractor plate, soil and sensors are saturated and positioned in the
pressure vessel.
2.
The lowest calibration pressure chosen by the user (>10 kPa) is applied
and the soil solution, which is held by the soil at an energy level less than
that applied, is allowed to leave the pressurized system.
229 Heat Dissipation Matric Water Potential Sensor
3.
Measurements of sensor temperature response are made periodically to
determine if equilibration is attained. This will require depressurization of
the pressure vessel if a pressure-tight feedthrough is not used. Prior to
depressurization, it is important that the effluent hose be blocked by
clamping or other method to prevent solution from re-entering the soil and
sensors.
4.
When equilibration is attained, the effluent hose is blocked, the vessel is
depressurized and 229 measurements are recorded.
5.
The next calibration pressure is then applied and the process repeated.
Pressure-tight bulkhead connectors are available for some pressure vessels.
Determining whether equilibrium has been reached is simplified when using a
feedthrough connector since the pressure vessel doesn’t have to be
depressurized and opened for each reading. A temporary connector can be
used to disconnect the datalogger from the other components installed in the
pressure vessel.
Pressurized readings can be used for determining equilibrium but should not be
used for calibration data. Thermal properties are affected by pressure and
calibration data should be collected at the same pressure the sensors will be
used—in most cases this is atmospheric pressure.
6.4.1 Wiring for Calibration using Pressure Plate Extractor
The wiring arrangement of Figure 6-4 depicts a datalogger with an AM16/32
multiplexer, a CE4 current source and a thermistor being used in a typical 229
calibration arrangement. This is similar to the wiring for program example #2
and #4 (see Section 5). All components except the datalogger can be placed
inside a 5 bar pressure vessel. If this method is used, the electronic
components in the pressure vessel should be protected against moisture
damage. Place the components in a container such as plastic bag with
desiccant.
25
229 Heat Dissipation Matric Water Potential Sensor
FIGURE 6-4. Datalogger and Peripheral Connections for 229 Calibration
7. Maintenance
The 229 does not require maintenance after it is installed in the soil. The
datalogger, current excitation module, and multiplexer, if used, should be kept
in a weatherproof enclosure. Periodic replacement of the desiccant in the
enclosure is required to keep the electronics dry and free of corrosion.
26
229 Heat Dissipation Matric Water Potential Sensor
8. Troubleshooting
Symptom
Possible Cause
Action
Temperature reading is
offscale (-6999 or NAN)
Thermocouple wire not
connected to correct
datalogger channel
Check program to see which differential input
channel 229 should be connected to and verify that it
has a good connection to that channel
Break in thermocouple wire
Use ohm-meter to measure resistance between blue
and red wires. The reading in ohms should be
approximately the cable length in feet. An open
circuit indicates a break in the wire
Multiplexer not operating
properly
Make sure that multiplexer has 12V between 12V
and GND terminals.
Check for a good electrical connection on the wires
that connect RES and CLK to datalogger control
ports.
Check for a good electrical connection on the wires
going from the common channels to the datalogger
and the current excitation module.
Check program to make sure that the control port
connected to RES is being set high and the control
port connected to CLK is being pulsed.
DeltaT reading close to
zero
Heater wire broken or not
properly connected
Check resistance between terminal screws for green
and black wires. It should read 34 - 40 ohms.
Current excitation module
not turning on
Check for 12V between +12V and ground terminals
Check for good electrical connection on wire
connecting CTRL with datalogger control port
Check program to make sure that control port
connected to CTRL is being set high
Temperature decreases
during heating
Thermocouple wires
reversed
Make sure blue wire is on the high side of the
differential input channel and red is on the low side
Readings for first sensor
on multiplexer are all right,
but all others read zero
All readings are being
written to the same input
location (Edlog dataloggers)
Check program to make sure measurement
instructions in the multiplexer loop are indexed
(-- next to the input location number. Press F4 or C
to toggle the --)
27
229 Heat Dissipation Matric Water Potential Sensor
9. References
Flint, A. L., G. S. Campbell, K. M. Ellett, and C. Calissendorff. 2002.
Calibration and Temperature Correction of Heat Dissipation Matric Potential
Sensors. Soil Sci. Soc. Am. J. 66:1439–1445.
Reece, C.F. 1996. Evaluation of a line heat dissipation sensor for measuring
soil matric potential. Soil Sci. Soc. Am. J. 60:1022–1028.
28
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