229 Heat Dissipation Matric
Water Potential Sensor
Revision: 5/09
Copyright © 2006-2009
Campbell Scientific, Inc.
Warranty and Assistance
The 229 HEAT DISSIPATION MATRIC WATER POTENTIAL
SENSOR 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 carrier prepaid. This warranty shall not apply to any
CAMPBELL SCIENTIFIC, INC. 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. CAMPBELL
SCIENTIFIC, INC. is not liable for special, indirect, incidental, or
consequential damages.
Products may not be returned without prior authorization. The following
contact information is for US and International customers residing in countries
served by Campbell Scientific, Inc. directly. Affiliate companies handle
repairs for customers within their territories. Please visit
www.campbellsci.com to determine which Campbell Scientific company
serves your country.
To obtain a Returned Materials Authorization (RMA), contact CAMPBELL
SCIENTIFIC, INC., phone (435) 753-2342. After an applications engineer
determines the nature of the problem, an RMA number will be issued. Please
write this number clearly on the outside of the shipping container.
CAMPBELL SCIENTIFIC's shipping address is:
CAMPBELL SCIENTIFIC, INC.
RMA#_____
815 West 1800 North
Logan, Utah 84321-1784
For all returns, the customer must fill out a “Declaration of Hazardous Material
and Decontamination” form and comply with the requirements specified in it.
The form is available from our website at
completed form must be either emailed to shanna@campbellsci.com
to 435-750-9579. Campbell Scientific will not process any returns until we
receive this form. If the form is not received within three days of product
receipt or is incomplete, the product will be returned to the customer at the
customer’s expense. Campbell Scientific reserves the right to refuse service on
products that were exposed to contaminants that may cause health or safety
concerns for our employees.
www.campbellsci.com/repair
. A
or faxed
CAMPBELL SCIENTIFIC, INC. does not accept collect calls.
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 Choosing a Reference for the Thermocouple Readings ...........................6
5.2 Adjusting for Thermal Properties of Sensor During Early Heating Times7
5.3 Datalogger Program Structure and Multiplexers......................................7
5.4 Temperature Correction............................................................................8
5.5 Example #1 — CR1000 with CE4 and Four 229s....................................9
5.6 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 Norma liz e d T emp era t u r e Cha n g e a n d Cor r e c t ion f o r S o i l Te mpe r a t u r e......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
List of Tables
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
4-1. 229 Sensor and CE4/CE8 Wiring........................................................... 5
5-1. Wiring for Four 229s with CR1000 and CE4......................................... 9
5-2. 229 Sensor and CE4 Wiring with CR1000 and AM16/32-series......... 11
5-3. 229 Sensor and CE4/CE8 Wiring with CR10XTCR............................ 13
5-4. 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 matr
ix 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, th
ceramic material controls the thermal conductivity. As water content in t
ceramic increases, water films are established between the solid particles
e
he
,
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 tempera
increase near the heat source will depend on the thermal conductivity of
ture
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 measure
the thermocouple will be reduced because conduction of the thermal ener
d by
gy
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 th
temperature rise will be greater. When 50 milliamps is passed through the
heating element for 30 seconds, the temperature increa
se ranges from
e
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 kP a
100 kP a
2.5
2
1.5
temper atu re increase (C)
1
0.5
0 5 10 15 20 25 30
FIGURE 1-3. Typical
50 kPa
10 kPa
hea tin g tim e (s)
Temperature Response of 229 Sensor in Silt Loam Soil
3
229 Heat Dissipation Matric Water Potential Sensor
2. Specifications
229
Measurement range: -10 to -2500 kPa
Measurement 30 seconds typical time:
hermocouple type:
T copper / constantan (type T)
3. Installation
Dimensions: 1.5 cm (0.6”) diameter
3.2 cm (1.3”) length of ceramic
6.0 cm (2.4”) length of entire se
Weight: 10 g (0.35 oz) plus 23 g/m (0.25 oz/ft) of cable
Heater resis
Resolutio ~1 kPa at matric potentials greater than -1
CE4/CE8
Output: 50 mA ±0.25 mA per channel, regulated
Output channel CE4: 4 CE8: 8 s:
urrent drain(while active):
C 25 mA + 50 mA * no. of 229’s connected to the
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”)
ght: CE4: 131 g (4.6 oz) CE8: 184 g (6.5 oz)
Wei
cylinder
nsor
tance: 34 ohms plus cable resistance
n: 00 kPa
CE4 or CE8 output channels.
4
3.1 Orientation
3.2 Contact
For best measurement results, the 229 should be installed horizontally at the
desired depth of the soil. This will reduce distortion of typical vertical wate
flux.
Good contact must exist between the ceramic matrix and the soil since the
measurement relies on water flux betwe
result if the sensor is ‘planted’ in a manner similar to that used for seed
Sufficient contact in coarse texture soils such as medium and co
be obtained by surrounding the ceramic portion with a slurry of
metimes referred to as silica flour).
(so
en the two. Adequate contact will
lings.
arse sand can
fine silica
r
229 Heat Dissipation Matric Water Potential Sensor
3.3 Equilibratio
Installation
4. Wiring
n and Saturation of the Sensor Before
The smaller the difference in water potential between the 229 ceramic an
surrounding soil, the sooner equilibrium will be reached. Filling th
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
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
for 12 hours fo
atmosphere for 1 hour results in complete saturation of the sensor.
Table 4-1 shows wiring information for the 229 sensor and CE4 or CE8
excita directly to the datalogger without
tion module when connecting sensors
the us sim tic of these
e of a multiplexer. Figure 4-1 shows a
conne
ctions.
e m 5 for de multiplexer wiring.
See th ultiplexer program in Section tails on
BLE 4- 229 Sensor and CE4/CE8 Wiring
TA 1.
llowed by soaking under a vacuum of ≥71 kPa (0.7 atm)
ple schema
d the
e ceramic
the
water
229 Wire Color
Blue gh side of differential channel Thermocouple Hi High
Red w side of differential channel Thermocouple Low Lo
Green excitation channel Heater High Current
Black Heater Low
Clear Shield G
CE4/CE8 Power 12V +12V
CE4/CE8 Ground G
CE4/CE8 Enable Control Port CTRL
Function
10(X), CR23X, CR800, CR850,
CR
1000, CR3000 E4/CE8
CR
C
5
229 Heat Dissipation Matric Water Potential Sensor
FIGURE 4-1. Schematic of Connections for Measurement of
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.
a 229 Sensor
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 calibratio n sect i on (Sect i o n 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
Loading...