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 .
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.
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 ................
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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(
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 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 Del
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
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.
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.
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 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 D
9 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.
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.
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.
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].
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.
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.
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.
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.
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.
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.
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.
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