Measuring sand electrical conductivity by cheap four-electrode probes in CanTho University, Vietnam L A Tuan1, V M Tri2 and G C L Wyseure3 Department of Environmental and Water Resources Engineering, College of Technology, CanTho University, Vietnam Department of Electronics Engineering, College of Technology, CanTho University, Vietnam Laboratory for Land and Water Management, Faculty of Biosciences and Engineering, Catholic University of Leuven, Belgium Correspondence to: L A Tuan (latuan@ctu.edu.vn) Abstract An electrical current system including four-electrode probes connecting with a Humusoft card and a computer have been made in CanTho University, Vietnam for measuring electrical conductivity (EC) of the salinity solution flowing through a horizontal sand column The whole system is rather cheap, compact and reliable for developing countries as Vietnam The experimental results with potassium chlorite solution show that there are good linear regressions between the calibrated EC measured by four-electrode probes and EC measured by Orion EC-meter Key words: four-electrode probes, electrical conductivity, solution, linear regression Introduction The methods of four-electrode have been used for evaluating soil water content and salinity under field conditions by McCorkle, 1931; Edlefsen and Anderson, 1941; Rhoades and Ingvalson, 1971 (Landviser, 2005) Then, these methods were combined practically with thermal conductivity probes for determination soil electrical conductivity in the field (Fritton, 1974, Nadler, 1980, 1981) and other soil properties (Pozdnyakov, 2002) Four-electrode methods have been applied widely not only in geology and soil sciences but also in the fields of archaeology and criminology when finding the tracers of burial places (William, 2003) and environmental pollution as surveying oil-mining contamination to the soil (Pozdnyakova, 1999) Commonly, four-electrode probe methods combined with geo-statistics methods are used for soil mapping (Halvorson, 1976) Parallel with the field experiments, the theory of four-electrode probe has been well developed and verified (Rhoades et al., 1976, 1989, 1999; Corwin and Lesch, 2005; Shmulik, 2005) Beside practices, many authors have tried to reduce the costs for making four-electrode probes (Austin, 1979; Rhoades, 1979) The four-electrode salinity sensor method has been being applied firstly in CanTho University (CTU), Vietnam for measuring soil electrical conductivity (EC) on the purpose of determining hydraulic characteristics of water movement on subsurface wetland This way can limit the time and effort involved of sampling and laboratory analysis and it may avoid the destructive nature in experimental column studies (Konukcu et al., 2002) Theory A simple four-electrode probe technique embraces four electrodes arranged with a fixed distance between them, as L illustrated in Fig The electrode can be made of any conductive metal as stainless steel or copper The conductivity of the soil can be determined by knowing the ratio of the electrical current source (I) passed the two outer electrodes (rod and rod 4) and the potential drop voltages (V) between the two inner electrodes (rod and rod 3) measured In this process, the solution acts as an electrical conductor, so the cations move to the negative electrode and the anions to the positive electrode Amperemeter, I Electrical alternating current A V Solution - - - - - - - - Voltage, V Potential drop - + + - - + - L + + + + + + + + + + L + + L Figure Simplified four-electrode probe theory of ions migration in solution In theory, EC - expressed in Siemens (S) - of the soil particles is determined by converting the value of electrical resistivity (ER): EC (1) ER ER of a material is defined as follows: A ÄV ÄV ER K KR (2) L I I where K = L/I is a geometrical coefficient, L is distance between the electrodes (cm) and A is the effective cross-sectional area of the electrodes (cm2) V is measured by a voltage (V) I is magnitude of current and determined by amperemeter, (mA) R is the resistance of the solution, R can be calculated by Ohm's law (V = R.I) and expressed in Ohms So, ER in (2) is in Ohm.cm and EC in (1) is in dS/m In fact, the bulk EC of a soil is not consisted of the contributions of the ions of the soil solution but also the contribution of ions of the solid soil particles (Nadler, 1980) Rhoades et al (1976) has described the relationship: ECa = ECw T + ECs (3) where ECw is the electrical conductivity of the soil solution, is the volumetric water content, T is a transmission coefficient, T = a + b with a and b being empirical coefficients, ECs is the solid phase surface conductivity Initial Experiments and Circuit Description On March 2004, some initial experiments on a simple four-electrode sensor have been done for establishing a relationship between the EC values measured from a Consort ECmeter and the calibrated EC values measured by four-electrode sensor, as in Fig Three replicates in randomized blocks experiment in electricity conductivities have been designed with two factors: frequency (Hz) of the circuit and water solution with potassium chlorite (gKCl/L), as presented in Table Figure 2: Four-electrode experiment applied in KU Leuven in March, 2004 Table 1: Factors in experiments Frequency (Hz) Solution (g KCl/L) F1 F2 F3 S1 S2 S3 110 230 340 0.5 0.745 2.5 S4 5.0 The system resistance was 10.17 Ohm, ECcalbr of standard water measured by the Consort EC-meter was 1.278 dS at 20C, the temperature correction coefficient was 0.019 As seen as Fig 3, the results show a good linear regression The experiment has also been conducted with replicates of treatments (Solutions: 0.00; 0.745; 1.50; 2.50; 3.50 and 4.50 g KCl/L), the constant frequency was kept at 330 Hz These results also show a good linear regression as presented in Fig At CTU then, three groups of four-electrode sensors were designed as drawn in Fig The sensor is a stainless steel rod of mm outside diameter, which is inserted perpendicular in the plastic block and reach cm outside of the block The distance between every two rods is cm ECm (dS/m) ECfes (dS/m) R2 = 0.990623 0.1000 0.0800 0.0600 ECfes 0.0400 Linear (ECfes) 0.0200 0.0000 200 400 600 800 1000 900 800 700 600 500 400 300 200 100 10.00 y = 161.32x E CCons ort (dS/m ) y = 0.000142x + 0.003699 0.1200 Linear regression analysis between ECefsT vs ECConsort EC mea sure d (ECm) vs Solution (g KCl/L) EC from 4-electrode sensor (ECfes) vs Fre quency (Hz) 0.1400 R = 0.9991 ECm Linear (ECm) 8.00 6.00 4.00 y = 69.491x - 0.2227 2.00 R2 = 0.9908 0.00 Frequency (Hz) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 ECfesT (dS/m) Solution (g KCl/L) Figure 3: Results of replicates in randomized blocks with factors 400 y = 158.48x + 9.0288 R2 = 0.999 0.00 1.00 2.00 3.00 4.00 5.00 0.12 0.12 0.10 0.10 ECmT (dS/m) 600 200 ECConsort vs ECmT Solution vs ECmT ECmT (dS/m) ECConsort (dS/m) Solution vs ECConsort 800 0.08 0.06 y = 0.0241x + 0.0031 R2 = 0.9958 0.04 0.02 0.00 0.00 0.08 0.06 y = 0.0002x + 0.0013 R2 = 0.9972 0.04 0.02 1.00 2.00 3.00 4.00 Sollution (g KCL/L) Solution (g KCl/L) 5.00 0.00 200 400 600 800 ECConsort (dS/m) Figure 4: Results of replicates in randomized with treatments Wires connected to rods cm cm cm cm Sensor Groove (hollow) Plastic block with groups of 4-electrodes (front and back looks) PVC plates; watertight glued together Figure 5: Plastic block with groups of four-electrode in designed (left) and made (right) There are measuring values consisting of the current though electrode and 4, the voltage between electrode and 3, and the temperature In fact, the current though electrode and is measured by reading the voltage drop through R1 This is the AC signal that is amplified and changed the form to DC voltage The voltage between the electrode and is principally measured the same way as the voltage between electrode and except the amplifier is connected in different from The thermal couple type K was employed here to measure the temperature, the micro voltages generated depending on temperature is amplified and sent to a personal computer (PC) In order to collect and store data, the measuring system was designed as conceptual Fig to use a commercial personal computer which has a data acquisition card inside The Graphical User Interface (GUI) is developed based on the powerful computational language, MATLAB SIMULINK which can program easily to connect the PC to the real world The cost-effective data acquisition card with extended real time tool box software form HUMUSOFT could meet the requirement of the measurement system The card was built with analog input channels, analog output channels with 12-bit resolution and up to 100Ks per second that is quite good for such slow process There are also digital outputs and digital inputs which are very useful for logical control 1: Personal computer with ADDA card inside 2: Signal conditioning and amplifying unit 3: Measuring process Figure 6: The schematic diagram of the designed measurement system The idea to design the measurement system is that every interval the data at one of 4electrode groups are collected and stored in the hard drive The data consist of the voltage across electrode second and third of the 4-electrode group and the voltage drop across the current-sensing resistor (Rcs) at Fig.7 Since there is only one sinusoidal power source is applied the electrode first and fourth, and due the limit number of analog input channels, the multiplexer is needed to change over form current group to the next group The switching circuit is taken into account in design and has to be synchronized at every interval The switching control circuit makes the measurement system scanned around all of 4-electrod groups cyclically as presented in Fig The other consideration is not too much important but need-to-have, that is a channel of analog input left for measuring the temperature The temperature is not necessary to map to the location of the electrode, it means that it not need to pass the multiplexer, it goes directly to the channel as in Fig The flexibility is made available in some cases, for instance, sometimes the response of the system needs to be observed when the process is subjected to different frequencies of the excitation, the sinusoidal source should be controlled form the program For that reason, the VCO (Voltage Controlled Oscillator) circuit is preferably used and one channel of analog output is employed to control the base frequency for VCO The allocation of the input/output of the data acquisition card is shown as Table 100k 102 OPAMP1 + D1 100 102 RLY2 RLY3 C9 1uF 102 100k RLY1 OPAMP2 100 + + 100k U6 10k VR20k VR20k2 J1 OPAMP3 + + 102 OPAMP4 100 l4 l2 l3 l1 m4 m2 m3 m1 current-sensing resistor C12 1uF + + - - V1 102 100k u4 Rcs D2 100 + 100k u2 102 u3 u1 100k J2 10k VR20k1 J2 VR20k3 Figure 7: The signal conditioning circuit for measuring Vdrop and V2-3 +12V DI0 DI1 DI2 DI3 RLY1 IC 74LS154 15 14 13 12 11 10 RLY2 RLY4 RLY3 RLY5 RLY7 RLY6 RLY8 RLY9 E1 E0 Q1 Q2 Q3 Q6 Q4 Q5 Q7 Q8 Q9 A3 A2 A1 A0 Figure 8: The switching-control circuit 4-20mA input J4 20k +3V 50k 100k 102 102 100k 10k + 100k 100 + 20k 100 + 10k 100 (-10V)-(+10V) output 100k 102 102 J5 100k VR20k4 102 10k J6 10k VR20k VR20k5 Figure 9: The signal conditioning circuit for measuring temperature The electronic circuit and the data display in computer are in Fig 10 Figure 10: Electronic circuit system (left) and data records display (right) Table 2: IO function and its occupations Pin number* IO function Occupation Pin AD0 V2-3(J1) Pin AD1 Vdrop(J2) Pin AD2 Temp(J6) Pin 20 AI0 VCO control(J3) pin 22 AGND Analog ground Pin 29 GND Ground Pin 30 DO1 Bit of the switching circuit Pin 31 DO2 Bit of the switching circuit Pin 32 DO3 Bit of the switching circuit Pin 33 DO4 Bit of the switching circuit (*) reference to the HUMUSOFT card AD512 User’s manual The ratio of the electric current (I) flowing through the outer electrodes to the voltage difference (Vdrop) between the two inner electrodes is measured The ratio I/Vdrop is inversely proportional to the electrical resistance of the measured medium, or proportional to its electrical conductivity The magnitude of electric current (I) through the two outer electrodes is obtained from I = V/Rf, where is known resistance inserted in the circuit The voltage difference V/Vdrop is automatically measured using a digital voltameter The proportionality constant between the output value V/Vdrop and the bulk EC depend on the shape and construction of the sensor, and is determined by measuring known EC-values of various water solutions at a known reference temperature This was done with all sensors using potassium chloride solutions in the range between and g KCl/L Tests and Results Randomized Block Design (RBD) for replicates of treatments has been done for establishing calibration equations Keeping a constant frequency 220 Hz, five solutions potassium chlorite concentrations have been prepared, i.e 0.00; 0.50; 0.745; 2.50 and 5.00 g KCl/L For EC measurements, an ORION conductivity meter model 105 has been used The three bands of electrodes were coded as Fig 11 H2V1 H3V1 H1V2 H2V2 H3V2 H1V3 H2V3 H1V1 H3V3 Figure 11: Coding of the bands of electrodes The different KCl solutions have alternately measured by the conductivity meter and the four-electrode sensors Trendlines were set up and the linear regression equations and Rsquare values between EcmT and ECOrion from the data results are presented in Fig 12 Ge ne r a l t r e ndl i ne H1 V Ge ne r a l t r endl i ne H 1V Ge ne r a l t r endl i ne H 1V 12 07 08 y = 0117x y = 0.007x 06 10 R = 9801 y = 008x 07 R = 9904 R = 9991 06 05 08 05 04 06 04 03 03 04 02 02 01 02 00 00 0.00 00 4.00 00 8.00 00 10 00 01 00 00 00 00 10.00 08 y = 0083x R = 9902 08 08 07 07 04 04 04 03 03 03 02 02 02 01 01 01 00 00 00 00 R = 9917 R = 9893 00 00 10 00 00 Ge ne r a l t r endl i ne H 3V 00 00 00 10 00 00 Ge ne r a l t r e ndl i ne H3 V 08 08 07 07 06 06 05 05 04 y = 0077x 03 03 02 02 02 01 01 01 00 00 00 00 00 E C O r i o n ( d S/ m) 00 10 00 00 10.00 05 R = 0.9907 R = 9887 00 06 y = 0078x 04 00 00 Ge ne r a l t r endl i ne H 3V 07 03 00 E C O r i on ( d S/ m) 08 y = 0079x 00 E C O r i o n ( d S/ m) E C O r i o n ( d S / m) 04 10.00 y = 0.0082x y = 0084x 4.00 00 05 05 00 00 06 06 05 00 Ge ne r a l t r endl i ne H2 V 00 00 E C O r i o n ( d S/ m) Ge ne r a l t r endl i ne H2 V Gener a l t r e ndl i ne H2 V 06 00 E C O r i o n ( d S/ m) E C Or i o n ( dS/ m) 07 00 R = 9903 00 00 00 00 E C O r i o n ( d S/ m) 00 10.00 00 2.00 00 00 00 10 00 E C Or i o n ( dS/ m) Figure 12: Trend lines in H1V1, H1V2, H1V3, H2V1, H2V2, H2V3, H3V1, H3V2, H3V The estimated cost for such a circuit system is approximately 600 EURO including the Humusoft card, electrical apparatus, an old PC and installation labor cost Otherwise, the four-electrode product is very light and rather small This cost can be considered as a competition price if compared with other commercial sensors for measuring soil salinity such as Time Domain Reflectometry (TDR) methods and it is rather fixed in Vietnam condition Acknowledgements The authors thank the VLIR-CTU project for financial support this research and thank all faculties and staff in the College of Technology, CTU for their helps during our experiments References Austin, R.S and Rhoades, J.D., 1979 A compact, low-cost circuit for reading fourelectrode salinity sensors Soil Sci Soc Amer J., 43, 808-810 Corwin, D.L and Lesch S.M., 2005 Apparent soil electrical conductivity measurements in agriculture Computers and Electronics in Agriculture, 46, 11-43 Fritton, D.D., Busscher, W.J and Alpert, J.E., 1974 An inexpensive but durable thermal conductivity probe for field use Soil Sci Soc Amer Proc., 38, 854-855 Halvorson, A.D and Rhoades, J.D., 1976 Field mapping soil conductivity to delineate dryland saline seeps with four-electrode technique Soil Sci Soc Amer J., 40, 571-575 Konukcu, F., Gowing, J.W and Rose, D.A., 2002 Simple sensors to achieve fine spatial resolution in continuous measurements of soil moisture and salinity Hydrology and Earth System Sciences, 6(6), 1043-1051 Landviser, Inc., 2005 Brief theory of the four-electrode method [on line], Available from: http://www.landviser.com:PDF/theory.pdf [Access April 25 2005] Landviser Inc., 2005 Four-electrode probe for detection of burial places of criminal origin.[on line], Available from: http://www.landviser.com/gorensic.htm [Access April 25 2005] Landviser Inc., 2005 Electrical geophysical methods to evaluate soil pollution from gas and oil mining [on line], Available from: http://www.landviser.com/oil.mining.html [Access April 25 2005] Pozdnyakov, A and Pozdnyakova, L., 2002 Electrical fields and soil properties In: 17th World Congress of Soil Science, 14-21 August 2002, Thailand, paper no 1558 Nadler, A and Frenkel, H., 1980 Determination of soil solution electrical conductivity from bulk soil electrical conductivity measurements by the four-electrode method Soil Sci Soc Amer J., 44, 1216-1221 Nadler, A., 1981 Field application of the four-electrode technique for determining soil solution conductivity Soil Sci Soc Amer J., 45, 30-34 Rhoades, J.D., Raats, P.A.C and Prather, R.J., 1976 Effects of liquid-phase electrical conductivity, water content, and surface conductivity on bulk soil electrical conductivity Soil Sci Soc Amer J., 40, 651-655 Rhoades, J.D., Manteghi, N.A., Shouse, P.J and Alves, W.A, 1989 Soil electrical conductivity and soil salinity: new formulations and calibrations Soil Sci Soc Amer J., 53, 433-439 Rhoades, J.D., 1979 Inexpensive four-electrode probe for monitoring soil salinity Soil Sci Soc Amer J., 43, 817-818 Rhoades, J.D., Chanduvi, F and Lesch, S., 1999 Soil salinity assessment Methods and interpretation of electrical conductivity measurements FAO Irrigation and Drainage paper 57, Roma Shmilik P.F., 2005 Soil properties influencing apparent electrical conductivity: a review Computers and Electronics in Agriculture, 46, 45-47 William J.J., 2003 Geophysical detection of graves – Basic background and case histories from historic cemeteries Council for West Virginia Archaeology Spring Workshop, Charleston, West Virginia, June 7, 2003 10 ... Figure Simplified four- electrode probe theory of ions migration in solution In theory, EC - expressed in Siemens (S) - of the soil particles is determined by converting the value of electrical resistivity... of four- electrode in designed (left) and made (right) There are measuring values consisting of the current though electrode and 4, the voltage between electrode and 3, and the temperature In. .. occupations Pin number* IO function Occupation Pin AD0 V2-3(J1) Pin AD1 Vdrop(J2) Pin AD2 Temp(J6) Pin 20 AI0 VCO control(J3) pin 22 AGND Analog ground Pin 29 GND Ground Pin 30 DO1 Bit of the switching