Measuring sand electrical conductivity by cheap four electrode probes in cantho university, vietnam

Measuring sand electrical conductivity by cheap four-electrode probes in CanTho University, Vietnam L.. Tuan latuan@ctu.edu.vn Abstract An electrical current system including four-elec

Measuring sand electrical conductivity by cheap four-electrode probes

in CanTho University, Vietnam

L A Tuan 1 , V M Tri 2 and G C L Wyseure 3

1

Department of Environmental and Water Resources Engineering, College of

Technology, CanTho University, Vietnam

2

Department of Electronics Engineering, College of Technology, CanTho University, Vietnam

3

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

1 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)

A simple four-electrode probe technique embraces four electrodes arranged with a fixed distance between them, as L illustrated in Fig 1 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 1 and rod 4) and the potential drop voltages (V) between the two inner electrodes (rod 2 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

Figure 1 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):

ER

1

ER of a material is defined as follows:

R K I

ÄV K I

ÄV L

A

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:

A

V

Amperemeter, I Electrical

alternating current

Voltage, V Potential drop Solution

+ + + + + + + + +

+ +

+ + +

+

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

1

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

3 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 EC-meter and the calibrated EC values measured by four-electrode sensor, as in Fig 2 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 1

Figure 2: Four-electrode experiment applied in KU Leuven in March, 2004

Table 1: Factors in experiments Frequency (Hz) Solution (g KCl/L)

110 230 340 0.5 0.745 2.5 5.0

The system resistance was 10.17 Ohm, ECcalbr of standard water measured by the Consort EC-meter was 1.278 dS at 20C, 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 5 replicates of 6 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 4

At CTU then, three groups of four-electrode sensors were designed as drawn in Fig 5 The sensor is a stainless steel rod of 5 mm outside diameter, which is inserted perpendicular in the plastic block and reach 1 cm outside of the block The distance between every two rods is 1 cm

y = 0.000142x + 0.003699

0.0000

0.0200

0.0400

0.0600

0.0800

0.1000

0.1200

0.1400

Frequency (Hz)

ECfes Linear (ECfes)

y = 161.32x

= 0.9991

0 100 300 500 700 900

Solution (g KCl/L)

ECm Linear (ECm)

ECefsT vs ECConsort

y = 69.491x - 0.2227

0.00 2.00 4.00 6.00 8.00 10.00

ECfesT (dS/m)

Figure 3: Results of 3 replicates in randomized blocks with 2 factors

Solution vs ECConsort

y = 158.48x + 9.0288

R 2 = 0.999 0

200

400

600

800

0.00 1.00 2.00 3.00 4.00 5.00

Solution (g KCl/L)

Solution vs ECmT

y = 0.0241x + 0.0031

R 2 = 0.9958 0.00

0.02 0.06 0.08 0.10

0.00 1.00 2.00 3.00 4.00 5.00

Sollution (g KCL/L)

ECConsort vs ECmT

y = 0.0002x + 0.0013

0.00 0.02 0.04 0.06 0.08 0.10 0.12

ECConsort (dS/m)

Figure 4: Results of 5 replicates in randomized with 6 treatments

Figure 5: Plastic block with 3 groups of four-electrode in designed (left) and made (right)

There are 3 measuring values consisting of the current though electrode 1 and 4, the voltage between electrode 2 and 3, and the temperature In fact, the current though electrode 1 and 4 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 2 and 3 is principally measured the same way as the voltage between electrode

1 and 4 except the amplifier is connected in different from The thermal couple type K

PVC plates; watertight glued together

Groove (hollow) Sensor

Wires connected to rods

1 cm

1 cm

1 cm

1 cm

Plastic block with 3 groups of 4-electrodes (front and back looks)

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 6

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 8 analog input channels, 2 analog output channels with 12-bit resolution and up

to 100Ks per second that is quite good for such slow process There are also 8 digital outputs and 8 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 4-electrode 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 8 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 do not need to pass the multiplexer, it goes directly to the channel as in Fig

9 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 2

1

current-sensing resistor

U6 VR20k

+

OPAMP1

102

102

+

OPAMP3

VR20k 1

D1

D2

C12

C9 1uF

10 2

VR20 k3

102

10 2

V R20k2 102

RLY3 RLY 2

RLY1

l l l l

u u u u m m m m

+ -+ -V1 J1

100 100k

100k

100k

100k

100k

10 0

10k

100

100

1 0k

Rcs

Figure 7: The signal conditioning circuit for measuring Vdrop and V2-3

74LS154

E1

A3

A1

15

13

11

9

7

5 3

1

+12V

DI3

DI2

DI1

DI0

Figure 8: The switching-control circuit

4-20mA input

(-10V)-(+10V) output 20k

50k

100

10k

102

+

VR20k 102

J6 +3V

102

VR20k5

+ 102

10k

100

20k

10k 10k

J5

J4

100k

102

+

VR20k4 100k 100k

100k

100k

100

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 30 DO1 Bit 1 of the switching circuit

Pin 31 DO2 Bit 2 of the switching circuit

Pin 32 DO3 Bit 3 of the switching circuit

Pin 33 DO4 Bit 4 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 9 sensors using potassium chloride solutions in the range between 0 and 5 g KCl/L

4 Tests and Results

Randomized Block Design (RBD) for 3 replicates of 5 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

Figure 11: Coding of the 3 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 R-square values between EcmT and ECOrion from the data results are presented in Fig 12

Figure 12: Trend lines in H1V1, H1V2, H1V3, H2V1, H2V2, H2V3, H3V1, H3V2, H3V

Ge ne r a l t r e ndl i ne H1 V 1

y = 0.007x

0 00

0 01

0 02

0 03

0 04

0 05

0 06

0 07

E C O r i o n ( d S / m )

Ge ne r a l t r endl i ne H 1V 3

y = 0 008x

0 00

0 01

0 02

0 03

0 04

0 05

0 06

0 07

0 08

E C O r i o n ( d S / m)

Ge ne r a l t r endl i ne H 1V 2

y = 0 0117x

0 00

0 02

0 04

0 06

0 08

0 10

0 12

E C O r i o n ( d S/ m)

Gener a l t r e ndl i ne H2 V 1

y = 0 0083x

0 00

0 01

0 02

0 03

0 04

0 05

0 06

0 07

0 08

E C O r i o n ( d S / m)

Ge ne r a l t r endl i ne H2 V 3

y = 0.0082x

0 00

0 01

0 02

0 03

0 04

0 05

0 06

0 07

0 08

E C O r i o n ( d S / m )

Ge ne r a l t r endl i ne H2 V 2

y = 0 0084x

0 00

0 01

0 02

0 03

0 04

0 05

0 06

0 07

0 08

E C O r i o n ( d S / m)

Ge ne r a l t r endl i ne H 3V 1

y = 0 0079x

0 00

0 01

0 02

0 03

0 04

0 05

0 06

0 07

0 08

E C O r i o n ( d S / m)

Ge ne r a l t r endl i ne H 3V 2

y = 0 0078x

0 00

0 01

0 02

0 03

0 04

0 05

0 06

0 07

0 08

E C O r i o n ( d S/ m)

Ge ne r a l t r e ndl i ne H3 V 3

y = 0 0077x

0 00

0 01

0 02

0 03

0 04

0 05

0 06

0 07

0 08

E C O r i o n ( d S / m )

H1V1

H1V2

H1V3

H2V2 H2V1

H3V2 H3V1

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

5 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

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