Field applications are not restricted to environmental monitoring and can include wastewater treatment plants, oil quality measurement and measurement of oil quality in fluid systems and hydraulic components. Some experimental results are also presented in this paper. Introduction and objective Removal of free oil and floating materials in industrial wastewater-treatment processes together with conductivity measurements are two important issues in water pollution control [].
Another important application of oil and conductivity measurements is in the field of water quality monitoring in rivers and estuaries especially in geographical areas nearby pollutant industrial plants. Nowadays, this subject is a major topic that affects life quality in our society.
This paper presents a measurement solution for oil-thickness and water conductivity. Oil-on-water thickness measurement is based on a capacitive sensor that takes advantage of the large difference between the dielectric constants of oil and water [4- 6]. Conductivity measurement is based on a three-electrode conductivity sensor that includes a reference and a measuring sub-cell [7]. A third sensor for temperature measurements thermistor Omega ON Series is also included in the system in order to compensate measurement errors caused by temperature variations.
Finally, it is important to refer that although this paper does not addresses in detail auto-calibration [8], sensor cleaning procedures, and choice of anti-fouling and anti-adherent electrode materials, these issues must be considered in a commercial design of the system to assure accurate measurements in aggressive environments like the ones provided by industrial polluted and contaminated waters. Sensor design Sensors are assembled together in a single structure as represented in Figure 1.
The conductivity sensor includes two parts: a reference sub-cell, connected between terminals 1 and 2 , and a measuring sub-cell, connected between terminals 2 and 3. The metallic plates are supported by a material with a low electrical permittivity acrylic in order to minimize the displacement current around the conductivity sensor.
Sensor assembly assures that the conductivity cells work always under the oil thickness layer, being the maximum expected oil thickness, doil max, lower than dimension W represented in Figure 1. The conductivity reference sub-cell is filled with a calibrated solution with a well-known conductivity value and is used for auto-calibration purposes and to minimize common errors that affect equally measuring and reference conductivity sub-cells ratio measurement techniques.
The oil-on-water sensor includes a single capacitor whose dielectric is filled with two different liquids: oil and water. In order to avoid mutual interference between conductivity and oil-on-water measurements an acrylic separator is used to reduce capacitive connections between both sensors. Sensing module: 1 2 3 - electrical terminals of conductivity sensor, 4 5 - electrical terminals of oil-on-water sensor, 6 7 - electrical terminals of the temperature sensor, 8 reference conductivity sub-cell, 9 - measuring conductivity sub-cell, 10 - acrylic separator, 11 - temperature sensor.
Sensors modeling Theoretical models of each sensor will be presented in this Section to underline its working principle. Oil-on-water sensor The electrical model of the sensor used for oil-on-water thickness measurement is represented in Figure 2.
Equivalent electrical circuit of the oil-on-water sensor Ceq- equivalent capacitor. Expression 1 represents a non-linear relation between Ceq and doil, which means that a non-linear relationship between the capacitive value Ceq and the measurement variable doil exists even when non-linear effects, like the ones caused by temperature variations, are considered.
Conductivity sensor The electrical model of the sensor used for conductivity measurements is represented in Figure 3. In this case, the equivalent electrical circuit, only considering resistive effects, is an acceptable approximation due to the high conductivity value of water. Equivalent electrical circuit of the conductivity sensor Rref- reference resistance, Rw- water resistance. To minimize polarization of the cell and double layer effects conductivity measurements are performed with alternate voltage or current sources.
In our case the working frequency used for conductivity measurements is equal to The coefficient B is the characteristic temperature of the thermistor and depends on its building material. Conditioning circuit and signal processing A. Figure 5 represents the internal block diagram of the UTI whose main elements include: an input multiplexer, an amplitude current-voltage divider, a voltage to period converter, a frequency divider and a logic control circuit.
The selection bus, connected to a microcontroller, contains 4 lines that are used to select one of the 16 operating modes of the UTI. In this mode, capacitive elements up to pF with a common electrode can be measured. The slow-fast mode pin SF is used to adjust testing frequency for capacitive fast mode and conductive slow mode measurements. In these modes, a ratio measurement of resistive values is performed and used to obtain water conductivity value 2 and temperature. Figure 6 represents sensors connections for each measurement mode.
Sensors connections: A oil-on-water measurement, B water conductivity measurement, C temperature measurement R1,2,3- measurement range adjustment of capacitive measurements, CP- cable parasitic capacitance, Ii- UTI sensors connections, Rth- thermistor resistance.
This type of exciting signals has three main advantages: reduction of low-frequency disturbing signals coming from power supply, cancellation of parasitic thermocouple junction effects and reduction of polarization errors associated with the conductive cell measurements. The use of a PIC is particularly efficient for frequency and duty-cycle modulated signals. Signal processing The main signal processing tasks performed by the microcontroller include time measurement of the output signal driven by the UTI, offset and gain errors compensation, and evaluation of oil-on-water thickness and water conductivity.
The output signal of the UTI is a three-phase periodic modulated signal. During the first phase, the offset of the overall system is measured Toff. In the second phase, a reference element, generally associated with full-scale amplitude FS , is used to obtain the reference period value, TFS.
Finally, during the last phase, the signal itself is measured Tx. Basically, the proposed sensing devices include a capacitive element used to measure oil thickness and a conductivity element to measure water conductivity. Temperature compensation of measured values is also provided by including an additional temperature sensor in the system.
Field applications are not restricted to environmental monitoring and can include wastewater treatment plants, oil quality measurement and measurement of oil quality in fluid systems and hydraulic components. Some experimental results are also presented in this paper.
Introduction and objective Removal of free oil and floating materials in industrial wastewater-treatment processes together with conductivity measurements are two important issues in water pollution control [].
Another important application of oil and conductivity measurements is in the field of water quality monitoring in rivers and estuaries especially in geographical areas nearby pollutant industrial plants. Nowadays, this subject is a major topic that affects life quality in our society. This paper presents a measurement solution for oil-thickness and water conductivity. Oil-on-water thickness measurement is based on a capacitive sensor that takes advantage of the large difference between the dielectric constants of oil and water [4- 6].
Conductivity measurement is based on a three-electrode conductivity sensor that includes a reference and a measuring sub-cell [7]. A third sensor for temperature measurements thermistor Omega ON Series is also included in the system in order to compensate measurement errors caused by temperature variations. Finally, it is important to refer that although this paper does not addresses in detail auto-calibration [8], sensor cleaning procedures, and choice of anti-fouling and anti-adherent electrode materials, these issues must be considered in a commercial design of the system to assure accurate measurements in aggressive environments like the ones provided by industrial polluted and contaminated waters.
Sensor design Sensors are assembled together in a single structure as represented in Figure 1. The conductivity sensor includes two parts: a reference sub-cell, connected between terminals 1 and 2 , and a measuring sub-cell, connected between terminals 2 and 3.
The metallic plates are supported by a material with a low electrical permittivity acrylic in order to minimize the displacement current around the conductivity sensor.
Sensor assembly assures that the conductivity cells work always under the oil thickness layer, being the maximum expected oil thickness, doil max, lower than dimension W represented in Figure 1. The conductivity reference sub-cell is filled with a calibrated solution with a well-known conductivity value and is used for auto-calibration purposes and to minimize common errors that affect equally measuring and reference conductivity sub-cells ratio measurement techniques.
The oil-on-water sensor includes a single capacitor whose dielectric is filled with two different liquids: oil and water. In order to avoid mutual interference between conductivity and oil-on-water measurements an acrylic separator is used to reduce capacitive connections between both sensors. Sensing module: 1 2 3 - electrical terminals of conductivity sensor, 4 5 - electrical terminals of oil-on-water sensor, 6 7 - electrical terminals of the temperature sensor, 8 reference conductivity sub-cell, 9 - measuring conductivity sub-cell, 10 - acrylic separator, 11 - temperature sensor.
Sensors modeling Theoretical models of each sensor will be presented in this Section to underline its working principle. Oil-on-water sensor The electrical model of the sensor used for oil-on-water thickness measurement is represented in Figure 2. Equivalent electrical circuit of the oil-on-water sensor Ceq- equivalent capacitor.
Expression 1 represents a non-linear relation between Ceq and doil, which means that a non-linear relationship between the capacitive value Ceq and the measurement variable doil exists even when non-linear effects, like the ones caused by temperature variations, are considered. Conductivity sensor The electrical model of the sensor used for conductivity measurements is represented in Figure 3. In this case, the equivalent electrical circuit, only considering resistive effects, is an acceptable approximation due to the high conductivity value of water.
Equivalent electrical circuit of the conductivity sensor Rref- reference resistance, Rw- water resistance. To minimize polarization of the cell and double layer effects conductivity measurements are performed with alternate voltage or current sources. In our case the working frequency used for conductivity measurements is equal to The coefficient B is the characteristic temperature of the thermistor and depends on its building material.
Conditioning circuit and signal processing A. Figure 5 represents the internal block diagram of the UTI whose main elements include: an input multiplexer, an amplitude current-voltage divider, a voltage to period converter, a frequency divider and a logic control circuit. The selection bus, connected to a microcontroller, contains 4 lines that are used to select one of the 16 operating modes of the UTI. In this mode, capacitive elements up to pF with a common electrode can be measured.
The slow-fast mode pin SF is used to adjust testing frequency for capacitive fast mode and conductive slow mode measurements. In these modes, a ratio measurement of resistive values is performed and used to obtain water conductivity value 2 and temperature.
Figure 6 represents sensors connections for each measurement mode. Sensors connections: A oil-on-water measurement, B water conductivity measurement, C temperature measurement R1,2,3- measurement range adjustment of capacitive measurements, CP- cable parasitic capacitance, Ii- UTI sensors connections, Rth- thermistor resistance.
This type of exciting signals has three main advantages: reduction of low-frequency disturbing signals coming from power supply, cancellation of parasitic thermocouple junction effects and reduction of polarization errors associated with the conductive cell measurements. The use of a PIC is particularly efficient for frequency and duty-cycle modulated signals. Signal processing The main signal processing tasks performed by the microcontroller include time measurement of the output signal driven by the UTI, offset and gain errors compensation, and evaluation of oil-on-water thickness and water conductivity.
The output signal of the UTI is a three-phase periodic modulated signal. During the first phase, the offset of the overall system is measured Toff.
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