This paper presents the development of a Twin-T oscillator comprising polymer coated parallel plates as a sensor for ocean water salinity monitoring.This sensor employs a parallel plate capacitor design, with sea water serving as the medium between plates. Novalac resin and a proprietary commercial polymer (Accuflo TW) were investigated as corrosion protective coatings for the copper electrodes of the capacitor. Electrochemical Impedance Spectroscopy (EIS) was employed to evaluate corrosion inhibition of polymer coatingin sea water. A detection circuit was designed and simulated using P-spice and then implemented in Printed Circuit Board (PCB). EIS results indicate that Accuflo exhibits better corrosion inhibition in ocean water than Novolac. Further, the use of Twin-T oscillator based detection circuit resulted in enhanced sensitivity and better detection limit. Experiments performed using ocean water samples resulted in oscillator frequency shift of 410 Hertz/power supply unit (Hz/PSU). Oscillator frequency drift was reduced using frequency-to-voltage converters and sensitivity of 10 mV/PSU was achieved.
The oceanic studies began in the 1930s with the search for petroleum, continued with the emphasis for improved naval warfare and more recently have been driven by a need to understand and protect the ecosystem. No discussion on oceanography is complete without a mention of parameters such as salinity, temperature, pressure and density. Extensive research has been conducted to understand the role of these parameters in regulating oceanic processes, [1,2] but there is still a lot of latent information that seems to remain elusive to the oceanographers worldwide. Salt concentration measurement in sea water is very important as it affect the weight of surface waters. Fresh water is light and floats on the surface, while salty water is heavy and sinks. Together, salinity and temperature determine seawater density and buoyancy, driving the extent of ocean stratification, mixing, and water mass formation. The density of surface seawater ranges from about 1020 to 1029 kg m−3, depending on the temperature and salinity. Salinity is commonly defined as the ratio between the weight of dissolved material in the sea water sample and the weight of the sample [
Salinity varies from place to place and it is lower where mixing occurs with fresh water runoff from river mouths or near melting glaciers, and found higher where high rates of evaporation, low precipitation and river inflow, and confined circulation occurs such as Red Sea. Chlorine (55.3%), sodium (30.8%), sulfate (7.7%), magnesium (3.7%), calcium (1.2%) and potassium (1.1%) are the major constituent of sea water salts [
Recently various principles and techniques have been reviewed for sensing sea surface salinity [10,11]. Menn et al. described the advances in measuring ocean salinity using optical sensors. Such sensor usually measure refractive index of seawater which is related to density and can therefore be used to measure absolute salinity [
To overcome the problem of external fields associated with inductive sensing capacitive principles for conduction sensing can be employed. Parallel plate capacitors rely on trapping the electric field in the region between the plates, [15,16] which provides an inherent confinement of the measuring field between the two plates, with little interference due to external fields/objects. Only a small portion of the field is exposed to the external media causing fringing effects at the corners and edges of the plates. Thus, due to the fringing fields at the plate edges the measured capacitance of a capacitor is generally higher than the calculated capacitance. Although, it is difficult to calculate the fringe field analytically, these are significant when the distance between the electrodes, is comparable to the smallest dimension of the electrode [
In this work attempts have been made to use parallel plate based capacitor for salinity measurement. Two commercially available polymers have been investigated to solve the problem of electrode fouling in sea water. Simulations have been conducted using P-spice software to simulate results and Twin-T oscillator design was employed to achieve higher sensitivity. Further, a reduction in frequency drifts has been achieved using frequency to voltage converter and Kelvin ring guard was employed to reduce the fringing field effects at edges of device.
Finite Element Modeling (FEM) using the FEM tool Comsol Multiphysics was performed to quantify the reduction in fringe field due to the incorporation of the Kelvin guard ring. Further, “Quasi-static” a sub-module of the “Electromagnetic” section was used to simulate appropriate sensors operational frequency range. Each
shape in the geometry was assigned a sub-domain name to facilitate the specification of materials used in the construction of the sensor. To simulate the seawater medium around the sensor, the capacitor was enclosed in a box with dielectric properties matching those of seawater. Next, the boundary conditions were specified by assigning voltage for each of the interfaces. This is the most important step prior to simulation, as the nature of the boundary dictates the electromagnetic equation used in the calculation of the electric fields in the system. The boundaries of the top metal plate were assigned a sinusoidal voltage, 5 Vp-p in magnitude, while those of the lower plate were assigned to ground. The junction of the liquid-dielectric interface was assigned as a continuous interface and is hence, governed by the equation,
where n is the normal vector and J1, J2 are the current density vectors of the two adjoining materials in consideration. In reality the space charge region in liquid dielectric junction with polarization contribute to the overall impedance. However, for simplicity these contributions were neglected in this simulation. Further, a mesh with “normal” mesh size was used for the mesh elements in simulation. The sensor was analyzed in the “time harmonic, small current” mode and solved using “UMFPACK”. The simulated system results for the potential distribution and electric field are shown in
Top in
Consequently, Equation (3) gives the capacitance from the energy density. The simulated capacitance for the modeled capacitor was obtained as 4.608 pF. Further, the capacitance of system obtained after the addition of guard rings (held at ground potential) around the capacitor plate (shown in bottom of
The capacitive sensor is part of a Twin-T oscillator detection circuit. The Twin-T circuit consists of two arms, the high pass arm and the low pass arm. The advantage of Twin-T oscillator over other single-capacitor circuits is a lower distortion sine wave output. This circuit functions as a notch filter by eliminating a particular frequency from the incoming signal, [
In present work, capacitors in the circuit were replaced with the capacitive sensors fabricated in house, which act as the salinity senor. Also, the resistors used have a tolerance of 2%, to minimize drift in the oscillator response. This kind of a circuit incorporating R and C components can also be utilized in applications requiring low frequencies of operation.
To compare the response of system employing Twin-T oscillator, an alternative circuit for capacitive salinity sensing was developed based on frequency to voltage converters (F-V). This design employs only one capacitor and does not require matching of circuit components. A block diagram of the circuit is shown in
Difference amplifier (LM 6142).
In the schematic shown (
The sensor consists of two parallel copper-coated FR4 plates. One of the plates had a dimension of 2 × 1 cm and the other plate has a slightly larger dimension (2.1 cm × 1.1 cm), for alignment tolerance and bonding. The sensor plates were fabricated using copper clad FR-4 substrate boards. The process flow steps for the fabrication are as depicted in
The first step in the fabrication of the sensor involves lithography. Electrode patterns were transferred using photolithography on positive photoresist coated FR-4. The pattern was developed in aqueous KOH (9 grams of KOH per liter of Deionized water) at 50˚C for 15 seconds. After development, the exposed copper was etched using Ferric Chloride solution. Subsequently, the photoresist over the copper was dissolved using acetone. Next, a thin layer of a dielectric material was coated on the plates. Two polymeric materials (Novolac and Honeywell’s Accuflo™) were evaluated to determine their effectiveness as corrosion inhibitors in ocean water.
Novolac resin was spin cast at 1000 rpm for 60 seconds to yield a thickness of ~5 microns. The resin was then hard baked on a hotplate at 140˚C for 90 sec. The top and bottom plates, fabricated separately, were aligned face to face with a spacing of ~600 mm, using spacers and bonded using photoresist. Similar methodology was adopted for Accuflo coating on electrodes.
Electrochemical Impedance Spectroscopy (EIS) is a powerful technique with a wide range of applications from material characterization to corrosion monitoring. Depending upon the type of EIS response and the prior knowledge of system under test, one can arrive at plausible conclusions about the state of the system. Usually for analysis, the real part of impedance is plotted against the imaginary part (complex plane impedance plot). Such a plot usually gives a semicircular arc, which in the context of corrosion can be interpreted as the electrochemical
response of a corroding metal in a conductive solution.
Further, the solution resistance is given by the high frequency intercept of the semicircle (closest to imaginary axis) and the charge transfer resistance is estimated by the diameter of the semicircle. Based on the estimation of these parameters, many physical quantities such as solution conductivity, exchange current density etc. can be determined.
In the present work, EIS technique was employed for the salinity sensor to understand the bulk and interfacial phenomenon. An Agilent 4294A impedance analyzer was used to record the impedance spectra of the parallel plate sensor system. Complex nonlinear squares (CNLS) fitting was used to analyze the generated experimental data.
The overall impedance of the equivalent circuit of
where, Rpore is the pore resistance, Rct is the charge transfer resistance, Ccoat is the coating capacitance, Cdl is the double layer capacitance and Rsol is the solution resistance.
where, ω is the angular frequency. The CPE1, is a constant phase element, which represents pore capacitance. The impedance of a constant phase element is given by the expression, 1/A*(j*ω)n, where A is the magnitude of the element, ω = 2*pi*f, is the angular frequency, and f is
the frequency in Hertz. The parameter n is such that if n = 1, the impedance of a CPE is that of an ideal capacitor and when n = 0, the CPE is a pure resistor. The parameter CPE2 represents the electrical double layer effects. Even though the use of CPE yielded better quality than using discrete capacitances, the CPE power factors, α, is close to unity, indicating proximity to pure capacitor behavior. The overall impedance response might also be affected by the polymer film relaxation, [25,26] introducing the slight frequency dependence to film and interfacial capacity.