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Eddy current testing (ECT), is a non-destructive method widely applied in detecting defects of metal plates based on the principle of electromagnetic induction. The flexible eddy current testing (FECT) has been developed from ECT. What’s more, it’s also an effective method that is given a higher priority in defect detection of metal with complex surface structure due to its flexibility and small size. An arrayed differential flexible butterfly-shape eddy current sensor is proposed to detect the surface defect of iron screw thread in this paper, which has one butterfly-shape coil, four sets of differential receiver (RX) coils and one top RX coil, and the proposed sensor is designed as butterfly-shape with a certain folding angle to match with the curvature of the internal and external screw thread, which can solve the lift-off problem of screw thread defect detection at the bottom. The theoretical analysis illustrates the design principle for the proposed sensors. The simulation based on COMSOL Multiphysics is applied to illustrate the probability of the proposed sensor, and the experimental testing system is adopted to verify the high performance of the proposed sensors. The results of simulation and experiment are identical, which show the flexible differential butterfly-shape array eddy current sensor can not only improve the sensitivity and reduce the error rate, but also orientate the defect on the surface of the screw thread.

In this paper, we design a smart pH sensor using untreated platinum sheet based on chronopotentiometry. This novel pH sensor is very suitable for applications in the deep sea, highly polluted water, and other harsh environments, where maintenance is difficult. In order to verify the long-term monitoring stability of the pH sensor, 17-day monitoring experiments are conducted in river water. We draw some conclusions for the properties of the pH sensor. First, the pH values obtained from the positive current agreed well with the pH glass electrode, indicating that it is suitable for pH monitoring. Moreover, the deviation derived from hysteresis is small. Second, the pH values obtained from the negative current could not reflect the actual pH of river water in long-term measurement. There may be two reasons for this: the changing conductivity in the river water and the unstable composition of the platinum sheet. Third, the conductivity may have an obvious impact on the potential obtained from the negative current; the electrochemical reaction where Pt is oxidized to PtO may be influenced by the ionic strength of the solution. Therefore, the pH values obtained from the positive current is more suitable for long-term pH monitoring.

Traditional methods for determining the ammonia nitrogen (NH3-N) concentration are the colorimetric method using Nessler’s reagent or salicylate. To prevent secondary pollution, minimise the time required for measurement, and reduce cation interference, new NH3-N measurement methods have been developed including fluorescence, gas phase molecular absorption spectrometry, and ammonia gas sensing electrode methods. However, in these techniques, the addition of a chemical reagent is unavoidable, so the real-time and accurate measurements of NH3-N are achieved hardly. In this study, we used an ammonium ion-selective electrode (ISE) for the real-time measurement of NH3-N with compensation for the ammonium ion (NH4+) component of the total NH3-N concentration. The basic mechanism is that an ammonium ISE can measure NH4+ concentration, and NH4+ ratio in NH3-N can be calculated based on pH and temperature. Experimental results indicate that the ammonium ISE exhibited near-Nernstian behaviour, having a fitted line with a slope of 56.4 mV/pNH4. The compensation effects and reproducibility are good. Furthermore, the difference in the temperature responses at different NH4+ concentrations agrees with the Nernst equation basically. Therefore, the proposed method can provide a real-time and environmentally friendly solution to determine NH3-N concentration.

In this paper, we describe a new turbidity sensing method based on the measurement of chromaticity coordinates. The new method measures turbidity using the transmission method, but omits the original light-intensity reference channel. The absorbance of a given solution, which is indicative of the absorption and scattering of the incident light by the undissolved matter, indicates the solution turbidity. Because the gradients of the absorbance curves increase at shorter wavelengths and differ for different turbidity solutions, the chromaticity coordinates of the solution shift with varying turbidity. The experimental results indicate that a good linear relationship exists between the chromaticity coordinates and the turbidity for a wide range of turbidity values. The slopes of the linear fits are 0.00037, 0.00034, and −0.00071/NTU for the x, y, and z chromaticity coordinates, respectively. Further, the turbidity is almost not affected by a change in the light intensity or the dark spectrum, which allows the omission of the original light-intensity reference channel. Therefore, it is not necessary to take into account the shift of light intensity and detector response in long-term measurement. The proposed method is suitable for colored water samples, provided the appropriate chromaticity coordinates are selected. In addition, the turbidity of real water samples is measured using the proposed method, and the results indicate the technique applicability.

A typical method for real-time and unpolluted measurement of chemical oxygen demand (COD) is UV–Vis absorption spectroscopy, however, contamination of the measuring system’s optical window surface affects the measurement accuracy during long-term monitoring. This study proposes an optical window surface contamination (OWSC) compensation method, and a multi-wavelength smart sensor for long-term accurate COD measurement was developed. This study uses equivalent absorption spectrum (EAS) to describe OWSC, and explores its spectral characteristics. Therefore, we can predict the EAS of OWSC between 250 and 380 nm based on the measured absorption spectrum between 380 and 440 nm, and subtract the EAS of OWSC to get the absorbance of COD to complete the spectrum compensation. Furthermore, we designed a smart COD sensor which has the advantages of real-time warning and maintenance-free. We proved that the OWSC compensation has excellent applicability through experiments using the COD sensor, and long-term accurate COD measurement can be achieved.

In recent years, ultraviolet–visible spectroscopy has been widely used for chemical oxygen demand (COD) measurements of water. However, turbidity in water can cause measurement deviations, so it is very important to compensate for the impact of turbidity. In this study, a novel method of turbidity compensation is proposed. The absorption spectra of standard solutions and their mixture were sampled in the wavelength range from 220 to 750 nm. We used a normalization technique to estimate the turbidity and dynamically simulate the absorption spectra of the turbidity. In addition, hydrogen bonds made the absorption peak blue shift in the mixture solutions. We established a numerical fitting curve to describe the relation between the blue shift and turbidity, and then we corrected the peak position. Furthermore, turbidity particles decreased the absorption peak of organic molecules. We introduced an impact index kN(λ) to describe the peak height reduction, and experiments demonstrated that kN is negatively associated with the absorbance of the standard COD solution. After the process of turbidity compensation, the root mean square errors (RMSEs) of the predictions were very small, which highlights the potential of this method for improving the accuracy of COD measurements based on ultraviolet–visible spectrum.

This study demonstrated the application of a tungsten oxide electrode to the measurement of ultralow liquid flow velocity based on streaming potential, using a pH glass electrode and temperature sensor to compensate for changes in open circuit potential induced by pH and temperature, respectively. A change in flow velocity on the surface of the tungsten oxide electrode caused the expected shift in its open circuit potential. Experimental results showed that the open circuit potential was linear relative to the logarithmic flow velocity, and the flow sensor exhibited excellent reproducibility and a large dynamic range, with the ratio between measurable maximum and minimum flow velocity surpassing 60:1. Moreover, a higher sensitivity was observed in the alkaline solution relative to the acidic solution, indicating that the alkaline solution is more suitable for this type of measurement. In conclusion, the tungsten oxide flow sensor provides a cheap, simple, adaptable, and energy-saving solution for the measurement of ultralow liquid flow velocity.

This study demonstrated the application of a metallic tungsten electrode for pH measurement. The metallic tungsten electrode exhibited an excellent linear response similar to the tungsten oxide electrode. Both the metallic tungsten electrode and tungsten oxide electrode used in our study exhibited a near-Nernstian response of −51.9 mV/pH and −56.8 mV/pH, respectively. No obvious interference was observed from the primary cations. The fitted curve for the metallic tungsten electrode showed a smaller slope (−1.87 mV/°C) and more linear (R2 = 0.999) temperature response than those of the tungsten oxide electrode, indicating its suitability in practical pH measurements due to its precise compensation of temperature. Moreover, the metallic tungsten electrode was almost uninfluenced by the hydrodynamic effect, and its open circuit potential change was less than 2 mV even at high stirrer rotation speeds (120 rev/min) in solution. Long-term stability could be achieved for the metallic tungsten electrode over the pH range 4.5–8.0. Owing to the advantages such as precise temperature compensation, insensitivity to the hydrodynamic effect, long-term stability, good selectivity, and intrinsic properties of the solid-state electrode, the metallic tungsten electrode has shown promise for long-term pH monitoring.

In this study, a novel solid-state pH sensor based on an untreated platinum sheet was investigated. Chronopotentiometry using positive and negative currents was applied to effect the interconversion of Pt and PtO. To eliminate the influence of concentration polarization, a galvanostatic double-pulse method was applied for pH measurement. The consecutively applied currents were +100, +3, −100, and −3 μA for excitation times of 0.5, 1, 0.5, and 1 s, respectively. Stable potentials under the small currents were obtained which could be used to determine pH. The platinum sheet electrode exhibited near-Nernstian behavior in the pH range 2–12, with fitted lines having slopes of −44.2 and −51.1 mV/pH unit under the positive and negative currents, respectively. Overall, excellent linearity and reproducibility could be achieved. Moreover, the temperature responses for the method in pH 4, 6.86, and 9.18 buffer solutions obeyed the Nernst equation, and further supported the proposed mechanism for pH determination. In conclusion, this new detection method provides a solid-state, long-lived, and adaptable solution for pH measurement.

Ultraviolet absorption spectroscopy is one of physical methods used for chemical oxygen demand (COD) measurements of water. The absorbances in ultraviolet band have a relationship to COD. However, turbidity in water could scatter emitting light and influence the absorbances. So it is very important to compensate for the impact of turbidity. In this study, the absorption spectra of standard COD solution (potassium acid phthalate), turbidity solution (Formazine) and their mixture are sampled in the wavelength range from 220 to 750 nm. The impacts of turbidity for COD measurement and compensation method are studied based on these data. The absorbance of mixture substract the absorbance of turbidity solution is less than the absorbance of standard COD solution. The result indicates that turbidity particles decrease the light absorption of organic molecules. Furthermore, we discover that the impact of turbidity is greater for the larger absorbance of the standard COD solution. Then attenuation coeffcient (AC()) is introduced and calculated based on exprimental results. In the process of turbidity compensation, the turbidity of solution is estimated using the absorbance of visible wavelength. The absorption spectra of the turbidity in the ultraviolet wavelength are simulated using normalization technique. The satisfactory prediction result of COD is achieved for the mixture after the turbidity compensation. In conclusion, the new turbidity compensation method could eliminate the influence of turbidity for COD measurements based on absorption spectroscopy.

In recent years, ultraviolet-visible spectroscopy has been widely used for measurements of water quality in seawater, because full spectrum contains large amounts of information. NO3 can be detected in 220–230 nm, and chemical oxygen demand (COD) and total organic carbon (TOC) can be detected in 280–290 nm. In visible spectrum, water color, turbidity and total suspended solids (TSS) can be measured. We presented a water environmental multi-parameter sensor developed for the simultaneous measurement of these parameters. In order to research their absorptive characteristics and explore the detection methods, we sampled the absorption spectra of artificial seawater solutions of NO3, COD and turbidity. In the measurement of NO3, COD and TOC, the cross sensitivity between NO3 and organic substances was avoided by selecting the appropriate characteristic wavelengths of partial least squares (PLS) models, and the interference of turbidity was eliminated by using fourth derivative spectrum. Turbidity and TSS was detected outside the spectral range around the absorption peak caused by water color. The water color was calculated to warn for harmful algal blooms. The feasibility of this instrument was demonstrated by a series of experiments.

A novel electrochemical surface plasmon resonance (EC-SPR) sensor has been developed based on the surface plasmon resonance (SPR) combined with a two-electrode electrochemical configuration. The theory of potential-modulated for EC-SPR was described, and several factors which can induce the change of the SPR resonance angle were revealed. Comparing with the conventional three-electrode electrochemical system, the reference electrode has been eliminated in this design, and the active carbon (AC) electrode employed as the counter electrode. Due to the large specific surface area, AC presents considerable double layer capacitance at the interface of electrode and electrolyte, which can provide a constant potential during the electrochemical reactions. Using an angle modulation SPR sensor and the resolution of that is 5x10-6 RIU (refractive index units), a real-time data-smoothing algorithm is adopted to reduce the noise of the data, which can guarantee an accurate result of the resonance angle of SPR. The EC-SPR setup was used for investigating the electropolymerization of polyaniline by applying a potential of cyclic voltammetry, both of the electrochemical current and the resonance angle shift of SPR are recorded to monitor the growth process of the polymer. Comparing with the three-electrode configuration, the novel AC two-electrode system can also obtain detailed information about the polymerization process from the resonance angle shift curves, including the change of thickness and dielectric constant, deposition and transitions between different redox states of the polymer film. Experimental results demonstrated that this two-electrode EC-SPR configuration is suitable for analyzing the electropolymerization process of a conducting polymer.

To obtain detailed information about electrochemistry reactions, a two-electrode electrochemical surface plasmon resonance (EC-SPR) sensor has been proposed. We describe the theory of potential modulation for this novel sensor and determine the factors that can change the SPR resonance angle. The reference electrode in three-electrode configuration was eliminated, and comparing with several other electrode materials, activated carbon (AC) is employed as the suitable counter electrode for its potential stability. Just like three-electrode configuration, the simpler AC two-electrode system can also obtain detailed information about the electrochemical reactions.

This paper describes a new turbidity transducer based on color measurement. The absorbance of solutions reflects the absorption and scattering of suspended particle for incident light which could determine the turbidity of solutions. The experimental results indicate that there are good linear relationships between chromaticity and turbidity. The new way is suitable for continuous monitoring of water turbidity in the wide range.