Sharmistha Bhadra

A Wireless Passive Sensor for Temperature Compensated Remote pH Monitoring

Temperature must be accounted for in order to provide accurate measurements in electrode-based pH sensors. We present an integrated wireless passive sensor for remote pH monitoring employing temperature compensation. The sensor is a resonant circuit consisting of a planar spiral inductor connected in parallel to a temperature-dependent resistor (thermistor) and a voltage-dependent capacitor (varactor). A pH combination electrode consisting of an iridium/iridium oxide sensing electrode and a silver/silver chloride reference electrode, is connected in parallel with the varactor. A potential difference change across the electrodes due to pH variation of the solution changes the voltage-dependent capacitance and shifts the resonant frequency, while temperature of the solution affects the resistance and changes the quality factor of the sensor. An interrogator coil is inductively coupled to the sensor inductor and remotely tracks the resonant frequency and quality factor of the sensor. The sensor is calibrated for temperature over a range of 25

A wireless embedded passive sensor for monitoring the corrosion potential of reinforcing steel

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Electrode Potential-Based Coupled Coil Sensor for Remote pH Monitoring

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Non-destructive detection of fish spoilage using a wireless basic volatile sensor

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Corrosion Potential Sensor for Remote Monitoring of Civil Structure Based on Printed Circuit Board Sensor

and pH over a 1.5–12 dynamic range. By employing temperature compensation, a measurement accuracy of less than 0.1 pH is achieved and the response time of the sensor is demonstrated to be less than 1 s. The sensor overcomes the pH measurement error due to the temperature dependence of electrode-based passive pH sensors and has applications in remote pH monitoring where temperature varies over a wide range.

Coupled coil sensor for detecting surface corrosion on steel reinforcement

Corrosion of reinforcing steel, which results in premature deterioration of reinforced concrete structures, is a worldwide problem. Most corrosion sensing techniques require some type of wired connection between the sensor and monitoring electronics. This causes significant problems in their installation and long-term use. In this paper we describe a new type of passive embeddable wireless sensor that is based on an LC coil resonator where the resonant frequency is changed by the corrosion potential of the reinforcing steel. The resonant frequency can be monitored remotely by an interrogator coil inductively coupled to the sensor coil. The sensor unit comprises an inductive coil connected in parallel with a voltage dependent capacitor (varactor) and a pair of corrosion electrodes consisting of a reinforcing steel sensing electrode and a stainless steel reference electrode. Change of potential difference between the electrodes due to variation of the corrosion potential of the reinforcing steel changes the capacitance of the varactor and shifts the resonant frequency of the sensor. A time-domain gating method was used for the interrogation of the inductively coupled corrosion sensor. Results of an accelerated corrosion test using the sensor indicate that the corrosion potential can be monitored with a resolution of less than 10 mV. The sensor is simple in design and requires no power source, making it an inexpensive option for long-term remote monitoring of the corrosion state of reinforcing steel.

Fluid Embeddable Coupled Coil Sensor for Wireless pH Monitoring in a Bioreactor

We present a coupled coil pH sensor for high-resolution remote pH monitoring. The sensor is based on a passive LC coil resonator whose resonant frequency is monitored remotely by measuring the change in impedance of an interrogator coil coupled to the sensor coil. The sensor resonator consists of an inductive coil connected in parallel with a voltage dependent capacitor and a pH combination electrode. When the pH of the contact solution changes, the electrode potential changes the capacitance, and therefore the resonant frequency of the sensor. A linear response with a 0.1 pH resolution is achieved over a 2-12 pH dynamic range at room temperature. The response time of the sensor is demonstrated to be less than 30 s and is limited by the response time of the pH combination electrode. Effects of varying separation distance and temperature change on the sensors performance are shown. The described sensor technology has potential application for remote pH monitoring in numerous fields such as biomedical sensing, environmental monitoring, industrial and chemical processing, and structural health monitoring.

Wireless passive sensor for pH monitoring inside a small bioreactor

A hydrogel-pH-electrode based near-field passive volatile sensor is described for real-time monitoring of fish spoilage. The sensor employs a varactor-based LC resonator that can be interrogated remotely using inductive coupling. The sensors resonant frequency varies in response to the basic volatile spoilage compounds (total volatile basic nitrogen, TVB-N) in the headspace of packaged fish. The sensor is shown to have a linear response to logarithm of the ammonia gas concentration with a detection limit of 0.001 mg L(-1) (1.5 ppm). Trials on tilapia at 24 °C and 4 °C, employing direct comparison of sensor measurements with microbial analysis, indicate that the sensor response is correlated with the bacterial growth pattern in fish samples. It is shown that the sensor can distinctly identify when the product rejection level (10(7) cfu g(-1) bacterial population) occurs for both 24 °C and 4 °C storage conditions. This demonstrates a potential for real-time monitoring of fish spoilage. The wireless sensor is suited to embedding in packaging material and does not require an integrated circuit, making it amenable to inexpensive mass production using printed electronic technology.

A wireless passive pH sensor based on pH electrode potential measurement

We present a printed circuit board-based wireless inductively coupled corrosion potential sensor for monitoring corrosion of steel-reinforced concrete civil infrastructure. The sensor is a passive LC coil resonator whose resonant frequency varies owing to the corrosion potential produced by two electrodes connected across a voltage-controlled capacitor. The junction capacitance of the voltage-controlled capacitor also varies according to the potential generated by the two electrodes, which is in turn responsible for changing resonant frequency of the sensor. The two electrodes are a stainless steel reference electrode and a steel reinforcement electrode. An external interrogator coil coupled with the sensor coil monitors the sensor resonant frequency shift remotely by measuring the impedance change from the source end. The sensor has a sensitivity of ~1.2 kHz/mV both before and after it is embedded in the cement-based mortar used for testing the sensor. The sensor can be used to sense the corrosion susceptibility of existing structures by embedding the corrosion electrodes in new grout within a small slot formed in an existing structure. Using this approach, weight concentrations of NaCl mixed in cement-based mortars of greater than 2%-3% were detected. Accelerated corrosion tests on embedded sensors in simulations of both new and existing structure demonstrated corrosion potential resolution of less than 10 mV and an uncertainty of less than 50 mV. The sensor is simple in design, inexpensive, and passive making it a battery-less option for long-term corrosion monitoring, and widely deployable for civil structure.

A wireless passive pH sensor for real-time in vivo milk quality monitoring

An embedded coupled coil sensor able to monitor the corrosion potential of reinforcement steel in concrete is presented. The sensor is based on coil resonator whose resonant frequency changes with an applied voltage. The sensor state can be monitored externally by a coupled coil. An accelerated corrosion test shows that it can provide useful information about the corrosion state and the level of chloride ingress into the concrete. The wireless sensor is passive and simple in design, making it an inexpensive, batteryless option for remote long-term monitoring of the corrosion state of reinforcing steel.