The thesis describes development and application of coupled resonant coil sensors, which is of growing interest for remote monitoring applications. An interrogation technique, which improves the accuracy and interrogation range of coupled resonant coil sensors, is introduced. The method uses time-domain gating to produce measurements that are dominated by the response of the sensor coil and are immune to surrounding object interference. For application in structural health monitoring a low cost embeddable coupled coil sensor, which is able to monitor the corrosion potential of reinforcement steel is presented. Results of an accelerated corrosion test using the sensor indicate that corrosion potential can be monitored with a resolution less than 10 mV and a sensitivity of 0.76 kHz/mV. The last part describes a coupled-coil pH sensor based on pH electrode potential measurement. A linear response over a 4 to 10 pH dynamic range and 50 kHz/pH sensitivity are achieved with a 0.1 pH resolution and 30 s response time.

This thesis purposes and develops inductively coupled LC (inductive-capacitive) pH sensors based on pH-sensitive electrode pair. The LC resonator circuit is based on a varactor and measures the low frequency potential difference. For wireless pH monitoring, the resonator circuit is integrated with a pH-sensitive electrode pair. This sensor demonstrates a linear response over 2 to 12 pH dynamic range, 0.1 pH accuracy and long-term stability. Accurate measurement of pH using electrode-based sensors is affected by temperature variation. A technique of simultaneously measuring two parameters, pH and temperature, with a single RLC resonator based sensor is presented. An algorithm is developed, which applies both pH and temperature measurement to incorporate temperature compensation in pH measurement. For in-fluid applications, an encapsulation method is applied to the LC resonator based sensor to reduce the influence of medium permittivity and conductivity on the sensor measurement. Non-invasive way to obtain reliable pH information from bacterial culture bioprocesses is demonstrated with the fluid embeddable sensor. The pH sensor is remodeled to an acidic and basic volatile sensor by embedding the electrodes in a hydrogel host electrolyte. Tests demonstrate that the volatile sensor has a detection limit of 1.5 ppm and 2 ppm for ammonia and acetic acid vapor, respectively. Application of the volatile sensor to fish spoilage monitoring shows that the sensor is capable of detecting the product rejection level with good sensitivity in real-time. It is important to develop low cost wireless passive pH sensor technologies for embedded applications such as bioprocess and food spoilage monitoring. The electrode-based passive LC sensor approach employed in this thesis overcomes drawbacks of some of the early developed passive pH sensors and can lead to an inexpensive implementation using printed electronics technology.

The thesis describes development and application of coupled resonant coil sensors, which is of growing interest for remote monitoring applications. An interrogation technique, which improves the accuracy and interrogation range of coupled resonant coil sensors, is introduced. The method uses time-domain gating to produce measurements that are dominated by the response of the sensor coil and are immune to surrounding object interference. For application in structural health monitoring a low cost embeddable coupled coil sensor, which is able to monitor the corrosion potential of reinforcement steel is presented. Results of an accelerated corrosion test using the sensor indicate that corrosion potential can be monitored with a resolution less than 10 mV and a sensitivity of 0.76 kHz/mV. The last part describes a coupled-coil pH sensor based on pH electrode potential measurement. A linear response over a 4 to 10 pH dynamic range and 50 kHz/pH sensitivity are achieved with a 0.1 pH resolution and 30 s response time.