A Wireless Passive Sensor for pH Monitoring Employing Temperature Compensation

We present an integrated coupled-coil passive sensor for temperature compensated remote pH monitoring. The sensor is a RLC resonant circuit consisting of an inductive coil connected in parallel to a thermistor, a varactor and a pH combination electrode. pH variation of the solution changes the voltage-dependent capacitance and shifts the resonant frequency of the sensor, while temperature change of the solution affects the resistance and changes the quality factor. The resonant frequency and quality factor of the sensor are tracked remotely by an interrogator coil coupled to the sensor coil. A temperature range of 25 degrees C-60 degrees C and pH over a 2-12 dynamic range can be measured simultaneously. With temperature compensation, the accuracy of the pH measurement is demonstrated to be 0.1 pH. The sensor overcomes the pH measurement error due to the temperature dependence of electrode based passive pH sensors.