A wireless passive pH sensor based on pH electrode potential measurement

We present a wireless coupled-coil passive 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 impedance of an interrogator coil coupled to the sensor coil. The sensor consists of an inductive coil connected in parallel with a varactor (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 over a 4 to 10 pH dynamic range and 50 kHz/pH sensitivity are achieved with a 0.1 pH resolution. The sensor has a response time of less than 30 sec.