Removal of Temperature and Earth's Field Effects of a Magnetoelastic pH Sensor

Magnetoelastic sensors are widely used for chemical and biological monitoring including measurement of pH, glucose, carbon dioxide, and Escherichia coli by applying a mass- or elasticity-changing coating that shifts the sensor's resonant frequency in response to the target analytes. However, the sensor's resonant frequency also varies with the ambient temperature and earth's magnetic fields, reducing the accuracy and reliability of the measurements. This paper presents a technique to eliminate the effects of temperature and earth's magnetic field on the magnetoelastic sensor by detecting the change in its higher order harmonic magnetic fields, which are generated by the sensor when excited by a low frequency magnetic field. The higher order harmonic response of the magnetoelastic sensor is a function of temperature and DC field but remains unaffected by the mass/elasticity change from the chemical or biological responsive coating, thus allowing the calibration of both interfering quantities. This paper illustrates the application of this technique on a magnetoelastic pH sensor, where the results show the calibrated measurements are independent from the ambient temperature and DC magnetic fields such as the earth's field.