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.