Measurement of temperature and liquid viscosity using wireless magneto-acoustic/magneto-optical sensors

Abstract

Remote query magneto-acoustic and magneto-optical sensors are used to measure liquid temperature, viscosity and density. Sensors comprising magnetoelastic Metglas(R) 2826MB thick-films, alloy composition Fe/sub 40/Ni/sub 38/Mo/sub 4/B/sub 18/, oscillate in response to an externally applied, time-varying magnetic field. The sensor oscillations are strongest at the characteristic mechanical resonant frequency of the sensor. Depending upon the physical geometry and surface roughness of the magnetoelastic films, the mechanical sensor-vibrations launch an acoustic wave that can be detected remotely using a hydrophone or microphone. Furthermore, the sensor oscillations act to modulate the intensity of a laser beam reflected from the sensor surface. The sensor vibrations were optically monitored using a photo detector placed in the path of a laser beam back-scattered off the sensor ribbon. Using a Fast Fourier Transform, the signal obtained in the time-domain from acoustical or optical detectors is converted into the frequency-domain from which the resonant frequency of the sensor is determined. The resonant frequency shifts linearly with temperature and, when immersed in a liquid, with the frictional damping forces associated with liquid viscosity and density, thus allowing a remote measurement of temperature and liquid viscosity.