Phase Shift Signal of Quartz Crystal Microbalance with 5th Harmonic Frequency for Monitoring Respiratory Rate
Real-time continuous monitoring of human respiratory rate has significant medical diagnostic value,a piezoelectric quartz crystal with a fundamental frequency of 10 MHz is used as a sensing element for respiratory monitoring,in which the direct digital frequency syn-thesizer(DDS)synthetic frequency source is applied to excite the piezoelectric quartz at 5th harmonic frequency and the phase shift of the piezoelectric quartz crystal is continuously measured under respiratory airflow in real time.Due to the sensitive mass response of the piezo-electric quartz to the adsorption and aggregation of water molecules on the surface of the crystal at high frequencies,the continuous monito-ring of respiratory frequency is successfully achieved by direct using a quartz crystal with a metal electrode(silver electrode)without the need for thick film modification on the quartz surface.Compared to the traditional frequency shift method,the proposed method has a fast signal sampling rate of down to 0.05 s/point,allowing for accurate monitoring of both slow and rapid breathing processes.The QCM phase shift respiration detection system not only meets the requirements for continuous respiration monitoring but also provides information on re-spiratory characteristics such as respiratory rate,respiratory strength,and respiratory disturbance for human respiratory monitoring.
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