Principles of pressure transducers, resonance, damping and frequency response


      The most common invasive pressure transducer we use is a strain gauge. This consists of short pieces of wire attached to a diaphragm which is distorted during transmission of the pressure impulse. Consequent changes in the length of the wire alter their resistance, which, when connected to a Wheatstone bridge circuit, allow us to produce an electrical signal for display. Resonance may occur when the frequency of the pressure waves in the incoming impulse matches the natural frequency of the transducer thereby causing superimposition of pressure waves. The natural frequency of the system may be increased by using a stiff diaphragm and short wide-bore tubing. Fourier analysis allows us to breakdown a complex waveform into its component harmonics. Faithful reproduction of the waveform is possible using the first ten harmonics. Therefore a transducer system with a natural frequency response of 30 Hz would allow us to reproduce pressure waves of 180 beats/min. Damping reduces the high-frequency noise to allow a more accurate reproduction of the wave form. Too little damping allows oscillations which distort the results while too much damping delays the signal.


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