Biomedical Applications of a Commercial Capacitance Transducer

A capacitance displacement transducer with a linear response and constant sensitivity for a frequency range of 0-1000 Hz is described. Its application to measurement of chest wall motions was verified using static displacements from flat and curved surfaces and both human tissue and metal. The transducer has been used to obtain recordings of apex motion, heard sounds, and brachial and radial pulses. Its advantages are that it is noncontacting and linear, and that it has a wide frequency response and can easily be calibrated to obtain quantitative data of actual motion. Problems encountered in use were the mechanical positioning of the probe including respiratory motions in the measurement and the need to insulate the probe with a film of thin plastic to insure against an electrical hazard to the subject.