A fluidic flowmeter for respiratory airflow measurements.

This thesis is concerned with the development of a laminated metal fluidic flowmeter for respiratory air flow measurements; both the inhalation and exhalation modes of breathing are detectable over a wide range of breathing rates. The principle of operation of the flowmeter is that a small confined planar incompressible jet, flowing normally across the respiratory tube, is deflected laterally by the respiratory flow. Two receiver ducts symmetrically positioned on the jet centreline, provide a differential output pressure proportional to respiratory flow, to improve output signal-to-noise ratios they are constructed with more than one parallel path. Different geometries have been investigated, including the effect of receiver spacing, variations in receiver and supply jet aspect ratio, and the application of positive feedback. Response tests have been carried out on the flowmeter by employing step flows from a large scale fluidic switch and comparisons with a Fleisch respiratory flowmeter have been made. It is concluded that a bidirectional flowmeter operating up to at least 450 litres per minute, with a dynamic response of the order of 14,500 litres/min/sec can be successfully constructed. An approximately linear relationship between receiver differential pressure and measured flow is achieved with a signal-to-noise ratio in the order of 160 at maximum output. This novel instrument has a faster response and covers almost three times the range of existing respiratory flowmeters