Design and Construction of an Ultrasonic Pneumotachometer

Pneumotachometers (PTM's) measure the instantaneous rate of volume flow of respired gas. Currently existing PTM's suffer from a number of limitations. Ultrasonic PTM's are superior in a number of ways to these PTM's, but have not yet achieved acceptance due to instabilities and inaccuracy. We investigated a new design which used cylindrical shell transducers to generate ultrasound and measured phase shift as a measure of transit time change due to flow. The results were linear and predictable over a wide dynamic range using a single gas under controlled conditions, but deviated from expected values when the gas composition changed. We evaluate our original model of UPTM operation and revise it in light of our findings.