Characterization and measurement of elastance with application to underwater breathing apparatus

The elastic loads inherent in underwater breathing apparatus (UBA) can affect diver performance. This work quantitatively examines elastance, its measurement by static tests, and its relationship to UBA. A rigid, fixed-volume container and a vertical water column with straight and parallel sides produce elastic loads that have application to closed-circuit UBA. We derived equations to describe the pressure-volume relationships of these elements from first principles and tested the equations experimentally for system pressure produced by a swept volume in a breathing machine. Our work demonstrates that simplified equations to describe elastance may not be sufficiently accurate for all situations. In addition, static measurements of elastance of a UBA or other element will not be reproducible unless all the volumes within the testing apparatus are accounted for; we provide experimental and mathematical techniques for doing so.