The present series of unmanned hyperbaric tests were conducted in order to investigate the oxygen fraction variability in semi-closed underwater rebreathing apparatuses. The tested rebreathers were RB80 (Halcyon dive systems, High springs, FL, USA), IS-Mix (Interspiro AB, Stockholm, Sweden), CRABE (Aqua Lung, Carros Cedex, France), and Viper+ (Cobham plc, Davenport, IA, USA). The tests were conducted using a catalytically based propene combusting metabolic simulator. The metabolic simulator connected to a breathing simulator, both placed inside a hyperbaric pressure chamber, was first tested to demonstrate its usefulness to simulate human respiration in a hyperbaric situation. Following this the metabolic simulator was shown to be a useful tool in accident investigations as well as to assess the impact of different engineering designs and physiological variables on the oxygen content in the gas delivered to the diver by the rebreathing apparatuses. A multi-compartment model of the oxygen fractions was developed and compared to the previously published single-compartment models. The root mean squared error (RMSE) of the multi-compartment model was smaller than the RMSE for the single-compartment model, showing its usefulness to estimate the impact of different designs and physiological variables on the inspired oxygen fraction. / <p>QC 20150903</p>

Vital components are more or less prone to fail in a diving apparatus. This thesis examines the performance of oxygen sensors, carbon dioxide scrubber monitoring and composite gas cylinders. A partial pressure of oxygen sensor authentication is suggested in a published patent and poster, weaknesses in carbon dioxide scrubber monitoring systems near surface are revealed in a published paper and potential harmful gas permeability properties of a composite gas cylinder, altering the gas composition and decreases the oxygen fraction, is measured and determined in a submitted paper.The importance of adequately and thoroughly performed safety tests that are standardized becomes even more relevant when managing personal protective equipment. The European Committee for Standardization have ratified relevant standard for the work in this thesis;EN-14143 Respiratory equipment – Self-contained re-breathing diving apparatus,EN-12245:2009+A1:2011 Transportable gas cylinders – Fully wrapped composite cylinders, andISO 11119-3:2013 Gas cylinders – Refillable composite gas cylinders and tubes – Design, construction and testing.These tests form a base-line for the methods, tests and result evaluations performed here and are considered safe; however improvements to the tests and standards can be made and are here suggested.