The following paper provides a comparison of the various breathing media commonly used in commercial diving and in hyperbaric support of pressure-faced tunnel boring machines. The effects of various inert gasses, the role of hyperbaric oxygen, and saturation at both high and low pressures are explained. The decision-making behind selecting a proper breathing medium for a given pressure and work scope is explored. Equivalent air depths, equivalent narcotic depths, oxygen toxicity, and air saturation are discussed, as well as a comparison of efficiency and productivity based on gas blends, associated costs, and decompression profiles.
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