Estimation of gas-phase diffusivities in hyperbaric environments

Diffusion of a particular gas in a mixture of three or more gases depends on diffusion characteristics and concentrations of the other gases and also on environmental pressure. Estimates of gas-phase diffusivities in hyperbaric environments can be calculated from binary coefficients by the Wilke equation. Sample calculations show that addition of carbon dioxide and water to inspired gas has very little effect on diffusivity of oxygen but that neglect of lesser components of a mixture, such as the nitrogen in 'trimix' or the helium in crude neon, would lead to errors of 10% to 20%. It is not possible to match a compressed air environment with a helium-oxygen or a helium-oxygen-nitrogen environment for both density and diffusivity. Diffusivities of oxygen and carbon dioxide in a helium mixture can be less than half the values in compressed air having the same density. In a plot of diffusivity vs. gas density, most useful mixtures are included in a hyperbolic-shaped band; diffusivity falls below 25% of the room air value when density is 5 times normal.