Three primary misconceptions about the air entrainment process used in some oxygen therapy masks are that (1) the Bernoulli principle causes air entrainment, (2) air is entrained by a pressure difference induced by the primary (jet) flow, (3) increasing the primary flow of oxygen increases the delivered oxygen concentration. The principle of air entrainment is constant-pressure jet mixing. The Bernoulli principle is not the basis for this mixing, because it states that pressure is reduced in exchange for an increase in velocity. Entrainment occurs at constant pressure; there is no mechanism to support a pressure gradient between the jet and the entrained air. Air entrainment occurs as a consequence of fluid viscosity. The amount of entrained air is proportional to the primary (jet) flow, leaving the concentration of oxygen unaffected by changes in primary flow.
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