sentences:  The term "oxygen window" was first used by Albert R. Behnke in 1967.Behnke refers to early work by Momsen on "partial pressure vacancy" (PPV) where he used partial pressures of oxygen and helium as high as 2–3 ATA to create a maximal PPV.Behnke then goes on to describe "isobaric inert gas transport" or "inherent unsaturation" as termed by LeMessurier and Hills and separately by Hills.