and helium and is often used during the deep phase of dives carried out using technical diving techniques. This term, first used by Sheck Exley, is mostly used by Technical Diving International (TDI). It is easily blended from helium and air and so has a fixed 21:79 ratio of oxygen to nitrogen with the balance consisting of a variable amount of helium. It is sometimes referred to as "poor man's trimix", because it is much easier to blend than trimix blends with variable oxygen content, since all that is required is to insert the requisite partial pressure of helium, and then top up with air from a conventional compressor. The more complicated (and dangerous) step of adding pure oxygen at pressure required to blend trimix is absent when blending heliair. Heliair blends are similar to the standard Trimix blends made with helium and Nitrox 32, but with a deeper END at MOD. Heliair will always have less than 21% oxygen, and will be hypoxic (less than 17% oxygen) for mixes with more than 20% helium. History as a diving gas 1919: Professor Elihu Thomson speculates that helium could be used instead of nitrogen to reduce the breathing resistance at great depth. Heliox was used with air tables resulting in a high incidence of decompression sickness, so the use of helium was discontinued. 1924: The US Navy begins examining helium's potential usage and by the mid-1920s lab animals were exposed to experimental chamber dives using heliox. Soon, human subjects breathing heliox 20/80 (20% oxygen, 80% helium) had been successfully decompressed from deep dives. 1937: Several test dives are conducted with helium mixtures, including salvage diver Max "Gene" Nohl's dive to 127 meters. 1939: US Navy uses heliox in USS Squalus salvage operation. Heliox usage, coupled with the absence of decrement in co-ordination and cognitive function in the salvage divers, confirms Behnke's theory of nitrogen narcosis. 1965: Nic Flemming's work to study sand ribbons in the English Channel becomes the first to compare diver performance while breathing air and heliox in the open water. 1963: First saturation dives using trimix as part of Project Genesis. 1970: Hal Watts recovers two bodies at Mystery Sink (126 m). 1979: A research team headed by Peter B. Bennett at the Duke University Medical Center Hyperbaric Laboratory begins the "Atlantis Dive Series" which proves the mechanisms behind the use of trimix to prevent High Pressure Nervous