In the beginning, people assumed that helium decompression was more dangerous and took longer than nitrogen decompression. This led to the discouragement of helium for diving and caused quite a bit of fear; all because of supposed facts that were actually fallacies. Today, we know that helium, used properly, is a gas that can be utilized for deep dives.
Modern Decompression Theory
When helium bottom mixes first came into use, the tendency was to switch from bottom mix to nitrox or air as early as possible in an effort to reduce overall deco time. Today, we know that if helium and nitrogen are decreased at about the same proportion as oxygen is increased, until a switch to enriched nitro mix in the shallow zone, deco differences are small and the diver feels much better than when using outdated protocols.
In fact, the newest field tests have shown that using helium mixes all the way to the surface with a switch to pure oxygen in the shallow zone can be even safer and more deco-efficient. NAUI Technical Diving Operations has constructed a new training regimen for instructors and divers, based on helium for technical diving; Helitrox courses and a full set of RGBM tables are now available.
Safe and reliable as a component of a technical mix, Helium is changing technical and exploration diving for the better. To understand why, it is important to look at the gas's properties.
Nitrogen has its limits, as anyone who has experienced narcosis can tell us; and while individual tolerance varies, it really isn't safe to use beyond 130 fsw. Beyond 500 fsw, loss of consciousness results. Oxygen also has its limits, and while a number of inert gas replacements have been tested, only hydrogen and helium provide desirable results. Hydrogen though, is explosive, making it riskier to mix, leaving us with only a single choice: Helium. The best gas for deep and saturation diving, it can be breathed for months without tissue damage; and, it saturates and desaturates approximately 2.7 times faster than nitrogen.
Since the middle of the 1940's, when mixed gas diving was first attempted, many errors resulting in grave injury and death have occurred. In 1945, Arne Zetterstrom made it to a depth of 500fsw on hdrox and nitrox, and then died of hypoxia and DCS after being hoisted to the surface too soon. A 1962 dive to 1000fsw by Hannes Keller and Peter Small resulted in both divers losing consciousness on the way up due to platform support errors, with Peter Small and a support diver named Christopher Whittaker, who was only 19 years old, ultimately meeting their deaths. In 1965, Robert D. Workman published decompression tables for both nitrox and heliox, with the nitrox version eventually evolving into US Navy tables. Then, as part of a study by Duke University Medical Center, the 3 man team of Atlantis III successfully made a 2250fsw chamber dive on heliox, with researchers discovering that adding 10% nitrogen to the heliox eliminated HPNS (High Pressure Nervous Syndrome).
Using Helium
Most divers who use helium report that they feel better and overall healthier than when using nitrogen mixtures. While these findings are of course personal and subjective, they are of great importance; post-diving decompression stress is also less than it is on nitrogen.
Finally, helium decompression is fast and efficient, as helium's bubbles are smaller, and they diffuse much more quickly. The gas's effects are much less narcotic, as well. In order to take advantage of these benefits, proper helium staging is crucial. There are many possibilities for switch depths, mixtures, and strategies; so knowledge is of critical importance. While helium isn't the cheapest gas to dive on, it is emerging as a leader for comfort and safety both.