Helium

Helium is so rare that it was not known to exist in significant quantity until 1895. Some estimates suggest that commercial helium sources will be exhausted shortly into the twenty first century. An extremely light gas (second only to hydrogen), helium is inert and doesn’t even combine with itself to form a twoatom gas molecule like nitrogen or oxygen.

In diving, helium is used as a substitute for nitrogen in the breathing mixtures of deep commercial, technical and military divers because it does not produce the narcotic effects under pressure that are associated with nitrogen. These mixtures include heliox (helium and oxygen), and trimix (helium, nitrogen and oxygen).

Because of its light molecular weight, helium mixtures are easier to breathe than nitrogen-oxygen mixtures at a given depth, but there are problems associated with helium use in diving. First, divers breathing heliox and trimix have to contend with extensive and complex decompression procedures. These procedures not only require special dive tables for the heliox or trimix in use, but may require the diver to switch to different gas blends during decompression. Obviously, this is beyond the needs and limits of mainstream recreational diving.

The second problem is a result of helium’s light molecular weight, which makes sound travel faster in a heliox mixture. This increased speed makes a diver’s voice sound garbled and unintelligible when using voice communication. Fortunately, electronic communication devices can restore this unintelligible speech to an understandable form.

Third, helium conducts heat well, so a helium mix is not used for dry suit inflation. This means that the diver must use a different gas for insulation (see Argon) in a drysuit. However, using a different dry suit gas raises a decompression concern in which gases dissolved in the body try to dissolve out of the body through the skin when the diver is surrounded by a gas different from the breathing gas. This theoretical problem (called isobaric counterdiffusion) occurs when a diver breathes a slowly diffusing gas while immersed in a rapidly diffusing gas. This is avoided by choosing an appropriate dry suit gas, (such as air or argon) or by using a wet suit.

Finally, at extreme pressures helium is associated with a physiological phenomenon call high-pressure nervous syndrome (HPNS), which causes muscular tremors, dizziness and nausea at extreme depths. This can be partially offset by putting nitrogen in the breathing mix, so that nitrogen narcosis “calms” the nervous system. Again, it should be noted that the use of helium and the effects on HPNS are well beyond the realm of mainstream recreational diving.