Originally posted 2019-02-22 12:34:35.
Scuba Q and A:
Why Do Deep Divers Use Helium?
There are three reasons why helium becomes a necessary addition to breathing gas as depth increases:
Oxygen Toxicity
Oxygen is known to cause central nervous system (CNS) toxicity at higher pressures. This toxicity presents as convulsions, which will inevitably drown the diver. The current standard maximum safe oxygen working pressure is 1.4 ata ppO2. This equates to 56.5m/185ft when breathing air (21% O2 / 79% N). Below that depth, the fraction of oxygen in a breathing gas must be reduced, maintaining max 1.4 ppO2, by the addition of helium.
Gas Narcosis
Both nitrogen, and probably oxygen, have strong anaesthetic properties at higher breathing pressures. This is defined by the Meyer-Overton Law of Lipid Solubility. As the pressure of breathing gas increases, the anaesthetic effect becomes stronger – causing a debilitating decline in cognitive ability, and eventually unconsciousness. Helium has a low lipid solubility and, thus, a low anaesthetic effect. For that reason, helium is added to breathing gases to reduce the fraction of nitrogen.
Gas Density
As the diver descends, their breathing gas becomes denser. Studies have shown that increasing gas density lowers respiratory performance and reduces capacity to expel metabolised CO2. Higher breathing gas density significantly increases the risk of hypercapnia (CO2 poisoning). The suggested limit, as shown in studies by a huge spike in CO2 retention, is a maximum breathing gas density of 6g per litre. On air, this equates to 36m/185ft. Helium has a very low molecular weight, so its addition to a breathing gas will lower its density. Much of the dive industry ignores this fact, as it stipulating that divers use expensive helium for dives below 36m/185ft would undoubtedly reduce business revenues.
About the Author
Andy Davis is a RAID, PADI TecRec, ANDI, BSAC and SSI qualified independent technical diving instructor who specializes in teaching advanced sidemount, trimix and wreck exploration diving courses across South East Asia. Currently residing in ‘wreck diving heaven’ at Subic Bay, Philippines, he has amassed more than 9000 open circuit and CCR dives over 27 years of diving across the globe.
Andy has published many magazine articles on technical diving, has written course materials for dive training agency syllabus, tests and reviews diving gear for major manufacturers and consults with the Philippines Underwater Archaeology Society.
He is currently writing a series of books to be published on advanced diving topics. Prior to becoming a professional technical diving educator in 2006, Andy was a commissioned officer in the Royal Air Force and has served in Iraq, Afghanistan, Belize and Cyprus.