Why Do Divers Use Helium?
Divers use helium for three reasons, all of which have a profound benefit to diving safety. Breathing mixtures that contain helium are commonly referred to as “trimix”.
What is trimix?
Trimix is a mixture that contains three different gasses; helium, oxygen and nitrogen. It is commonly used by technical divers, although recreational trimix courses are becoming more popular over recent years.
Trimix has three sub-classifications:
- Normoxic trimix
- Contains a ‘normal’ amount of oxygen
- The dive community considers 18-21% O2 normal
- Whereas in medal terms, normoxic means specifically 21% O2
- The mix is breathable at the surface and at shallow depths
- Hypoxic trimix
- Contains less than 18% oxygen
- The mix is not breathable at the surface
- The %O2 determines a minimum safe depth for breathing
- It is breathable once the ppO2 equals 0.18
- Contains more than 21% oxygen
- It is enriched air nitrox (EANx) plus helium
- Typically used for 30-40m/100-130′ dives.
What are the reasons why divers use helium?
Oxygen is known to cause central nervous system (CNS) toxicity at higher pressures. This toxicity presents as convulsions (clonic seizures) which will typically drown the diver.
The recommended maximum safe oxygen exposure is 1.4 ppO2. When breathing air (21%O2), this equates to 56.5m/185ft. Below that depth, the fraction of oxygen in a breathing gas must be reduced to maintain 1.4 ppO2. Helium is added to reduce the %O2.
Nitrogen has strong anaesthetic properties at higher breathing pressures. This is defined by the Meyer-Overton Law of Lipid Solubility.
As the partial pressure of nitrogen increases, the anaesthetic effect becomes stronger. That anaesthetic effect is known by divers as “nitrogen narcosis”.
Nitrogen narcosis causes a debilitating decline in cognitive ability; which is very hazardous when diving.
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 and mitigate the effects of nitrogen narcosis.
As the diver descends, their breathing gas becomes denser. Studies by Simon Mitchell, in conjunction with the US Navy Experimental Diving Unit (NEDU), have demonstrated 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), along with CO2 narcosis when diving.
The recommended gas density limit by DAN is 5.2g per litre. When breathing air this equates to 31m/102′.
Helium has a very low molecular weight, so its addition to a breathing gas will lower the gas density.
Dive industry attitudes to gas density
Sadly, the recreational dive training industry ignores the issue of gas density. That’s purely a profitability consideration, because the DAN gas density recommendations would necessitate using expensive helium for Deep Diver and entry-level technical courses.
Which diving agencies promote the use of helium?
Scuba training agencies have differing policies on the use of helium between the depths of 30m/100′ and 55m/80′:
- Mandated use of helium below 30m/100′:
- Offer recreational-level helium courses (30m-40m/100-130′):
- Do not recommend or offer helium on recreational courses:
About the Author
Andy Davis is a RAID, PADI TecRec, ANDI, BSAC and SSI-qualified independent technical diving instructor who specializes in teaching sidemount, trimix and advanced wreck diving courses.
Currently residing in Subic Bay, Philippines; he has amassed more than 10,000 open circuit and CCR dives over 27 years of diving across the globe.
He has published numerous diving magazine articles, designed courses for dive training agencies and tests/reviews dive gear for scuba equipment manufacturers. He is currently writing a series of advanced diving books and creating a range of tech diving clothing and accessories.
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.
Originally posted 2019-02-22 12:34:35.