May 2015

Different Diving Gases

By Mike Waddington

Many non-divers believe that the cylinders that we use contain pure oxygen. Once you have completed your Open Water level course you will learn that this is not true, our cylinders are filled with natural air unless we are specially trained to dive with other gasses. So, what are these other different diving gasses that can be used? And why would we need to use them?

Diving with Nitrox

Enriched air nitrox (EANx), or simply called Nitrox is the second most commonly used gas in diving after air. When we dive our bottom time is limited by two main factors, our gas consumption and our nitrogen loading. Nitrox is simply a blend of Oxygen and Nitrogen that has higher levels of oxygen than air does. Common mixtures are 32% oxygen 68% Nitrogen, or 36% Oxygen 64% Nitrogen, although for recreational diving you can find nitrox levels with oxygen levels varying from 22% to 40%. By reducing the amount of nitrogen we breathe while diving we allow ourselves to stay longer at depth, however because of the increased levels of oxygen we cannot dive as deep as we would be able to with air.

Diving with nitrox requires special training, a gas analyser and someone to correctly and accurately blend your desired gas. Nitrox is commonly used on Liveaboards, where the customers and dive guides may be completing five or six dives in a day, for over a week.

Trimix Diving

Trimix is a mixture of Nitrogen, Oxygen and Helium used to increase depth limits. There are three main types of Trimix: Normoxic, Hypoxic and Hyperoxic. Normoxic means that is has similar or the same level of oxygen in the mix as air. This means that the diver is limited to the same depths as air. Hypoxic means that there is less oxygen in the mix than there would be in normal air, this allows the diver to go deeper than they could on air, but it also means it may not be suitable to breathe at the surface, therefore needed a ‘travel gas’ to get to a depth when the diver would switch onto the hypoxic Trimix. Hyperoxic (also called ‘Helitox’) contains more oxygen than standard air, which means the diver cannot dive as deep as they could be able to for air.

Diving with Helium

Helium is used in the breathing mix because unlike nitrogen, it isn’t narcotic. This means a diver could use Trimix to dive to depths of 100 + metres and still be clear headed. As a diver descends over 100 metres using a helium based breathing gas, they may get ‘High Pressure Nervous Syndrome’ (HPNS) which causes the diver to shake uncontrollably while at depth. Helium conducts heat six times faster than air, so if used while drysuit diving it could potentially lead to hypothermia.

Helium based breathing gasses such as Trimix are only used by serious technical diving who use it to either go deeper than air would allow, or to stay ‘sober’ while completing complicated dives at depths greater than 30 metres, such as long cave/wreck penetration, or scientific research.


Heliox is another helium based breathing gas, but unlike Trimix it does not contain nitrogen. It is used for the same reasons as Trimix, so it is only ever encountered in the worlds of technical and commercial diving.


Oxygen is the key to life, without it we would die within minutes. However under pressure oxygen can be toxic, therefore oxygen is only used by technical divers who use it to accelerate their decompression. Oxygen is also highly reactive and can make other substances burn with very high intensity. Proper training and special equipment is required to work with pure oxygen.

Despite its dangers, a cylinder of pure oxygen is an essential piece of a first aid kit for divers. Usually this cylinder will have a different valve on it, and be clearly marked so it is not accidentally taken underwater. It is used in a variety of emergencies such as near drowning or cardiac arrest, and it is the primary first air for divers surfacing with signs and symptoms of ‘Decompression Illness’ (DCI)


Argon is used for diving, however it is not used as a breathing gas. Argon is an inert gas and is far more narcotic that Nitrogen, so is not suitable for use. There is a gas mix called Argox which is composed of Argon and Oxygen, which is being used for decompression research. The main use for Argon while scuba diving is for dry suit inflation by Trimix divers because it offers good thermal insulation.


Most people believe hydrogen to be an explosive dangerous gas, which it can be. However a gas mixture of less than 4% hydrogen will not burn, and with more than 75% in air (94% in oxygen) will also not burn. So 100% is safe, as long as it is contained. If it leaks however, and there is an ignition source, it can be very explosive (remember the Hindenburg disaster)

So what use could it have in diving? As we descend to depth the gas we breathe becomes denser, making it harder to breathe. As we start to go really deep we use helium as it is much less dense than air, therefore easier to breathe, but what happens when we go so deep that even helium becomes difficult to breathe due to pressure? We move to the only gas that is lighter than helium, hydrogen.

Trimix has allowed commercial divers to descend to 686 metres however a hydrogen-oxygen-helium mixture has beaten that record with 701 metres! Unlike helium, hydrogen is narcotic, although less so than nitrogen. The difference is that the effects of narcosis from nitrogen is somewhat like mild alcohol intoxication, whereas hydrogen has effects similar to that of LSD when used at great depths. Some divers have even claimed out of body experiences while conducting deep dives using a hydrogen based mix, which is the last thing you want while working deeper than 600 metres underwater!

The Future of Diving?

For recreational diving the future will stay the same as it is now. Most divers will simply use air, and some may choose to use Enriched Air Nitrox. It is the technical and commercial diving world that will see changes in the gasses used.

Helium is becoming more and more expensive as the years go by, and there is one reason for that. We are running out of it, and we are expecting to fully deplete our stocks within the next 25 to 30 years. It is not just used for diving, but it is very important for hospitals, who use it in liquid form to cool MRI scanners, and anti-terrorist authorities who use it in radiation monitors.

Hydrogen could become a serious contender in gas selection for deep diving in the not too distant future, or maybe other exotic gasses could be used. Let’s just make sure we don’t fill our scuba cylinders with Nitrous Oxide!

‘Different Gases used for Diving’ was written by Mike

Photo Credit: Perth Scuba

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Mike Waddington

I first discovered diving in 2008 after going snorkeling on the Great Barrier Reef in Australia. After trying diving at a flooded quarry in England I decided to head out to warmer more interesting waters in Thailand where I ended on the Island of Koh Tao completing my Open Water course. Instantly addicted with money to spend and plenty of time on my hands I decided to continue until I became a Divemaster so I could live what seemed as the perfect life.

After that I headed to the Caribbean to an island called Utila to complete my instructor course, I spent several months out there completing the MSDT internship, teaching students and leading dives. This is also where I discovered my interest in the technical side of diving, taking part in equipment repair courses and learning about blending gasses and running compressors.

With all my new qualifications it was time to head back to where it had all started, Back to Koh Tao where I intended on living the dream. Once I arrived I quickly found a job and started teaching straight away. During my time on Koh Tao I took part in all many technical diving courses, learning how to dive with re-breathers, in caves and even going down to 90m/300ft!


PADI Master Scuba Diver Trainer
PADI/DSAT Tech Deep Instructor
PADI/DSAT Gas Trimix Gas Blender
PADI/DSAT Trimix Diver
TDI Intro to Cave Diver
TDI Advanced Wreck Diver
TDI Inspiration rebreather Decompression Procedures
PADI Professional Videographer
BSAC Compressor Operator
TDI Equipment Service Technician

Dream Dive Locations:

Silfra, Iceland
Cenotes, Mexico
Chuuk Lagoon, Micronesia
Ice Diving in Russia