Scuba diving: Gases and mixes
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Diving naturally revolves around gases: We have to breathe the appropriate gas at the correct depth if we want everything to go smoothly: this includes planning for the correct Oxygen/Nitrogen ratio and in more advanced cases for Helium (and sometimes even hydrogen).
On top of being a key point for our well-being underwater there are also other uses for gases in diving, such as inflating buoyancy devices including BCDs and drysuits, and thermal insulation. There are also harmful gases we should be aware of so we can prevent their formation and put ourselves out of harm's way.
This is all to say that as divers, we should know which gases are being used and why. For this exact reason, I wrote this article to explain in detail the use of each gas and gas mix!
Oxygen
Oxygen is the main and most important gas. I say it is the most important with full confidence because it is what keeps us alive. When we breathe, the fresh air gets into our lungs and our alveoli. There is then a gas exchange that takes place between those alveoli and the capillaries surrounding them to supply the blood with new oxygen. The blood is then pumped throughout the body to fuel the whole body and your cells through cellular respiration. The oxygen is somewhat the fuel of your body.
We have to make a distinction between the Oxygen we breathe and the Air we breathe, as when we talk about Oxygen, we talk about the molecular form of Oxygen: Dioxygen, present at a rate of 21% in the Earth's Atmosphere. The Air, however, designates the mix of all the gas in the atmosphere that we breathe, but more on that later.
Even if we need oxygen to stay alive, as it turns out, too much of it is harmful to our system. When talking about the toxicity of Oxygen we'll use PPo2 as a measurement for how much Oxygen we're currently breathing. PPo2 stands for Partial Pressure of Oxygen, which represents part of the absolute pressure of the breathed gas that is only produced by the Oxygen. For instance, on the surface, we breathe a PPo2 of 0.21 bar because there is 21% Oxygen in the air and the surface pressure is 1 bar, this results in PPo2 = 1 Bar x 0.21 = 0.21 Bar. If we dive at 40m on air, the PPo2 would be PPo2 = 5 Bar x 0.21 = 1.05 Bar.
In recreational diving, we don't exceed a PPo2 of 1.4 Bar, and during accelerated decompression stops the limit is at 1.6 Bar of PPo2. The depth at which a gas reaches a PPo2 of 1.4 Bar (or 1.6 for a gas used for accelerated decompression) is called MOD, or Maximum Operating Depth of this gas. The MOD of air is 57m, and to go deeper than that, you would have to start breathing gases with less than 21% Oxygen content. Such a gas is known as a hypoxic gas.
Nitrogen
Nitrogen makes up the bulk of the air we breathe, at a proportion of around 78%. It is distributed throughout our body through our blood and before it diffuses into our tissues. Nitrogen is known as an inert gas: On top of not being nearly as reactive as Oxygen, it isn't metabolically active and is just stored in our bodies.
The form of Nitrogen present in the Atmosphere, that we breathe is N2, or Dinitrogen -It is a molecule made up of two Nitrogen atoms bonded together-.
Nitrogen poses two main issues that affect divers: First is Nitrogen Narcosis, which is akin to a drunken state when the partial pressure of the breathed-in Nitrogen is high enough -it usually happens on dives exceeding 30m/98ft of depth-.
The second issue is that Nitrogen saturates the tissues of the body in proportion to its partial pressure, which increases with depth, and if the tissues have too much Nitrogen, the diver will be forced to perform decompression stops at gradually shallower depth to allow the nitrogen to diffuse out of the tissues to prevent the formation of bubbles when ascending. This is what is known as decompression diving.
Helium
Helium is also used as a breathing gas and is mainly used to reduce the Oxygen and Nitrogen content of a gas that is used. Helium is not as narcotic as Nitrogen and not as toxic as Oxygen and is therefore a good substitute.
Although I say that it is a good substitute, after about 150m (492 feet) of depth, another problem arises: High-Pressure Nervous Syndrome, also known as HPNS. This physiological condition occurs when breathing gas under pressure containing Helium; symptoms range from tremors to nausea and a decline in mental capacity. The severity of the symptoms will vary depending on the depth, Helium content, and speed of compression. There have been attempts to reintroduce a little bit of Nitrogen in the breathing mix to reduce HPNS, with successful results with an N2:He ratio of 1:10 to depth up to 1000ft (~304m).
Helium is also easier to breathe at deeper depth, -when gas is more compressed- due to its low density.
Hydrogen
Hydrogen is used as an experimental breathing gas to go further than what Helium allows us to on its own as it has known potential to mitigate HPNS. It is lighter than any other gas but has a high reactivity: Hydrogen-containing gas can explode if they come into contact with an ignition source.
A lesser-known effect induced by breathing Hydrogen under high pressure is Hydrogen Narcosis: Hydrogen is a narcotic gas, although it only has 0.55 the narcotic potency of Nitrogen. Because it is rarely used, it is quite hard to find information on Hydrogen Narcosis.
Argon
Due to its relatively high narcotic potential (narcotic potency 2.3 times higher than that of Nitrogen), Argon isn't the ideal choice for a breathing gas. Although it isn't toxic, Argon isn't backed by enough research to be properly used as a breathing or decompression gas. There have, however, also been attempts to use it to reduce HPNS in ultra-deep dives.
The main purpose of using Argon is for drysuit inflation as it has better thermal properties: Because argon has a thermal conductivity of 33% less than air, it helps the body to keep its warmth if used in a drysuit!
Mixes
Mixes, as covered in the following paragraphs, are blends of multiple other "singular" gases. For instance, Air is a mix, because it is a blend of Oxygen, Nitrogen, and Argon.
Air
Starting with the most common gas blend, Air, as just mentioned is a mix of multiple other gases in the following proportions:
- 21% Oxygen
- 78% Nitrogen
- <1% Argon
- Traces of water vapor, CO2, and other gases
Air is the most common mix used to dive because it is cheap and adequate to use for shallow dives. Air becomes noticeably narcotic at depths greater than 30m (98 feet). The maximum operating depth of air, as mentioned at the beginning of this article is 57m, after wich depth, the partial pressure of Oxygen will surpass 1.4 bar.
Nitrox
Nitrox designates an Oxygen/Nitrogen mixture with a higher Oxygen part than 21%. Due to the risks of Oxygen Toxicity happening shallower due to this increased oxygen content, Nitrox tanks should be analyzed and clearly marked. Special cylinders are used for Nitrox. The usual color codes for nitrox are Yellow and Green.
We also call Nitrox gas "Enriched Air Nitrox" or EAN. It is usually followed by the oxygen content of the mix; for example, EAN 28 designates a gas mix with 28% Oxygen and 72% Nitrogen. EAN 32 would be a gas mix with 32% Oxygen and 68% Nitrogen.
Since proper precaution has to be taken for any gas containing a high Oxygen concentration -as it facilitates combustion-, any EAN 40 or above is treated as pure Oxygen and should only be used with Oxygen-clean equipment.
But why use Nitrox instead of air? We use it to increase our bottom time and decrease our decompression time. To avoid having to do decompression stops, we only have limited time at certain depths (e.g. Maximum 5 minutes on air at 40m/131 feet of depth without having to do decompression stops), this is because it takes some time for nitrogen to spread throughout our body and saturate our tissues. The more nitrogen is in the gas we breathe, the faster our tissues will get saturated. Inversely, if we have less nitrogen in our gas, then it will take more time for our tissues to get saturated. Because we have more Oxygen in Nitrox mixes, we consequently have less Nitrogen and saturate slower, this gives us prolonged bottom time.
This extended bottom time does come with a drawback: The higher the oxygen content of the gas mix is, the shallower its Maximum Operating Depth is.
Here are some frequent Nitrox mixes with their corresponding Maximum Operating Depth:
- EAN 28 - MOD of 40m/132 feet
- EAN 32 - MOD of 34m/111 feet
- EAN 36 - MOD of 29m/95 feet
- EAN 40 - MOD of 25m/82 feet
Trimix
Trimix is a mix of Oxygen, Nitrogen, and Helium. It is called Trimix because it is a blend of three gases. Introducing Helium reduces the Narcotic potency of the gas and makes it possible to go deeper while keeping full mental capacity. Adding Helium also makes it possible to reduce the Oxygen content and go deeper without exceeding the maximum PPO2.
Every Trimix blend inhaled at a certain depth has what we call Equivalent Narcotic Depth or END for short. It represents what depth you would have to go to, while on air, to be as "narced". This depth will vary depending on whether people consider Oxygen as Narcotic or not. This might be a little bit tricky to understand, so here is an example:
A diver is planning on going to 44m of depth with a 25% Oxygen and 20% Helium mix (this means, the other 55% will be Nitrogen). The Equivalent Narcotic Depth for this mix at 44m would be:
28m of depth if you don't consider O2 as Narcotic
33m of depth if you consider O2 as Narcotic
In the above example, a diver going to 33m of depth while breathing air will be as "Narced" as a diver going to 44m breathing the Trimix blend described.
Helium is an inert gas, like nitrogen, and it is also dissolved in the solution in our blood and tissues and should be taken into account in decompression schedules. For this reason, we'll avoid switching from a gas mix with a high Helium percentage to a gas mix without helium to avoid having a big concentration gradient and risking bubble formation.
Other Mixes
Even though Air, Nitrox, and Trimix are the main breathing gases used, other blends exist, and as it turns out, divers like giving names to each specific mix! So here are other existing mixes and their use:
- Hydrox: Hydrox is a blend of Hydrogen and Oxygen. Hydrox is only used in extremely deep dives with a very low Oxygen percentage, as any such mix with a higher Oxygen concentration than 3.8% is considered an explosive mix.
- Heliox: Heliox, as its name might suggest, is a blend of Helium and Oxygen. Also mainly used for deep-diving purposes, taking Nitrogen out of the equation reduces the narcotic potency of the mix, although Helium poses its own problem on deep dives. The fact that helium is easier to breathe due to its light nature (and not as explosive as hydrogen) makes it useful in the medical domain for patients who have trouble breathing.
- Hydreliox: Hydreliox is a combination of Hydrox and Heliox! It is a blend of Hydrogen, Helium, and Oxygen. This mix isn't common and belongs to the realm of research rather than recreational (or even technical) diving.
- Argox: Argon is a blend of Argon and Oxygen. As I have mentioned before, argon isn't used so much for breathing as it is for thermal insulation. Mixing Argon with Oxygen reduces the costs (as Argon is expensive) while only slightly decreasing the insulation capacity of the gas.
Harmful Gases
Let's talk about harmful gases a little bit. It is important to understand which are the potential harmful factors and how to avoid them!
Carbon dioxide
Carbon dioxide, or CO2, is a product of cellular respiration. It is discharged from the body with each exhalation. If the concentration of CO2 in our blood is too high, it will lead to convulsions, passing out, and eventually death.
Of course, it doesn't happen instantly and we feel the buildup of CO2 in our body -it is the notable feeling that forces you to breathe when you hold your breath for too long- and our natural reaction will be to inhale.
Divers have to take that into account when they breathe underwater: there is a "dead air space" that is created between their mouth and the regulator where air can stagnate if the diver breathes too fast (it will mainly happen to hyperventilating divers). This will lead to the diver breathing the same air he just exhaled and may lead to inhaling too much carbon dioxide.
Another possible cause of CO2 poisoning in divers is enclosed airspaces: in overhead environments, especially caves where there are "air pockets", it is possible for CO2 to get trapped and buildup. At a high enough concentration level, even a single breath in such a CO2-saturated environment can lead to instant loss of consciousness.
How to avoid CO2 poisoning: Breathe slowly, deeply, and regularly. Don't take off your regulator when surfacing in an enclosed airspace.
Carbon monoxide
Carbon monoxide, or CO, is similar structurally to carbon dioxide, with the big difference that it has one Oxygen atom removed. It is a product of incomplete combustion and is very poisonous. Carbon Monoxide is colorless, odorless, and tasteless, making it tricky to detect.
Carbon monoxide binds itself with hemoglobin, preventing hemoglobin from binding itself with Oxygen and depriving our body of it: Carbon Monoxide poisoning leads to asphyxiation.
In the realm of diving, Carbon Monoxide contamination might happen due to an unmaintained compressor or filter.
How to avoid CO poisoning: Only get tank fills from trusted dive centers. Get a confirmation that they have a Carbon Monoxide detector and that their compressor, and filters are up to date with their maintenance.