Learn the science that rules the world of diving, one topic at a time. In this installment, we’re exploring gases.
Speak to a non-diver or a novice diver, and you’ll find them often referring to what is in diver’s tanks as “air” or, worse, as “oxygen”.
The latter is downright wrong, as recreational divers never dive on pure oxygen. The former is usually correct, as most dives worldwide are done on normal atmospheric air.
However, the proper term, that you’ll often hear more advanced, and in particular technical divers use to call air for diving, is “gas”.
But wait, isn’t gas toxic?
It’s a gas, gas, gas
A “gas” in the strict, scientific sense, and the sense we use it in diving, is simply the aerosol form of a type of element or combination of elements.
In science, everything can be in three states or forms: solid, liquid, and gaseous. Take water as an example, it’s solid form is ice, it’s liquid is plainly water, and its gaseous is steam.
So when we talk about “gas” in diving, we’re not speaking of gas as we know it from stoves. Strictly speaking, that is typically a methane gas, so a form of gas.
But any element can be a gas. So when we speak of the gas called “air”, we’re actually talking about a combination of primarily the gases nitrogen and oxygen.
Partial pressure
When we speak of gases in relation to diving, we often use the term “partial pressure”, especially with oxygen.
Partial pressure is the pressure a single gas in a mix, relative to the amount of gas in the mix and the depth, or pressure, we’re breathing it at. The reason why this is relevant, is because oxygen at too great a partial pressure is actually toxic. The maximum partial pressure where oxygen is considered safe is 1.6, with 1.4 being the recommended partial pressure for diving, and 1.6 used only as a backup.
Partial pressure is as mentioned relative to depth and pressure, meaning that the more oxygen in a mix, the shallower a depth do we reach the maximum partial pressure it is safe to breathe it. The partial pressure of a gas is found by timing the amount of gas in the mix with the pressure it is breathed at.
So the partial pressure of oxygen in normal surface air, with 21 % oxygen, breathed at 30 meters/90 feet is:
0.21 (oxygen content) * 4 (bars pressure at 30 meters/90 feet) = 0.84 bars of partial pressure.
Air – the most common breathing gas
Air is really just a combination of gases, 78 % nitrogen and 21 % oxygen, with the remaining 1 % made up of a number of gases, but in such small quantities that they’re typically not factored in.
We use the oxygen to fuel or cells, while the nitrogen is a passive gas that we don’t use.
Unfortunately, nitrogen binds very well in our body’s tissues, which is where the risk of decompression illness comes from. Our tissues are saturated with nitrogen as we dive, more so the longer and deeper we dive (more on the combination of gas and pressure in a later installment) and needs to be released in a controlled manner.
Essentially, nitrox, the second-most common gas for diving, is not that much different than air. It contains the same gas components, just in different ratios. In nitrox, some of the nitrogen is replaced by oxygen, allowing for longer dives, but at the same making the mix unsuited for very deep dives, due to oxygen toxicity.
Other breathing gases
For diving, we also use other gases, though. The one gas we cannot be without, is oxygen, though we don’t want too much of it, as it increases the risk of oxygen toxicity.
Also, the deeper we go or the longer we want to stay, the more it becomes relevant to want to replace some of the nitrogen with a less troublesome gas. The common “replacement” gas is helium, as this gas is completely inert, and doesn’t cause problems one way or the other.
Here are tips on How To Manage Your Air Consumption for a deeper and longer dive.
For long dives in relatively shallow water, we may also use helium to replace a certain amount of the nitrogen in the mix, to reduce the amount of nitrogen being stored in our tissues, and through that, reduce the risk of decompression illness.
If we go deep enough, and we might opt for adding even more helium to also replace some of the oxygen, to reduce the risk of oxygen toxicity. The former mix (21 % oxygen but added helium to replace some of the nitrogen) is known as helitrox and the latter mix (varying levels of oxygen based on dive depth, helium added to replace this and some of the nitrogen) is known as trimix.
Trimix and helitrox are thus essentially the same, just air and nitrox, it is just the mix that differs.
Take a moment to look at this.
