This is the second in a short series of pieces about gases and gas behavior.
This series of articles are excepts from my book, Twenty Lectures on Technical Diving, due this summer. This particular piece is based on a presentation prepared for an Advanced Nitrox / Decompression Instructor Program in 2008
“In the natural sciences, and particularly in chemistry, generalities must come after [earning] detailed knowledge of each fact and not before.”
Joseph Louis Gay-Lussac, French chemist whose work on gas behavior was visionary and is misquoted by dive instructors the world over, 1778 – 1850
For divers, certainly for technical divers, a little detailed knowledge of oxygen and its behavior is important. Why? Well, breathing too much of it can be fatal. Breathing too little of it can be fatal. And as though that’s not enough reason to pay attention, oxygen has to be stored, transported and delivered with some care otherwise it can cause real damage to property and people! In short, if we forget or neglect to follow the rules, oxygen can be a real menace; however, the rules are straightforward and easy to remember!
Let’s start off with some basic chemistry and character assessment.
Oxygen makes up approximately 21 percent of air by volume… this compared to nitrogen at roughly 79 percent. These figures are fudged because air has many other components including things like water vapor, carbon dioxide, and traces of several Noble Gases like neon, xenon, and so on. But regardless of these facts, divers and diving texts simplify matters and quote the 21 percent figure. In truth, we can make this approximation without causing a fuss or compromising our safety. But it is worth remembering that it is unlikely that the percentage of oxygen in the air around us right now is 21 percent… it’s certainly less and it varies under the influence of humidity, temperature, the season, location and the environment.
Oxygen is non-flammable — which strikes some people as counterintuitive — but it is highly reactive. This means that on its own at atmospheric pressure, oxygen behaves itself, but introduce another substance into an oxygen-rich environment or increase the pressure and you have a potentially dangerous situation because oxygen bonds eagerly with almost everything. With the slightest encouragement that “bonding” process can take the form of an aggressive, all consuming fire.
For example, high-pressure oxygen delivery systems — the vessels, valves and lines used to fill scuba cylinders — must be designed and built with no sharp corners in the hoses or sudden restrictions that might cause adiabatic compression, and thereby start a fire. Oxygen fires in these environments are notoriously difficult to extinguish and often burn until the oxygen runs out or there’s nothing left of the system to burn.
In oxygen delivery systems, needle valves are used rather than ball valves so that oxygen flow can be finely controlled and the likelihood of sudden pressure increases is lessened. All scuba gear used for mixing and delivering hyperoxic gases should be composed of materials suitable for use in a high-pressure oxygen environment. These components must be cleaned of hydrocarbons, lubricated sparingly with special lubricants, and be carefully stored and used specifically to prevent contamination with dirt and grease. So, don’t eat a sausage and bacon breakfast burrito while putting together your decompression cylinder!
In addition, decompression cylinders of high–test nitrox or pure oxygen must be filled slowly. I have seen the high-pressure seat inside a tank valve vaporized during a hurried fill. The cylinder looked fine from the outside but the gas it contained was pure oxygen contaminated with the gases formed as the nylon burned. (Two lessons learned that day. The second being always pre-breathe gases that are going to be used on a dive, before the dive begins.)
Oxygen is more compressible than nitrogen. Its molecules are so “friendly” that they cram up nice and tightly when being pushed into enclosed spaces. So for a given pressure inside a scuba cylinder, one is able to put a greater quantity of oxygen than say, air or most certainly helium. This is important information for those divers who blend their own gases. Without fudge factors taking into account variations in gas compressibility, or calculations modified via Van der Waals’ or Beattie-Bridgeman equations, their mixes will contain higher than planned levels of oxygen.
For those of you who like details, oxygen has a density of roughly 1.43 grams per litre at normal room temperature and pressure (20 degrees, one atmosphere).
OK, so that covers some basics about handling oxygen, now what about breathing it?
Of course oxygen is the “active” ingredient in air and necessary for our body to function. One part of our circulatory system’s job is to deliver oxygen to the tissues within our body, and over millions of years, that transport system and the rest of the human body it serves has evolved to function comfortably breathing a gas with an oxygen fraction of about 21 percent.
At sea-level an oxygen fraction of 21 percent translates into an oxygen partial pressure of 0.21 bar or 0.21 atmospheres. The wonderfully adaptable engine that it is, the human body is able to acclimatize to attitudes where there’s a significant drop in atmospheric pressure and therefore in the partial pressure of oxygen.
The communities of La Paz, Bolivia and Lhasa, Tibet are both above 3,600 metres or 11,800 feet. The air at that altitude is approximately two-thirds as dense as it is at sea level. Since the fraction of oxygen remains unchanged, we can use Dalton’s Law to calculate that the partial pressure of oxygen available to the folks walking along Avenue Camacho, in the Bolivian capital or the tourists at Jokhang Temple in Lhasa is about 0.15 bar.
Without doubt, if we could magically and instantly transport everyone in this room to either of those spots, most of us would pass out and risk death as a result of severe high altitude pulmonary or high altitude cerebral edema.
But what about the people who live there… and what about the tourists? The key of course is time. Time to acclimate to the lower partial pressure by ascending gradually, giving the body time to make adjustments to less available oxygen. Visitors also get the help of anti-mountain sickness drugs.
Even with these precautions, a significant proportion of “sea-level” tourists never truly get used to being at altitude and every year, some have to be evacuated to lower altitudes. Attrition rates vary but up to half the folks on trekking holidays in Nepal and Peru fall foul of altitude sickness.
This is wonderfully interesting but somewhat misleading for divers. We have to be extremely careful to avoid low partial pressures of oxygen, because there is no acclimatizing to hypoxic mixes for us. If someone pumped a gas mix into this room containing 15 percent oxygen, we’d all fall asleep. If we breathed that same gas with 60 kilos of dive gear strapped to us, and we had to move through a medium 800 times denser than air, there might be a few of us for whom the sleep would be infinitely long and dreamless.
Recreational divers do not and can not adapt to hypoxic mixes. Divers have to be particularly careful to pay this heed. Our bodies need a partial pressure of at least 0.16 bar to sustain a base-level of activity… 0.18 if we hope to swim or make sense of the world. Less than that and the brain begins a slow samba towards siesta time.
This is bio-physics or physiology and so the variables of individual susceptibility come into play when we talk about hypoxia. Its effects may be more or less pronounced depending on the person and even with the same person at different times. My personal comfort with this aspect of dive execution is conservative. I’ve seen divers using trimixes with less than 14 percent oxygen, breathing them on the surface. Their practice is to get quickly to a depth where the oxygen partial pressure or their mix becomes normoxic (0.21 bar). In the case of a 14 percent mix, this would be at approximately 5 metres or 16 feet.
I’m not comfortable with that practice at all. For me it’s tantamount to playing Russian roulette. It only takes one instance where something goes slightly wrong… a very minor thing… that requires a little extra effort, and there’s Mr. Sleepy tapping you on the shoulder. That’s just not the way to start a dive to a depth that requires hypoxic back mix.
I’m more comfortable breathing a decompression mix that’s hyperoxic on the surface and then switching to back mix at some convenient point before reaching that decompression mix’s Maximum Operating Depth (MOD).
Hyperoxic? A gas containing a greater fraction of oxygen than air. And that’s a good a transition as any into defining best practice when there’s lots of oxygen.