Which diving gases to use for archaeological work in 45m depth?

Friday 5.8.2016

We are going to dive with "back gas" air, and "deco gas" pure oxygen (O2). This means that during the normal dive we breath plain compressed air, and during the last 6m on ascent we use pure oxygen. Term back gas comes from the fact that (usually) we carry that gas on our backs in two 12 liter 200 bar tanks. The decompression gas is carried in a separate 11 liter (and 200 bar) tank hanging in front under left elbow. This is not quite what I expected, but it is a workable solution. Better solutions do exist, but I guess they would be too expensive or too difficult to organize.

The basic solution (Air + O2) has a few consequences. First of all, plain air has about 79% nitrogen, and than tends to be slightly narcotic already starting at 30m depths. At 45m depth the narcotic effect is such that you must take it into consideration for all divers. Doing simple things will become difficult. For example, just making a simple bowline ("paalusolmu" in Finnish) requires lots of concentration and time even if you can do it in a few seconds on surface. If you just learned how to do it on surface, it might be really difficult to remember how to do it at 45m. Tasks for such divers must be really simple: go there, do this and then this, and finally come back.

You could alleviate this nitrogen narcosis problem if you would use nitrox as your back gas. Nitrox is enriched air, where some of the nitrogen is replaced with oxygen. For example, Nx28 has 28% oxygen and 72% nitrogen. With that, you could dive to 40m, and it would have the same narcotic effect as diving with air to 35m. The trouble with nitrox is, that when you put in more oxygen then your maximum operating depth will decrease. After such depth limit oxygen will become poisonous. So, for 45m depth your oxygen content can be no more than 25%, and that gives you very little gain as compared to plain air. Narcotic effect of Nx25 at 45m is equal to that of plain air at 42m. That does not help you very much. It is like the difference between having 6 or 7 beers.

Another advantage with nitrox is the effect on your ascent time. Just 25 minutes diving with air at 45m depth would (with certain other settings) give you in 67 minutes of ascent time. With Nx25 the ascent time is cut to 49 minutes. That is clearly less and it also reduces the total amount of consumed gas significantly. It is especially nice because the back gas tanks would not last for 67 minutes.

Having O2 for decompression will speed up ascent significantly. With air as back gas, ascent time would be reduced from 67 minutes to 36 minutes. With Nx25 back gas, ascent time would be reduced from 49 minutes to 30 minutes. So, with pure oxygen for decompression, using air as back gas (as compared to Nx25) will cost you only 6 minutes of ascent time. That is tolerable. However, if you lose your own O2 gas for any reason, your do not have enough back gas to decompress safely. That would ruin your whole day. So, you need to have extra O2 tanks staged somewhere on the ascent line, just in case.

How else could you reduce nitrogen contents, when you can not increase oxygen proportion too much? You could use helium, and such mixture of helium, oxygen and nitrogen is called trimix. For example Tx20/40 would have 20% oxygen and 40% helium, and only 40% nitrogen. Helium is not narcotic, and diving with such trimix at 45m would feel like diving with air at 28m. You could do a bowline with your eyes closed.

Why would we then not use trimix? There are three reasons: cost, training and making it. First, trimix is expensive. Such mixes cost like 50-150€ per dive, when plain air costs some 5-10€. Multiply that by 20 divers per dive, and you start talking real money. Using trimix requires more training than using nitrox, and the requirements for participating this fieldschool did not include a trimix certificate.

Making trimix requires a special skill called gas blending, and that might make logistics difficult. You also need to have a special gas blending site suitable for making trimix, and blending gases properly takes time. It is so much simpler just to pump plain air in your tanks. You can run such compressor anywhere. The same reasoning applies also to nitrox, even though it is easier to blend than trimix. Both require handling of pure oxygen, which by itself requires strict safety protocols to be followed.

Why are we using pure oxygen for decompression. I would personally rather use of 50% oxygen, which would result to slightly smaller ascent times than with O2. You could start using the decompression gas earlier and it would also avoid the sudden oxygen concentration jump from plain air to O2.

Even better solution might be to carry both 50% oxygen and pure oxygen tanks for decompression, one under each elbow. However, having so many tanks of front of you might become cumbersome while working on the site. It might be better to stage both decompression gases somewhere close to the ascent line, and then pick them up before starting the ascent.

But again, making 50% oxygen requires gas blending. Topping up O2 tanks from larger O2-containers is much simpler. Also, using just one decompression gas is simpler than using two, and requires less training.

Summa summarum, we will be using air as back gas and pure oxygen for decompression. Both gases are easy to fill up, and relatively cheap. Our ascent times are reasonable.  We will all be in slight narcosis while working at 43-45m depths, but hopefully we are not asked to do anything requiring deep thinking, like tying a knot. We are strongly discouraged from drinking alcohol during the fieldschool, but I guess we all will get our kicks from sipping nitrogen at 45m instead.