Understanding trimix
Beyond normal recreational diving limits, use of mixed gases allows extraordinary projects to be undertaken. Painstaking planning is the key, says John SimenonEveryone can be a technical diver, but its up to you how technical you want your diving to be.Most people have neither the interest nor the aptitude to use the most technical end of the spectrum of techniques available. At the other end of the curve, finning about in shallow, warm, clear water, enjoying the freedom of being weightless and carefree with the fishes, is enough for many divers, and why notMany divers add redundancy or back-up to their kit, starting with a double (Y or H) valve on the tank which allows them to connect two complete regulators. Others add a back-up computer. Real tekkies use a BC with a second redundant bladder. Once you include the invisible barrier of a decompression-stop in your dive, you should not have to rely on one sole item of equipment.

However, entering the water with extra equipment does not in itself make you a technical diver. What is important is the level of theoretical knowledge and ability needed to use the various techniques safely. Divers need to be well-practised in easy conditions until those techniques are second nature.

Some people get their pleasure merely from using equipment. After all, we buy complicated hi-fi equipment without necessarily knowing much about music, and our cars dont always reflect our driving ability or even our travel requirements.

Ask many amateur mixed-gas divers what they achieved during their dive, and returning safely to the surface would be the honest answer.

My favourite definition of a technical diver Someone who sees a task to be done, weighs up the requirements, and gathers the equipment and expertise needed to carry out that task.

One of the most unfortunate consequences of recent advances in technology was the introduction of the dive computer. This wonderful device led divers to lose their planning skills and rely on the unit to run their dives for them, with potentially disastrous consequences in case of computer failure (yes, it does happen!) or of accidental transgressions of the limits of recreational diving.
In technical diving, which for our present purposes means diving beyond normal recreational depths, the most important skill is planning for decompression stops, for new contingencies and redundancy and for much greater gas consumption.
All technical diving training programmes start with a nitrox course in which we learn how to replace with oxygen some of the nitrogen found in the air we breathe. With less nitrogen in the gas mix, there is less nitrogen uptake in the body.
Within the limits of oxygen toxicity, we can spend more no-stop time under water, but O2 toxicity still limits depth. Oxygen becomes toxic to the human body at partial pressures about six to seven times those found in the air we breathe at the surface
So the next course trains those diving on air to handle time-induced decompression, at familiar recreational depths of no more than 40m, and to use oxygen-enriched mixes (minimum 80 per cent O2) during deco stops, understanding their advantages and limitations, to shorten decompression time.
Diving with new equipment configurations (extra tanks, oxygen-clean equipment) must also be learned
The next step is to enroll on an extended-range course, in which we apply the techniques and procedures already learned to depths of more than 40m, but not exceeding those at which nitrogen in the air we breathe becomes dangerously narcotic
Many agencies felt in the past that divers could be trained to handle nitrogen narcosis down to 60-70m, but recent doctrine suggests that 55m is probably the most extreme limit not to be exceeded on air. Some suggest that other mixes should be breathed as shallow as 40m.
To illustrate the planning and trade-offs involved in technical diving, lets assume that our dream is to explore an English wreck in 90m of very cold water, with usually strong currents
Our nitrox course taught us that beyond a partial pressure of 1.6 bar, oxygen becomes dangerously toxic. In cold water, faced with potentially strenuous exercise, it is best not to exceed 1.4 bar. A simple formula tells us that maximum O2 content should be 14 per cent at the bottom. If I was particularly concerned, I would opt for the US Navy limit of 1.3 bar or less.
This would be achieved at the expense of a higher percentage of inert gas in the mix, so increasing decompression time and the need for additional gas. But isnt this well below the minimum 18 per cent oxygen we are told our body needs to sustain life
In fact, the body reacts not to percentage but to partial pressure of gases, and needs a minimum of about 0.18 bar of O2, which at the surface translates into 18 per cent. With a PO2 of 1.4 bar, I wont lack oxygen.
If nitrogen incapacitates just about every diver beyond 40m, I need to choose my equivalent narcotic depth - the depth on air to which I want my mixed-gas dive to equate in terms of narcosis. I am easily narked, so I choose a conservative 30m
With the help of Daltons Law (Partial pressure of a gas in the mix = absolute pressure x percentage of that gas in the mix), I find that at 30m on air, PN2 = [0.79 x 4] = 3.16 bar. If I want PN2 at 90m to be 3.16 bar, the percentage of N2 in the mix should not be more than [3.16 x 10] = 32 per cent.
So I now have 14 per cent O2 and 32 per cent N2. What do I use to fill the remaining 54 per cent
A convenient gas happens to be helium. Lighter than air, it is best-known for letting balloons and modern zeppelins float. It also conducts sound faster than air, which makes for that Donald Duck voice we love to hear after inhaling the gas from toy balloons.
More important to divers is that, for reasons not fully understood but also linked to its lighter molecular weight, helium has a narcotic effect close to zero. By filling the missing 54 per cent with helium, we obtain our ideal bottom mix. As we now have three gases in the mix, we call it trimix 14/32/54, the percentage of oxygen coming first, then nitrogen, then helium
Unfortunately, there are several trade-offs involved with trimix. Its main disadvantages are that it is very expensive - which might lead some divers to reduce its percentage in the mix, thus increasing narcotic exposure - and that it requires special tables or computer models for decompression, often resulting in longer stops
Thankfully, there are many computer programs available nowadays, such as Abyss or Z-plan, that will remove the tedium of most of the planning process.
We have determined our bottom mix, but should we use it on our way down and up again Dalton warns that a mix with 14 per cent O2 is dangerously hypoxic shallower than 3m
A fast descent could take care of that, but what about the time spent during descent and ascent breathing a gas of which 86 per cent is inert It would make our deco schedule unbearably long. We need to find what is called a travel mix
We know that eliminating helium and increasing oxygen content will shorten deco schedules, so we choose a mix that will carry us down to 30m and back up again from that depth
Why 30m A good nitrox for 45m (24-25 per cent O2) would not help much at 30m, and we dont want to overburden ourselves with too many different mixes and tanks. Further, decompression programs tell us that, except for a few deeper stops recommended by empirical evidence and starting at perhaps 45m, it is after 30m that most of the decompression will take place
Turning to good old Dalton again, we find that the best mix at 30m is 33 per cent (again, depending on the oxygen toxicity risk we are ready to take). We now have our travel mix.
However, if we run a deco schedule based on the above two mixes, we will spend a long time under water. Using 80 per cent O2 from 12m will speed the process considerably. During deco at that depth, as I am resting and the water is warmer, I can choose a PO2 limit of 1.6 bar. We have our decompression mix.
Other steps include calculating the decompression profile (number and length of each stop), and figuring how much of each mix to take based on this profile, on our surface consumption rates (SCR, the amount of gas we breathe at the surface while resting and exercising) and on contingency reserves (usually a third to a half of the total gas is set aside for emergencies)
Finally, equipment configurations are chosen to provide the best combination of redundancy and simplicity, typically twin 12 or 15 litre tanks for the bottom mix, redundant regulators, a backplate with a wing BC, 8-10 litre travel and deco tanks, each with its own oxygen-clean regulator, and redundant computers or bottom-timers, masks, knives etc.
The drysuit and/or a 25kg (minimum) liftbag will provide redundant buoyancy. Considerable training is required in using all this new equipment, especially in moments of stress.

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