me, breathing air at 50 metres is roughly the same as drinking four pints of beer. The narcosis strips away your ability to understand and rationalise situations – and robs you of the ability to deal with things when they go wrong. The ‘narcs’, as they are affectionately known, affect people in different ways. Some get euphoric – some get paranoid. Some people get tunnel vision; others lose control and panic when the simplest thing goes wrong – something that could easily be dealt with normally by the same person on the surface.
Rather than breathing compressed air at any depth greater than 40 metres, nowadays I always dive on a trimix diluent, which replaces a large element of the dangerous nitrogen in the breathing mix with helium, which has no discernable narcotic effect. Although nitrogen narcosis is no longer an issue for me, if you want to get an idea of what nitrogen narcosis can do, I recounted a hit I got in the chapter entitled ‘Bail out on the Cushendall’ in The Darkness Below. This was a 58-metre air dive in 3-metre visibility, in a current, on the wreck of the World War II casualty SS Cushendall which lies off north-east Scotland. Oh, the things we do when we are young …
Whereas oxygen is very therapeutic and beneficial in normal use, as the aqualung feeds a diver increased volumes of breathing gas on descending, this means that in addition to getting higher partial pressures (or concentrations) of nitrogen, the diver also gets increased volumes of oxygen.
Oxygen, the very stuff that keeps us alive on the surface (and of course underwater as well) becomes increasingly toxic in the larger volumes breathed by divers as they venture deeper. The risk of an oxygen toxicity hit becomes a very real danger. This starts off with twitching and spasms but rapidly develops to uncontrollable convulsions where a diver will amongst other things, rip off their mask and spit out the breathing regulator. In water, a hit nearly always results in drowning, unless the diver is wearing a full-face mask. A number of leading technical divers have sadly ‘ox-toxed’ over the years and died of the uncontrollable convulsions – so deadly underwater. Some had mistakenly breathed their shallow water oxygen-rich decompression gases at too great a depth, quickly bringing on a fatal oxygen toxicity hit.
The trick is to use a nitrox mix which has the right amount of oxygen to safely accelerate decompression – and to use it at the right depth where the nitrox does not become toxic. The consequences of getting it wrong can be fatal.
A commonly used enriched air nitrox (EAN) mix that is suitable to breathe and shorten decompression (compared to breathing standard compressed air all the way to the surface) from a depth of 20 metres upwards is EAN50, which comprises 50 per cent oxygen and 50 per cent nitrogen. The increased amount of oxygen and reduced amount of dangerous nitrogen shortens (or accelerates) the time needed for decompression before surfacing.
At a depth of 20 metres, the water pressure on your body is three times what the atmospheric pressure on your body is as you read this right now. So the aqualung feeds the diver three times as much compressed breathing gas to keep the pressure in the lungs exactly the same as the surrounding water pressure – and avoid a lung collapse. This means that in every breath the diver is breathing in three times as much oxygen as on the surface. If each breath is 50 per cent oxygen, or half of the total mix, we could say that at the surface that the partial pressure of oxygen (abbreviated to PO2) is 0.5. At 20 metres, breathing three times as much oxygen the partial pressure is 3 x 0.50 = PO2 of 1.5 bar.
Trials have shown that a PO2 of 1.4 is relatively safe, but above a PO2 of 1.6, you are entering an area where the oxygen concentration in your body is starting to become toxic – and if the levels increase or if that same level is breathed for more than a certain time, you risk an oxygen toxicity hit, convulsions and death. That’s why we put a maximum depth limit on breathing EAN50 of 20 metres, where the PO2 is 1.5 bar.
But EAN50 has a fixed percentage of oxygen in it at all times – 50 per cent. Thus, at 10 metres, the PO2 is twice 0.5 = 1.0 bar. There’s less therapeutic oxygen in the breathing mix compared to breathing, say, EAN80 with 80 per cent oxygen, where the PO2 is 1.6 bar. So, although EAN50 is good because you can start breathing it deeper, at 20 metres, and start reducing the level of nitrogen in your body early, in the shallows it is not giving you as much oxygen as you could safely breathe. You can breathe pure 100 per cent oxygen from 9 metres to the surface, which is extremely good at accelerating decompression. Thus, in the shallows EAN50 is not such an effective decompression gas.
Many divers do in fact use EAN80 (80 per cent oxygen and 20 per cent nitrogen) for decompression. This higher oxygen level is very beneficial for decompression, but can only be breathed from about 12 metres up to the surface. To breathe EAN80 deeper than 12 metres or to breathe EAN50 deeper than 20 metres, or EAN100 (100 per cent pure oxygen) deeper than 9 metres, risks a potentially fatal oxygen toxicity hit. Thus, every deco mix, be it EAN50, EAN80, EAN100 or whatever, all have their own depth limits where the amount of oxygen in the mix becomes toxic – and potentially fatal.
In a contrast to open-circuit diving, Paul and I, in common with the majority of technical divers, have for a long time been using closed-circuit rebreathers (CCRs).
Whereas in open-circuit diving the exhaled breathing gas is vented to the surface, a closed- circuit rebreather continuously recirculates the same breathing gas – there is no venting to the surface. One of the benefits of using a rebreather is that a diver can program their onboard computer to never let the PO2 in the breathing gas loop drop below a certain level.
As a diver rebreathes through a CCR, during each breathing cycle (that is, one inhale and one exhale) the diver’s body metabolises some of the oxygen as it passes through the body, producing carbon dioxide (CO2). The expired breathing gas thus contains less oxygen than the gas the diver inhaled. In a CCR, that expired gas is cleaned of the dangerous CO2 in a scrubber canister filled with sofnalime and then analysed in a chamber in the rebreather by onboard oxygen sensors. The results trigger a solenoid switch to open and bleed just the right amount of oxygen into the breathing gas to keep the PO2 at the desired level of say 1.3 bar, no matter what depth the diver is at. So, on the ascent, all the way to the surface, the rebreather is trying to inject oxygen to keep the PO2 at say, 1.3 bar. At 20 metres, a CCR diver can be breathing a PO2 of 1.3 bar – but in contrast to breathing EAN50 on open circuit, at the final deco stop at 6 metres a CCR diver is breathing almost pure oxygen. The diver is getting the optimum amount of oxygen, so beneficial to decompression, at any point.
A modern rebreather, the A.P. Diving Inspiration Vision, popular with technical divers. The corrugated hose leading from the mouthpiece over the diver’s right shoulder is the ‘exhale’ hose of the breathing loop. The corrugated hose leading over the left shoulder to the mouthpiece is the ‘inhale’ hose. The wrist-mounted computer handset is in the foreground. © Bob Anderson
Rear view of a popular closed circuit rebreather (CCR).
The diver’s exhaled breath moves from the mouth through the exhale hose that runs over the right shoulder and into the bottom of the central stack between the two cylinders. From there the exhaled breathing gas passes upwards through a canister holding the ‘scrubber’ sofnalime material, which strips the dangerous carbon dioxide out of the exhaled breathing gas.
After passing through the scrubber, the cleaned, exhaled gas passes into a chamber at the top of the stack, where three or more oxygen sensors analyse the resulting breathing gas to determine how much oxygen the diver has metabolised in the last breathing cycle. Two electronic controllers (essentially minicomputers) then trigger a solenoid (switch) that injects the correct amount of high-pressure oxygen into the breathing mix to raise the depleted oxygen level back up to the desired level (the PO2 ‘setpoint’).
The cleaned, analysed
