By Bill Nadeau

In my last column I wrote that some of us occasionally speed on the highway and that we get away with itno ticket, no accident. The first time we broke the speed limit it was a revelation'hey the car didn't blow-up!' Once we figured out that we did not immediately get a ticket or into an accident the speeding thing became easier to do. In diving when we slip past the 'no -decompression' limit, if for a brief minute or two, we can likely recover without suffering the so often preached consequences that the DCS gods will strike us down with a permanent neurological hit. We notice that our computers gave us a simple to read message that you must stop at 15 feet for five minutes. 'This is pretty straight forward,' we say to ourselves, 'there really isn't anything at all to this decompression stuff. As long as I follow what my computer says I can safely perform decompression dives.' If you believe this then you seriously misunderstand the dynamics of decompression, underestimate its complexity and increase your risk level.

Yes, you can safely make decompression dives but not because your computer says you can. Divers do not graduate as technical divers when they purchase a computer capable of providing decompression profiles. On an ascent from a recent trimix dive, a couple of students and I noticed another pair of deep divers descending a popular wall site with twin tanks on their backs and a single deco bottle. While we were completing the last 25 minutes of our 90 minute dive they joined us at our stop at fifteen feet. One of the pair approached us to indicate he had a problem. Apparently arriving at his 20-foot stop he turned on his decompression gas and found that he had a malfunctioning regulator. The disturbed diver showed me that his computer read an obligatory 30 minutes of decompression and the remaining air in his bottom mix was a little less than 400psig. Seeing that each of the divers in our group was carrying four tanks, the distraught diver figured that perhaps he would just 'borrow' some gas from us. These divers got themselves into trouble because they did not fully understand what was involved in making a decom pression stop.

1) First of all these divers were not trained in any decompression techniques so they had no idea of how important advanced dive planning, contingency planning, risk management and decompression physiology are when doing these kind of dives.

2) The divers had no idea what their breathing rates were (or how to calculate them) and depleted nearly all of their bottom gas before reaching their deco stop.

3) The divers did not carry any redundant gas, did not plan for any contingencies, like losing their deco gas, and failed to employ proper gas management rules for decompression diving by keeping a 33% reserve in their bottom mixes.

4) Yes each diver in my group had four tanks BUTtwo of the tanks had trimix and were hypoxic (not enough oxygen) at 15 feet, the third tank was an EAN36 with 33% remaining and the other an oxygen mixture (EAN90) which we were breathing. The 33% reserve remaining in our EAN36 stage bottles was just thata reserve should we prematurely exhaust our EAN90 supply or have a regulator failure and need to return to the EAN36. By asking either of my students or me to give up our redundant deco gas meant increasing our risks. I eventually donated my nitrox gas but I kept the cylinder attached to me in the event I would need my supply back.

5) The divers had difficulty maintaining their decompression depth, bouncing back and forth between 25 and 10 feet seri ously predisposing themselves to DCS.

6) On the surface I discovered they did not have any form of backup plan to follow should their computers fail during the dive and they did not realize that hard tables were available for decompression diving.

Fortunately this story has a good endingthe divers completed the full 30 minutes decompression with a little gas to spare and despite the shivering they experienced on the last ten minutes, everyone was fine. The problem is that many divers assume that the practice of decompression is fairly straightforward despite depth, time and the gases being used.

On this particular dive my students and I were actually using four gases to complete the dive; a trimix gas with a selected proportion of oxygen, helium and nitrogen that prevented hyperoxia and narcosis at depth, but actually required more decom pression than if we were on air. We also had a nitrox mix of 36% to travel to and from the depths with and to start our decompres sion profile on, an oxygen supply to decompress on and argon for our suits to help prevent the cold from settling in on those long hangs. This decompression dive was planned using both trimix tables and a computer program that could generate our contin gencies. At this level of diving the decompression techniques need to be exact as a variation of as little as two or three minutes from the schedule (the decompression stops actually started at 120fsw) could result in a DCS hit.

Before planning a decompression dive, individuals must understand how and where dive tables, computers and decompres sion programs they are using come from. Some are based on 12 tissue groups while other are based on 16. Some have a built in safety factor based on a predetermined index of conservatism while others allow you to choose how conservative you wish the table or program to be.

And then there is the oxygen factor; it is true that too much of anything is not necessarily good for you. Traditionally decompressing on 100% oxygen was common but today many technical divers are using lower concentrations of oxygen to help reduce their CNS load (Central Nervous System Oxygen Toxicity based on NOAA's maximum oxygen exposure limits). By using lower oxygen mixes for decompression divers can also switch to their decompression gas at deeper depths reducing decompression time and stress.

Finally there is the actually hang itselfthose long, cold, boring and sometimes exhausting stops where the current fights you every minute of the way. Divers need to consider these factors as well for it is an unsettling thing to be blown off an ascent line in the middle of the ocean with 30 minutes of decompression to go. The diver must maintain his depth while exerting minimal energy and maintain a constant breathing rate. Floating aimlessly on a lift bag and reel or on an up-line can not only be boring but the lack of movement predisposes a diver to hypothermia, a condition not ideal for off-gassing nitrogen.

I have touched on only a few of the many considerations that involve the physics, physiology, dynamics and practice that encompass the 'Decompression Stop'. It's complexity is debated, researched, debated, refined and debated by thousands on this planet yet one thing remains as a fundamental tenetthat performing a decompression stop requires respect, training, attitude and experience. It is also a rewarding mechanism that opens many doors to recreational, technical and commercial divers.