Affordable insurance for deco diving
The way it looks to me, recent advances in the design and function of the Personal Dive Computer (PDC) have made several positive changes to the way both sport and technical divers plan and conduct their dives. Scuba Diving International™ divers from their open water training on up are required to wear a PDC to track their nitrogen loading, and this common-sense approach to sport diving seems to have pushed manufacturers, at least in part, to designing and producing a huge selection in simple, user-friendly and reliable models for the recreational sport market. But the technical diver has benefited too. The release of inexpensive dual-gas decompression computers has been particularly important to those trained as TDI Advanced Nitrox divers.
In these cases, dual-gas computers allow divers to take advantage of rich nitrox mixes (or pure oxygen) used during staged decompression stops. Carrying a second gas rich in oxygen is the universally accepted way to optimize off-gassing. The practice of running decompression on the same gas that was used on the bottom – once the norm among divers – has long fallen out of favor.
Luckily there is another “habit” common in the early days of technical diving that dual-gas PDCs have virtually eliminated. When the only computers available were single gas it was common for divers to leave their computers at 10 or 20 feet to “finish” the decompression while the owner surfaced. This, just for the record, is NOT a recommended practice!
The fact is that a single-gas computer is unable to take the optimized or shortened decompression time afforded by special gases into account and therefore does keep divers in the water longer than necessary.
Of course, there is nothing inherently wrong with extending decompression times but following the ascent profile computed using back gas while actually breathing high-test nitrox or pure oxygen can cause problems.
The first and perhaps most obvious is that breathing partial pressures of oxygen during ascent increases central nervous system (CNS) oxygen toxicity loading. As any Advanced Nitrox trained diver knows, CNS toxicity must be accounted for and levels kept within acceptable norms… Technical Diving International™ recommends that divers keep CNS levels at no more than 80 percent of the allowable single dive time for ALL stages of the dive. This includes CNS loading netted during bottom time as well as during ascent and decompression.
It is possible to work these figures manually using NOAA oxygen exposure tables or with dive-planning software or a PDC in simulation mode during the dive planning steps. However, the process is much more accurate if these “estimates” are backed up and confirmed during the dive in real time via a multi-gas PDC.
A second argument against using a single-gas computer to run a staged decompression ascent where the diver is breathing decompression gas (a 50 percent nitrox for example) is that the computer would stop the ascent at sub-optimal depths for sub-optimal duration. A dive planned with plenty of conservatism might allow 9 out of 10 divers to get away with this, but when planning decompression dives, it is always best to weigh the odds in ones favor as much as practical.
Frankly a multi-gas computer is good insurance and a strong back-up for correct training and experience.
Many of the models on the market today use modern decompression algorithms to calculate ascent schedules and include deep stops or micro-bubble stops in those calculations. The efficacy of these stops coupled with the use of decompression gas, adds an element of security for divers planning technical dives. The risks remain, but their identification and mitigation is better managed with the use of modern dive computers. I would not dive without one… or two.
Sean Harrison is V-P Training for Scuba Diving International™ Technical Diving International™ and Emergency Response Diving International™