tonyc:
Hello all,
I'm a very new diver, but interested in DIR. My question is concerning DIR and tables. I understand (I think
) that DIR is anti- computer, and I have been using the SSI tables when I log and plan dives. I also have some navy tables that are pretty much the same with a little less conservatism. I wear a suunto vyper, but log and plan dives just using the tables.
Thanks in advance,
Tony.
Tony,
I snipped the rest of your question, but in short it seems that as a new diver you are concerned with conflicting information that you hear from various shops and/or agencies respecting computers and the DIR position. You are correct to ask the questions, and we very strongly encourage you to ask WHY!!!! If someone can't answer the WHY, then you probably shouldn't listen to them.
That being said, since you are new to diving, let me tell you up front that I am a GUE instructor, and this conversation has been very heavily discussed in the past so feel free to do a search, but realize that after a few exchanges more often then not the threads tend to denigrate.
In short, we have a few issues with divers that rely solely on computers. Specifically, most of the earlier models of computers use an algorithm that we disagree with. Most tables, and consequently, most computers use the Buhlman algorithm. The Buhlman algorithm has been around since approx. 1958 and in various forms is the mathematical model used to generate most of the recreational dive planners [ FTR, PADI uses a different model] and as computers became readily available Buhlman's model worked it's way into the early computers. Accordingly, what a diver was then faced with was "planning" a dive using a Buhlman model, this is the tables that you learned in your OW class. The benefit of computers was that they allowed for in-water calcualtions based on real-time dive information such as depth and time. This advance made it easier for divers to extend their bottom times when their profiles allowed for multi-level diving, it also allowed for divers to rely on the pixels to compute repetitive diving and multi-day diving. Over a period of time what you began seeing was that divers no longer paid much attention to their dive planning and became solely reliant on computers. At GUE we teach that divers should turn their brain on underwater, rather then rely on a device that could fail and should it fail it would leave the diver without access to critical information. Lately you've even seen agencies go so far as to no longer teach basic decompression theory in OW classes, but teach divers how to use a computer. That is tantamount to not teaching your child math in school because eventually they'll use a calculator anyway..
Moreover, as I noted, the earlier computers relied upon a algorithm that we believe is outdated. As more and more modeler's studied decompression theory it started to become clear that a diver that incorporates deep stops into their dive profile actually achieve better results, whereas the Buhlman model would penalize you for incorporating deep stops into your profile. So in esesence your computer was working against you. Recently the dive industry has embraced the "newer" decompression algorthims that provide for a bubble mechanic model(s), ie; VPM and/or RGBM..
In really brief terms, the Buhlman model tracked the amount of nitrogen in your tissues and used that as a basis for your M-value, the amount of nitrogen loading a given tissue group could tolerate and still allow a diver to surface without experiencing signs or symptoms of DCS. The VPM and/or the RGBM models are what are called a dual-phase model meaning that in addition to tracking the nitrogen loading they also track the size and formation of bubbles. As a result of concerning their models with bubble size and formation they've found that incorporating deep stops into your profile you allow the bubble size to shrink as you ascend, whereas if you juxtapose that concept against the Buhlman model, as you are stopping on your ascent the Buhlman model provides for the fact that you are still "on-gassing" and accumulating nitrogen loading, and thus shortens your time at depth and/or makes you stay at your safety stop longer. That's a very brief overview of a complex topic, but the point of course being, is that if a diver solely relies on a device such as a computer, and that device fails and the diver has no background, education or experience in learning basic decompression theory he endangers himself, he may be forced to sit out a day of diving all because they lacked the basic information we teach.
Fundamentally, that is the rationale behind our belief that divers are better off turning their brains on underwater rather then relying on expensive devices that may provide faulty information. Notwithstanding the fact that generally speaking, most computers set the conservation factors in their NDL's to the lowest common denominator and doesn't allow the diver to adjust for the fact that perhaps they don't smoke, perhaps they are physically fit, perhaps they've had a PFO test, or are beyond the scope of many of the limiting factors that contribute to DCS..
Hope that explains it, but if not let me know and I'll be happy to help..
Later
