Re: Are computers Really Safe?, the first place to look for a dive instructor

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Posted by Kendall Raine on October 01, 2002 at 15:27:40:

In Reply to: Are computers Really Safe? posted by Captain Tim on October 01, 2002 at 10:34:50:

Computers are neither safe nor unsafe. Computers move numbers around real fast. That's all.

Bottom Time Advantage

Computers offer divers doing multilevel dives longer NDL's than divers using tables. That's because computers depth averaging whereas table instructions specify that the deepest depth reached during the dive be used to compute depth. If you depth average and use a table, your NDL's mimic those produced by the computer. Depth averaging with tables on a multilevel dive removes some conservatism. Put another way, a computer offers no NDL advantage on a true square dive profile.


Most computers use neo-Haldanian models (i.e. Navy, Buhlmann, Spencer). These models assume bubble formation is controlled by the metastable limit. This is probably hogwash. Spontaneous bubble formation shouldn't happen the way Haldanian models suggest under the kinds of pressure changes encountered in diving without violating physical laws. The piece these models fail to capture is the pre-existence of bubble micronuclei in the body. Various theories exist about what creates the nuclei, but one of the most promissing is simple muscle movement can cause nuclei formation through cavitation/tribonucleation. Models which recognize the pre-existence of these bubble seeds, like RGBM and VPM, prescible NDL's, repetative NDL's and ascent schedules which are different than those of neo-Haldanian models. Since error terms compound, failure to account for bubble seeds grows in consequence over multiple dives and multiple days. This is evident in just about every Doppler study done by DAN.


Should you chuck your computer? I have, but it's not necessary. Should you chuck your Edge, Datamax, whatever and buy a Vyper with RGBM? That would be an improvement, but again, not necessary. Most computer users never get bent, even when they do stupid things. The algorithms stuffed into computers are heavily "padded" to account for old, fat cigarette smoking divers who enjoy a beer or two between dives. No. No need to chuck the computer so long as you recognize its limitations and do the following:

1. Get fit
2. Get rest
3. Treat every dive as if it were a deco dive. It is. Pull "deep" stops as well as shallow ones.
4. Skip the dive if you don't feel right
5. Stay hydrated
6. Avoid getting chilled
7. Get plenty of surface interval even if your computer says it's OK to dive.

Other things like using enriched air help if used appropriately.

The thing to avoid is blindly following the computer. If you feel lousy or fatigued after a dive, it could be many things. One of those things could be asymptomatic decompression stress. Forget what the computer says and listen to what your body is saying.

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