Random Jottings

Fundamental dive 'laws', real or compromise?

As divers, or not, we all know that there are fundamental rules which are writ in stone by some organisations but being an engineer sometimes you ask, well where is the evidence?

There are a few sacred cows to consider but here is a flavour...................

Deep Diving

Many organisations have a 'deep' limit on certification of 40m, which is a reasonable arbitary figure but how was it derived? Bearing in mind that most dive injuries and fatalities occur in significantly shallower water how is 40m safe?

Those who would have you think it was scientifically derived by The US Navy and adopted by recreational bodies as law are right but your reasoning is fundamentally flawed! The figure was adopted because at this depth you are limited to a notional 12 minutes bottom time without incurring any deco obligation and the USN decided that it would take that long for a diver to look at a job and just make a start, so not safety governed at all, rather put in place based on useful work output from a diver who is effectively a commercial guy.

Ascent Rates

Where did this figure of 30 feet (10m) per minute come from then? Oh it's USN again! This figure was agreed in one of their regular working groups 'way back' and it was a figure that was a compromise based on the demands of what we can call scuba divers versus hard-hat divers. The scuba-divers were looking at a figure of 30m a minute whilst the hard-hat divers wanted 5m per minute, well lets be more specific the hard-hat divers tender operators......who were handcranking the diver plus stage up from the depths......ahhhh yes, kerching as the penny drops!

Now while I can't recommend streaking to the surface I would ask that people look at some of the tech diving texts where the initial ascents are at up to 30m per minute to minimise further deco obligation and to reach depths where off-gassing would start. My own diving pattern for anything over 25m in depth is to move to deep-stop at 13m as fast as comfortable and then adopt a much more leisurely ascent to the surface with the 3m to air section taking a minute as I slowly wind myself up the DSMB line.

Reverse Profile Diving

One of my bug-bears, having put up with loads of trips where the first dive of the day is to some broken up lunar landscape at 40m just to make sure it's deep dive first, oh how the guides would get upset with me bimbling along at 20m taking photographs of living reef!

Following years of arguments with PADI being regularly challenged and responding that it was USN policy therefore 'good', see above there was a meeting of minds and it was demonstrated that it wasn't and never had been USN policy, there was no emperical evidence that it was 'bad' and caused cases of DCS. This was demonstrated over a two day meeting of 49 dive organisation leaders via 29 presentations at the Smithsonian institute, at the end all agreed that there should be no reason that reverse profile diving be 'prohibited'.

I used to take the argument to the extreme with the PADI indoctrinated guides on liveaboards, it was a weeks diving holiday so damn the shallow check dive the first time we got wet should have been for that 40m dive and the itinery would need to be changed to do the deep stuff at the start of the week.........I loved seeing them implode!

1.6 is the maximum PPO2

Another sacred cow, the 1.6 figure was derived following research which found that 45 minutes of exposure to a PPO2 of 1.6 posed a very low risk to the average diver, this figure was then dropped to 1.4 for active diving and 1.6 for doing nothing.

Now the problem with a high PPO2 selection is that it will kill you, an oxygen toxicity hit in the water kills whereas buggering around with ascent rates may cause treatable DCS so I am loathe to challenge for the average joe. Personally when on my rebreather I used a PPO2 of 2.0 diving and 2.8 on shallow deco and didn't experience problems, you may!

The other problem when researching PPO2 levels is that commercial dive operations are very guarded with their dive details as bottom time = cash, having seen some data I know of one reputable company where a PPO2 of 3 is used for a working diver, however this company tests the divers in a chamber working on a exercise bike with a PPO2 until they have a hit, simply so the diver gets to know the signs.

I would recommend trying to get hold of a copy of 'Oxygen and the Diver' by Dr Kenneth Donald, it's a really heavy read but data rich and written by the guy in charge of the UK navy dive research establishment so authorative. the only problem is that it's out of print and in demand therefore expensive!

Anyway that's it for my ramblings, remember to challenge anything you are told and ask 'Why' then don't accept anything less than a reasoned argument, there are a huge amount of credible sources of data available free or at low cost, read and decide yourself!

But always......dive safe!!

RichW

 

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