What's the deal with these numbers?

[DuboisP]

I am assuming this is the nearly universal PPO recommendation of 1.4? Also, what is a good comprehensive reading recommendation for oxygen toxicity issues? Thanks!

http://www.scubaboard.com/forums/ba...navy-diving-manual-rev-6-15-april-2008-a.html

 

[AndyNZ]

The recreational limit for scuba diving is 130ft. But oxygen doesn't pose a substantive toxicity issue until 218ft. BUT the record for compressed-air scuba diving is 509ft! So WTF?!?!

The recommendations are based on perceived risk.

Take the 18m (60ft) recommendation for new divers that some agencies use. This makes perfect sense - the theoretical models that Lynne refer to start to change at around that depth, so that the NDLs become much shorter. At 18m, the typical breathing rates of new divers are such that they are unlikely to be able to spend sufficient time at those depths to put them close to the NDL... something that is true for deeper dives. Variation in gas consumption is linear, reduction in NDLs is exponential.

So it is just a tool for managing risk. It doesn't mean that it's a law. My first post-OW dive was to 27m... there's nothing stopping that, though in retrospect it was perhaps somewhat foolish.

Think of it as a speed limit - you can go faster, but if you do there are implications... in terms of potential for an accident, fuel consumption and so on.

Similarly the 30m (100ft) recommendation makes sense - this is the depth where nitrogen narcosis starts to become apparent for the majority of divers. So again, another guideline is a appropriate.

The 40m (130ft) recommendation is, to my mind, the deepest you can make a sensible dive on standard(ish) scuba gear - taking into account the effects of narcosis, short NDLs and gas consumption. It's the point where really, any miscalculation in how long you spend at that depth is going to have serious implication. As soon as you have a decompression obligation and you haven't calculated how much gas you need for the ascent - then you are playing a lottery, but one you will never win... just someday you will lose.

Yes, the record for breathing compressed air at depth is way beyond this limits. I suspect that it wasn't done on "standard" recreational scuba gear. Yes, it is way beyond the point at which air becomes toxic - but what was the bottom time? Hyperbaric chambers regularly run patient schedules with a ppO2 of 2 or even 3ATA. Those patients tox, but it's not straight away. It takes time - and that time will vary from individual to individual.

 

[MaxBottomtime]

Same thing goes for the whole DCS thing. I would think all sorts of factors would come into play determining a persons "threshold" for nitrogen. E.G. I would think fit people are less succeptible than out-of-shape people, people with lots of diving experience have bodies that have "figured-out" how to dispell nitrogen more efficiently than "newb-bodies", the extent to which you're relaxed or stressed would play a role, etc. I guess I'm not really looking for an answer, I just can't help but feel that these numbers are, to a large extent, made up.

The few divers I know who have been bent experienced it on dives much shallower than their deeper dives. Some were described as "underserved hits", although I feel if it happens, you probably did at least something to trigger the event. I've only had one buddy die under water, but it was at a depth 150 feet shallower than his deepest air dive. Sometimes it's a roll of the dice.

 

[spectrum]

Blah blah blah! Diving is different for everyone. Blanket statements like these above simply are not true for everyone.

 

[flots am]

. . . with lots of diving experience have bodies that have "figured-out" how to dispell nitrogen more efficiently than "newb-bodies", the extent to which you're relaxed or stressed would play a role, etc. I guess I'm not really looking for an answer, I just can't help but feel that these numbers are, to a large extent, made up.

The numbers are all "made up," however they work for nearly all divers in most circumstances.

Some are the product of empirical testing and a fudge factor, some are based on algorithms that model gas loading based on various theories and some are based on observations like "we haven't seen many people die under these conditions."

The end result is that if you dive within recreational no-deco limits using available current training and procedures, you'll be pretty safe. Once you start stretching things, you'll be less safe, but nobody can tell you that any particular person will be "safe" when doing "X" and will be injured and die if they do "X+1".

flots.

 

[Superlyte27]

The numbers are all "made up," however they work for nearly all divers in most circumstances.

Some are the product of empirical testing and a fudge factor, some are based on algorithms that model gas loading based on various theories and some are based on observations like "we haven't seen many people die under these conditions."

The end result is that if you dive within recreational no-deco limits using available current training and procedures, you'll be pretty safe. Once you start stretching things, you'll be less safe, but nobody can tell you that any particular person will be "safe" when doing "X" and will be injured and die if they do "X+1".

flots.

I agree.

 

[Hatul]

I think Mike Boswell's answer is a good one. It's a combination of narcosis, air supply, and getting close to deco that get's more problematic the deeper you go. The number itself is a general rule and no one will arrest you if you violate it, but your risks increase dramatically.

Adam

 

[elmer fudd]

I actually think they're pretty good guidelines and that's all they are. No one can stop you if you want to dive to 200' without tech training or even an OW cert.

Here's why I think they make pretty good sense though. First off the 60' limit for new divers. At 60' or less things are pretty simple. So long as you watch your pressure gauge and don't hold your breath as you ascend you're not too likely to get hurt. There's no noticeable narcosis, NDL's are long and it doesn't take much gas to get there or get back.

The 100' limit for AOW divers also makes sense. There's still not much noticeable narcosis, but past 60' NDL's become much shorter and getting bent becomes a real possibility if you don't watch your time and depth. Doing a CESA from these depths also becomes much less of a possibility.

The 130' limit makes sense because when you go beyond it the likelihood of mandatory decompression becomes very high. At these depths you're definitely affected by narcosis. NDL's are very short and gas management is very important.

Past 130' you're looking at likely decompression, either noticeable narcosis or trimix and probably lots of new gear like doubles and deco bottles.

 

[Rhone Man]

It's all been said above. 130 feet is not a law, it is guideline. But it is pretty good one. If you want to go below that, get the right training and the equipment.

Don't even think about 509 feet. More people have died than survived making world record attempts for depth on air.

 

[String]

The so called "recreational depth" is just an arbitrary definition used by SOME agencies. It's not a law, its not a rule and its far from universal. The reasons they use ranging from narcosis, air consumption at depth but mainly as these agencies tend to train no-decompression diving and at 40m the bottom time is so short to go deeper as no stop would be a pointless bounce dive.

Other agencies just have different recommended max depth which increases as you go through the grades in a nice sensible progression and don't make arbitrary "rules" about a maximum depth.

BSAC for example use 20m for entry level, 35m for next level and 50m for the next level.

As for the term "recreational" i take that to mean i dive someone is doing for fun, as a hobby regardless of complexity or depth. "Commercial" is a dive done for money.

So i could have a 60m recreational "fun" dive and a 6m "commercial" teaching dive.