Planned deco on a recreational dive?

[boulderjohn]

In the 2016 BSAC Incident Report 35%of DCS was 'within limits'. Only 2% was 'missed stops'. Just because you are within NDL does not mean you are safe.

I'm surprised the total was that low--I had heard that other studies had it closer to 50%.

We have to remember what these statistics mean. Only a very, very tiny percentage of dives end in DCS--something like 0.002%. Of that 0.002%, about 35% were within NDLs. That means that the odds of getting DCS while within NDLs are extremely low, but they do exist.

We had a similar case in terms of the understanding of statistics in the political scene a month ago, when it was revealed that a certain court had nearly 90% of its cases overturned by a higher court. That sounds horrible. It turned out that nearly 90% of the cases that the higher court reviewed were indeed overturned, but they only reviewed fewer than 1% of all cases, and they only reviewed the cases because they had been given convincing evidence that there would be a reason to overturn them. Thus, it was actually very rare to have a case overturned, not as common as the 90% figure sounds.

 

[KenGordon]

There were 75 incidents (plus about 30 'illness' which is probably dci) so 25 ish 'within limits' and about 2 who missed stops. Of course a large part of this population are trained to do deco dives. The scariest bit is that about 20% involved a rapid ascent. There is no need for that. > 30m and repeat dives are similar proportions.

The reporting population is between 30k and 50k Mostly these were BSAC members but nearly half the incidents were for people either not members or their status was not known.

That looks like between 1 in 300 and 1 in 500 chance of a bend each year, and maybe a 1 in 1500 chance it was diving 'within limits'.

In the BSAC context I'd guess 'within limits' will include having done the required stops as well as NDL dives.

 

[DevonDiver]

. For example, I once had a buddy do a panicked, sprinting ascent to the surface from 85 feet because he had looked at his Suunto computer and seen that he had only two minutes to go before he reached NDL. He seriously believed that meant he had about two minutes to reach the surface before he got full blown, paralyzing and possibly fatal DCS. Don't be that guy.

This is an issue with training.

Firstly, it illustrates insufficient knowledge of how a dive computer calculates N2 loading and off-gassing. The diver SHOULD have been aware that ascending in depth would have increased their NDL time.

Secondly, it indicates insufficient dive planning and weak situational awareness. The diver should enter the water knowing their outline bottom time AND should understand their proper ascent time from depth AND should have set a minimum NDL at which to begin a controlled ascent with safety stops.

Thirdly, it indicates an insufficient knowledge of decompression and emergency decompression procedures. The diver should have known how and why DCS would present and been able to prioritise threats to define the safety course of action when faced with a potential emergency deco scenario.

None of this justifies a polar swing in logic to underplay the seriousness of entering deco.

What it does illustrate, from my perspective, is a gross failure of agency course syllabus to adequately prepare the diver for computer diving AND for conducting more challenging dives which closer approach no-stop limits.

When we look at the syllabus for continued education in the diving programmes from major scuba agencies, we can see how progressive courses (AOW, Deep, computer/multi-level) utterly fail to tackle the knowledge and procedural deficits which this example illustrates.

These continuing education courses should represent a staged increase in both knowledge, skill and procedure relative to the risks and demands of increasingly challenging (closer to limits) dives being conducted. They do not.

Deep Diver courses are a poignant example of this failure. Dives are devoted to demonstrating pointless examples of theoretical concepts (color loss, compression) so shockingly fundamental that they're actually first taught.. and 'mastered' during OW theory chapter #1.

Rather than belittling the prudence of staying within no-stop limits, perhaps the more positive (and safety conscious) debate would be to ascertain how and why divers are progressing through multiple courses of scuba training; and yet still remain oblivious to the core knowledge needed to safeguard themselves from the risks they encounter.

 

[redacted]

I can agree with this. I just have a hard time drawing a line/figuring out where the cut off is between them. For instance, if people think 2 minutes of deco is no big deal, how about 3? How about 4? 5? 6? 10? where do you draw the line? I just think untrained average OW divers need to completely stay away from deco and not accept advice that "a couple of minutes" is probably fine, safe, and no big deal. A couple of minutes can quickly turn into a large deco obligation at depth if you don't know what you are doing.

It is a dangerous logic process. By induction, if one minute of deco does not matter and one more minute never mattersw, then deco does not matter. That is obviously wrong. So if we accept the premise that one minute does not matter then there must be some subsequent minute that does matter. That is, the minute that crosses the line. That seems like a dangerous idea to try to work with. NDL should be the line. Less than or equal to NDL is acceptable risk. Greater than NDL is unacceptable risk that must be managed to reduce that risk to an acceptable level. Given that even within the acceptable risk category, DCS has occurred, it is questionable to support the idea that even blowing off a couple minutes of deco should be percieved as unacceptable risk, regardless of how small the increase is.

 

[MXGratefulDiver]

DECOMPRESSION IS ALL JUST A THEORY ANYWAY
To unequivocally state that "this or that" will happen following a missed two minute stop, or a blown "safety stop" is just plain wrong.

Well, yes to the latter ... but saying that "decompression is all just a theory" is like saying "weather is all just a theory". It's quite real, I assure you ... and the consequences of overstaying your welcome are quite well known. What isn't known is exactly what combination of physiology and circumstances will result in a bad outcome. It's far better to say it's an inexact science ... or perhaps that it's an exact science, but that we are incapable of exactly measuring all of the variables that combine to produce a given result. As a consequence, we rely on mathematical models that simulate certain idealized conditions which may or may not apply to you on any given dive.

Or, to put that in language that the average diver can understand ... "what it all comes down to, is we haven't got it all figured out just yet" (Alannis Morissette) ...

... Bob (Grateful Diver)

 

[MXGratefulDiver]

I see lots of comments by folks with minimal diving experience and zero tech experience... some seem very cynical of the need for deco training before deco dives.

Seriously, just because someone did a few crappy specialty courses before, doesn't mean they should write-off the validity of further training for advanced dives.

Doing a dive like the OP describes just ONCE probably has a low risk of catastrophe. It's a horrid 'trust me' dive, but if nothing goes wrong, then it's eminently survivable.

The real danger stems from doing such a dive and allowing oneself to become convinced that it proves some sort of competency... and, thus, validates doing further dives beyond one's training.

I've seen this a lot with marginally experienced divers... especially those skeptical of training... who firmly succumb to "normalisation of deviance" and fall foul of a typical delusion that uneventful dives somehow prove a capacity to assuredly survive dives that are patently beyond their ability and training.

The need for specialist training for deeper and/or decompression dives isn't based on nothing going wrong... it's based on providing a reassurance that you'd survive compound factors occurring that'd otherwise kill or injure you.

You can trust to luck for your safety with recreational dives... as they're ultimately very forgiving. The parameters are deliberately kept forgiving... thus preserving safety even with minimal, or ineffective training.

Technical dives aren't forgiving. Increases in depth and bottom time drastically increase the severity of consequences for any situation you're unable to deal with.

You might 'get away with it' once. Or twice. Or 50 times. But eventually you'll be faced with a scenario that you can't handle using recreational level equipment and protocols. When that certainty occurs.... you'll have zero options available to preserve yourself.

Is it really worth the risk, just to save a few $$$ on training and equipment? Or because you don't have the moral fortitude to resist temptation or slight peer pressure?

Well put ... a simpler way to say it might be "As long as everything goes right, you should be fine. But as soon as something goes wrong, the likelihood of you being able to deal with the problem are not good." A good dive plan doesn't assume that everything will go right ... which is why most of your dive training involves learning how to deal with things going wrong ...

... Bob (Grateful Diver)

 

[MXGratefulDiver]

For example, I once had a buddy do a panicked, sprinting ascent to the surface from 85 feet because he had looked at his Suunto computer and seen that he had only two minutes to go before he reached NDL. He seriously believed that meant he had about two minutes to reach the surface before he got full blown, paralyzing and possibly fatal DCS. Don't be that guy.

That's a different problem ... someone who doesn't understand what his computer is telling him. Several years back I was diving with a guy ... a DM, no less ... who took off up a wall from 120 feet without any warning. I followed him up to about 40 feet before I stopped and watched him go ... because the profile we'd been diving made it imprudent for me to go any closer to the surface (in my estimation) without some off-gassing. I wasn't in deco, but being an old dude I just didn't feel comfortable following him any further. After a couple minutes I started heading up slowly, and at about 20 feet I encountered my dive buddy heading back down. He indicated to me that we should turn around and descend back down the wall, but I overruled and signaled that we should hang at this depth for a bit, then head shallower. After doing the usual ascent from there, we surfaced. I asked him what that was all about and he replied that his computer had told him to go to 10 feet for 2 minutes ... which is exactly what he had done. Once back on the boat I had a chat with him about ceilings ... and what his computer was actually indicating.

... Bob (Grateful Diver)

 

[scubadada]

It is a dangerous logic process. By induction, if one minute of deco does not matter and one more minute never mattersw, then deco does not matter. That is obviously wrong. So if we accept the premise that one minute does not matter then there must be some subsequent minute that does matter. That is, the minute that crosses the line. That seems like a dangerous idea to try to work with. NDL should be the line. Less than or equal to NDL is acceptable risk. Greater than NDL is unacceptable risk that must be managed to reduce that risk to an acceptable level. Given that even within the acceptable risk category, DCS has occurred, it is questionable to support the idea that even blowing off a couple minutes of deco should be percieved as unacceptable risk, regardless of how small the increase is.

I have volunteered this information before, but it is apropos to this thread. I am mainly a recreational diver and dive within NDL, but, about 5-6% of my dives are planned with short decompression, these are technical dives and are treated as such.

My primary computer is an Oceanic VT3 running DSAT. My backup computer is a Dive Rite Nitek Q running Buhlmann with GFs chosen to match the VT3. The algorithm match is close, but not perfect. I always clear either or both computers when they enter deco. I don't pretend that blowing off the more conservative computer is a good thing to do. For me, NDL is the line dictated by my more conservative computer on that dive. Not being as young and invincible as I once was, I pad my last stop by several minutes, something like a safety stop. So far, so good.

 

[Beau640]

I've interpreted multiple posts on here as DCS is "uncommon" and a "minor" issue (because it's so uncommon) and people stating that "a couple of minutes" of deco is no big deal and unlikely to cause an issue. I've interpreted multiple posts on here as stating it's "probably" okay for an OW diver to do "lite" deco.

At this point I've given up with the arguing and just have agreed to disagree because even 1 minute of deco to an untrained OW diver, to me, in my own opinion, breaches training and should not be condoned. But it seems as if multiple people on here disagree with me and so again, agree to disagree. I will not change my mind however and will continue advocating for training before undertaking even "lite" deco.

Again this isn't about saying some of the divers here are crazy for doing what they do. Many of you have a lot of experience. The argument has been stop saying it's okay for and average diver, not you, to undertake even "lite" deco dives.

 

[Stoo]

I've interpreted multiple posts on here as DCS is "uncommon" and a "minor" issue (because it's so uncommon) and people stating that "a couple of minutes" of deco is no big deal and unlikely to cause an issue. I've interpreted multiple posts on here as stating it's "probably" okay for an OW diver to do "lite" deco.

Beau, there is absolutely nothing wrong with your approach, and I agree completely that DCS is a serious issue potentially, and for those lacking in a solid understanding of the physiology and the many other factors involved, then sticking to the "rules" is an entirely appropriate and sensible approach.

What a number of us are saying however, and @NWGrateful Diver perhaps put is best... the science of decompression is a really inexact science. Perhaps a really simple example to illustrate:

Say you take two identical twins. For a week, they eat and drink and sleep and pee and dress in exactly the same way. Your put them in identical dive gear and they dive arm in arm so their profiles are identical.

There is only one difference. Diver A is wearing Computer Brand A, known for it's conservative algorithm. Diver B is wearing computer Brand B... a "technical diving" computer and Diver B has changed the gradient factor in the computer to a fairly aggressive ratio,. He has places to go after the dive and wants to be out as soon as he can be.

These two do a normal dive to X feet for Y minutes. Diver A, with the conservative computer indicates that his NDL is getting short and thumbs the dive. Diver B, looks at his computer and it indicates that he has another 8 minutes of bottom time left, but being the good buddy, heads up, still arm and arm.

When they get to 70', a herd of Lesser Known Pygmy Hawksbill Dolphins circles them. Being rather rare, the two divers linger for a few minutes. Suddenly Diver A realizes that his computer is indicating that he now needs a mandatory 2 minute stop at 10'. Diver B's computer is still fine, and in fact his NDL is growing in leaps and bounds as they ascend.

Two minutes later, they are at 10'. Computer A shows 2 minutes of deco. Computer B shows '99' or unlimited time.

Forgetting "safety stops" for a moment, if both divers ascend to the surface at that minute, is anyone here going to tell me that Diver A is going to absolutely get bent, but Diver B is going to dance a jig on the boat, chug a beer and head in unscathed? Of course not... anything could happen. They could both be fine (and likely will be) OR, they could both get bent OR Diver B will get bent and Diver A will be fine.

And the reason for this is because the computer algorithms are AT BEST estimating what is going on in those divers bodies. And depending on who wrote the programme and how the diver set the conservancy of the computer "something" might happen, or it might not.

And that is because decompression "theory" is a really inexact science. Lot's a smart people have been trying to understand it for years and they know a LOT, but they also know that don't know everything. And then on top of that, there are researchers who are looking at really different models on how our bodies deal with being full of inert gas. Who knows, maybe 10 years from now, we will have an entirely different book of knowledge.

And I think it's important to understand that the amount of research done on DCS is pretty small, relative to the amount of research done on say heart disease, or cancer. DCS has the potential to only effect a relatively small number of people. There just aren't that many of us out there.

Truly "unearned" hits are pretty rare. Most of the time, there is some underlying cause... dehydration, exertions... something like that that provides that "a-ha" moment. I friend of mine has been taking hits of skin bends from time to time for several years. Medical investigation showed no areas of concern. Until last month when it was discovered she has TWO holes in her heart... the "a-ha" moment. She is going in to have them surgically repaired in a few weeks and should be fine to resume diving a few weeks later.

Anyway, my point is simply this... Computers and bodies are all different. Computers give us a convenient digital readout of what some programmer (who likely doesn't even dive) has coded to reflect some old guy's calculated tables,and they do an awesome job of leading us to believe that they are magically reading what's happening in our bodies, which of course they aren't doing. Until there is a transponder suppository, that you shove "where the light of day don't shine" they will remain valuable toys that help to keep us safe... but they are only offering a decent estimate.

And my second point is as good as I can offer... Don't get into deco without a thorough understanding of what's going in in your body (in theory). Period.

But also don't freak if you do cross that line. Do the best you can, but if you happen to bob to the service 1.5 minutes before you have completed your stop, do not lose your mind.