Contradictions in conservatism (or safety)?

[woodcarver]

In reading a bunch of threads, on a variety of topics, something keeps coming up that seems to be contradictory in nature. In some, there is a overriding air (pun intended) of erring on the side of being conservative in planning. This makes sense as running out of gas is a really bad thing. But in regards to NDL, time, depth, light deco, what is light deco, there seems to be a lot less of a consensus regarding what is considered safe.

A number of complaints of this algorithm or that being too conservative. Some preferring this computer over that because it won't penalize you as much if you're doing a lot of diving each day, such as on a LOB. Or you can set this one to be more liberal where the other only has one choice. Choosing to change the PO2 setting to 1.6 instead of the more conservative 1.4. Are computers better than tables because they give you more BT.

Safety is a subject that seems to be very high on most people's list, mine included. So if we are trying to conduct ourselves in as safe as manner as we can in this sport, why do quite a few of you intentionally use a more liberal algorithm? By definition, wouldn't this push you closer to that fuzzy line of DCS? How does one decide one algorithm is better than another? Aren't all of them based on somebody's educated guess on how long it takes to off-gas? Whose guess is better?

Just trying to learn here, not pushing any opinion. The more I've learned, the more I've learned I need to learn. This is just an attempt to expand my knowledge.

 

[KenGordon]

Possible reasons:

People like to justify their own purchasing decisions.
They don’t think the line is fuzzy.
They think the line is fuzzy but a really long way to the right.
They have mitigations in place in case they fall the wrong side of the line.
They are misinformed about the actual difference between conservative computers and less conservative computers.
They are really doing deco dives, including stops, just within the NDL of an aggressive computer. Aka, padding the safety stop.
They don’t see that they can achieve the same goal by other means.

Dives have limits. Those can be gas, temperature, tide, bladder capacity, deco time, run time (ie the boat’s limit) and some I haven’t thought of. If the limits are short, for example 20 minutes at 30m, and the dive interesting you look for ways to increase the limits, carry more gas, get a dry suit, heated vest, fit a pee valve, use Nitrox, chose a different boat etc. After doing the practical things the limit starts to be the deco for dives at any significant depth. Everyone has a limit on deco. It might be none at all, it might be quite a lot. Now the game starts to be how to optimise that deco while doing a long dive, either to avoid it or to get out before freezing, wetting yourself or all the cake having been eaten before you get back on the boat. There are three ways to do this, change the profile - do a shallower dive, start up sooner, get down later - change the breathing mixture - less nitrogen uptake, or get out of the water with a higher nitrogen load.

Consider 3 levels of conservatism, GF70/70, 80/80 and 100/100.

For a 40 minute 30m dive the following profiles are available:

Air, square profile
gf70/70 results in deco of an hour.
Gf 80/80 cuts 17 minutes,
Gf 100/100 cuts the same again. 22 minutes of deco.

32% gf70/70 22 minutes of stops
80/80 cuts 7 minutes
100/100 cuts 10 minutes, leaving about 5

Air, 15@15, 10@30, 15@10
70/70 has a ceiling and a TTS of about 8 (3 ascent, 5 stop) minutes at the end of the 30m part but no deco after the 10m part.
80/80 reduced that to 3 minutes of stops
100/100 never has a ceiling, 90/90 is about the edge.

Use 32% and the 70/70 dive has 13 seconds of stops at the end of the deep section.

The choice of gas can make a dive possible as much as the choice of conservatism.

Once accelerated deco is used the time advantages of an aggressive setting shrink even more. The difference between 70/70 and 100/100 is stops of 16 vs 10.

The real algorithms are not based on guessing. They bent people and set the parameters based on that. The implementations found in computers take that information and then add some guesses and assumptions which ought to make them a bit safer than the originals. The huge advantage sport divers have over the navy divers used in the testing is that they are not usually working hard at depth.

The computer is a secondary thing in staying safe. Good diving practices are key. I was doing a dry dive earlier in the week. They get about 60 or 70 bents divers a year and most of them were diving within the limits of their computer or tables.

 

[Blasto]

But in regards to NDL, time, depth, light deco, what is light deco, there seems to be a lot less of a consensus regarding what is considered safe.

There's no defined range of light deco, so it's just an adjective and a noun, which can mean different things to everyone.

Can even mean different things to the same person depending on the circumstances. Say, I'd consider light deco in rec gear to be anything that still fits within NDL at GF 100/100, but a lower GF is observed out of conservatism. In tec gear, light deco would instead be deco that can be done entirely on backgas with enough reserve for a major failure.
And someone will surely disagree with both - good thing they aren't definitions, just personal margins for what one considers to separate their lower-risk and higher-risk brackets.

So if we are trying to conduct ourselves in as safe as manner as we can in this sport, why do quite a few of you intentionally use a more liberal algorithm?

The answer is, we're not. We're trying to conduct ourselves in a manner that's sufficiently safe for everyone's risk perception to be within their risk tolerance.

How does one decide one algorithm is better than another? Aren't all of them based on somebody's educated guess on how long it takes to off-gas?

They're not. The only algorithm based on somebody's educated guess is RD.
The rest of the algorithms are based on:
* A dataset of Navy dives, which have or haven't resulted in DCS;
* Measurements of bubbles in test diver tissues;
* Dissolved gas or bubble behavior mathematical models.

 

[Centrals]

The only sure way to avoid DCS is to stay DRY.

 

[Blasto]

The only sure way to avoid DCS is to stay DRY.

So, as long as I wear a dry hood with my FFM, I'll be fine?

 

[RainPilot]

I believe conservatism should be applied to the diver not the computer. As an example, using Bühlmann as the algorithm (only GF High as these are NDL dives)

Starting with a surfacing value of 100 as being shown to be “acceptably” safe with fit young Navy divers, I need to decide where I fit in. I’m not so young or fit anymore so I go to 95. I also smoke and am usually a bit dehydrated (desert living problems) so I come down to 90 for single, relaxed dives.

Now that I have what I believe to be a suitable baseline for my body, I also add a certain amount of padding depending on the dives themselves. Repetitive dives over multiple days with some deeper stuff? Down to 80. Far from a chamber or medical help? Down to 70 I go.

That’s MY conservatism set. Now I need a computer that I can set to match that and STICK WITH.

I feel way too many divers feel that conservatism is a computer thing and so go to any lengths to circumvent the padding. Sure there are some algorithms and implementations that don’t match what you feel you need as a diver but that’s not where the conservatism lies. You need to decide how far you lie from the deco ideal and act accordingly, even if that means you don’t get all the bottom time you want.

My personal situation is such that I need to be very conservative. My solution was to do deco training so that I don’t have to decide between bottom times and safety factor anymore.

 

[uncfnp]

So if we are trying to conduct ourselves in as safe as manner as we can in this sport, why do quite a few of you intentionally use a more liberal algorithm?

Like many activities in life it comes down to a personal choice in risk tolerance. There is no absolutely safe line. As divers and researchers have progressed in their understanding of DCS that fuzzy line has shifted a bit and new algorithms are develpoed and then implemented by the manufactures. The informed diver has the option to select the algorithm that they believe best matches their personal diving factors and risk tolerence. Since few of us know all the variables and many of the dive computer manufacturers use proprietary versions of the algorithms, the choice is still somewhat arbitrary.

 

[Magnus Lundstedt]

From reading a lot of discussion on this subject, I conclude that many do not understand how the algorithm works.

It seems like many divers think that it is as safe to dive a liberal algorithm as to dive a conservative algorithm, you just got more NDL from some magic ndl-creating pixies. They do not understand that it is a mater of how big the risk is, and that 1 in 1000 000 is less safe than 1 in 1000 001, only by a little, yes, but still a higher risk.

 

[CWK]

After a lapse of 20 years, I returned to diving and purchased a Suunto in April last year. Three months later, I purchased a Perdix because I knew by then that I would be doing more advanced training.

For, recreational no deco dives I use both my computers on different settings. I use my Suunto on a setting of 0 which is similar to a Perdix setting of xx/95. For my Perdix, I use a setting of 50/80. I use my Suunto to give me maximum NDL at the bottom, but after I start to ascend, I follow the Perdix and do not ascend beyond 6m until my ceiling has cleared. Then I go to 5m and follow the Suunto for the safety stop.

You could say that I did a conservative deco dive based on my Perdix setting of 50/80 (and I am certified for deco). Alternatively, you could say that I did a non-conservative no-deco dive based on my Suunto. I could happily replace my Suunto with a computer that allows me xx/100 to give me more NDL at he bottom, but I would still be diving conservative because I would still follow the ceiling dictated by my Perdix setting of 50/80. It is like having my cake and eating it.

 

[boulderjohn]

One thing to remember is that the people who participate in ScubaBoard are on average not representative of the scuba community as a whole. As an example, past surveys about what kind of BCD people use have usually come out as about 50% backplate and wing, yet industry statistics indicate that BP/Ws represent about 1% of total sales, and many, many dive shops do not even offer them for sale.

In that total SB group, you will find some who are convinced that NDLs are very conservative. They feel that all established algorithms are plenty safe enough for their diving, and they logically prefer the ones that do not send them to the surface immediately. It would be a mistake to assume that they represent a large portion of the general diving population. It would also be a mistake to assume that this attitude contradicts an attitude toward having lots of gas, because the two groups may be entirely different--it would be like seeing some people on FaceBook expressing a faith in Jesus and others expressing a faith in Allah, assuming they are the same people, and wondering about the contradiction.