Suggestions for getting my first dive computer

[scubadada]

You seem to be claiming that diving is safe and that those following computers will not get bent. But people do get bent. I am saying that following a computer is no guarantee of safety but that some profiles are riskier than others. The longer the bottom time the higher the risk. That is widely accepted. Do you dispute that? Is it too much of a leap to see that those exploiting the longer NDLs are at greater risk, all other things being equal?

Do you have evidence that longer NDLs are as safe as shorter ones? That is the advice you give.

My advice is buy the cheapest one because these things do not actually matter.

I have tried several times to correct your faulty statements and logic. This is like discussing religion.

 

[KenGordon]

I have tried several times to correct your faulty statements and logic. This is like discussing religion.

If you want to do that then try a formal representation of the logic in question.

You ask for evidence of X. I ask for evidence of !X and point out that the accepted basis of the system in question => X.

You say long NDLs are as safe as short NDLs. I say if enough people dive long NDLs some will get bent that would not on short NDLs. I say that is not a favour as a recommendation. However I also say that it does not matter as often dives are engineered to be shorter. Thus the supposed advantage of longer NDLs is moot.

 

[scubadada]

If you want to do that then try a formal representation of the logic in question.

You ask for evidence of X. I ask for evidence of !X and point out that the accepted basis of the system in question => X.

You say long NDLs are as safe as short NDLs. I say if enough people dive long NDLs some will get bent that would not on short NDLs. I say that is not a favour as a recommendation. However I also say that it does not matter as often dives are engineered to be shorter. Thus the supposed advantage of longer NDLs is moot.

Ken, you have no evidence that more conservative, commercially available, decompression algorithms, are safer, have lower rates of DCS, than more liberal ones. I have pointed out at least 3 times, that all the algorithms may simply be very safe. It's extremely easy to reduce your risk of DCS to zero, avoid diving.

 

[Chavodel8en]

Yeah... Love your profile pic, too. So very apropos.

Great, I chose the cartoon pic to entertain the kids while the rest of us discuss grown folks' business.

 

[stuartv]

Do they really care if that requires 3 minutes compulsory deco on a Suunto or 3 minutes of DSAT safety stop for the same bottom time?

Yes, because one is a violation of their training and the other is not.

You seem to be claiming that diving is safe and that those following computers will not get bent.

Nobody has ever even implied that.

If you want to do that then try a formal representation of the logic in question.

You ask for evidence of X. I ask for evidence of !X and point out that the accepted basis of the system in question => X.

You say long NDLs are as safe as short NDLs. I say if enough people dive long NDLs some will get bent that would not on short NDLs. I say that is not a favour as a recommendation. However I also say that it does not matter as often dives are engineered to be shorter. Thus the supposed advantage of longer NDLs is moot.

Nobody said that longer NDLs are as safe as shorter NDLs. What thread have you been reading? What we have said is that there is no evidence presented that contradicts the following: Longer NDLs from liberal recreational computers increase the chance of DCS, as compared to the shorter NDLs from conservative computers. But, the difference is so small that it is noise, drowned out by other factors that have a MUCH larger influence on getting DCS. So much so, that the difference in algorithms is inconsequential, with respect to the chance of getting DCS.

On the other hand, the difference in algorithms is very meaningful to many divers in terms of how much bottom time they get, as longer bottom time directly correlates to more enjoyment of the dive.

So, a miniscule, teeny-tiny difference in the chances of DCS versus (on some dives) a very large difference in the bottom time allowed (while staying within PADI, et al, OW training).

Assertion: There is no statistically significant difference in the likelihood of DCS for people doing NDL dives, between a mainstream "liberal" computer (e.g. Oceanic Geo running DSAT) and a mainstream "conservative" computer (e.g. Cressi Leonardo).

Evidence in support: None of the mainstream computer manufacturers are known to have a notably higher incidence of DCS among their computer users.

Of course, this evidence is not rigorously researched hard data. But, what do you have to refute the assertion made? Anything?

 

[Chavodel8en]

Frankly, it seems like some folks want others to dive the way THEY think is correct, or barring that, at least to deny that new divers have the ability to make choices for themselves, because there's no way a new diver could possibly have the breadth of precious scuba knowledge.

And what seems lost to many here is that new divers don't stay new divers. They gain experience and grow their diving. There's no reason they shouldn't have their initial computer that's appropriate for their continuing diving. (And Im not arguing for an expensive model - I've still got my cheap, but liberal Geo)

I've seen a number of comments that people doing "resort" diving will be limited by things besides NDL, so it doesn't matter. Well, several of my vacation dives have been limited by the computer - and I don't think my experience is rare. Its not unusual for a new diver - 1 yr or 2 yrs post OW - to now be in a position to make an informed decision as to whether they want a conservative or liberal computer.

 

[Edward3c]

...
Evidence in support: None of the mainstream computer manufacturers are known to have a notably higher incidence of DCS among their computer users.

I love it when people resort to "evidence". Which research establishment is going to take on a manufacturer in the courts by saying one model is more dangerous than another?

 

[KenGordon]

Assertion: There is no statistically significant difference in the likelihood of DCS for people doing NDL dives, between a mainstream "liberal" computer (e.g. Oceanic Geo running DSAT) and a mainstream "conservative" computer (e.g. Cressi Leonardo).

Consider a graph of probability of a bend (y) against bottom time (x) for a given depth.

On the left, 0 bottom time (just a bounce), on the right some long time such that we might clearly expect a bend. Let's say 2 hours.

For t=0 the pDCS is very nearly zero, for t=120 pDCS is very nearly one.

For all points on that line pDCS at t is less than at t+1

The only goes up, it never dips down again. So the pDCS at an NDL of n is lower than at n+4 (4 being the sort of difference we are talking about in NDL)

We know that pDCS for dives shorter than typical NDLs do result in DCS (see the BSAC incident reports) so we know that pDCS is not zero around the contentious part of the curve.

You also say, other factors dwarf the profile. That is true, but the profile is under the control of the diver. Those factors move the curve left and right and by doing a shorter dive the diver can mitigate those factors.

Experienced divers ought to be thinking about these extra risk factors and might take other steps to mitigate them, less experienced divers are likely to be oblivious to those factors.

 

[Storker]

The answer to short NDLs is not longer NDLs. It is Nitrox, and potentially doing stops.

Personally, I have never understood the need for buying the most liberal computer on the market. Maybe it's because I don't go on liveaboards and thus don't do more than two dives per day (the logistical challenges of bringing enough gas for more than two dives for the crowd aren't attractive to me), maybe it's because I'm usually gas limited if I'm on air (most of my dives are dry, and that means a higher RMV/SAC than warm water wetsuit diving), maybe it's because I'm generally content with 45-60 minutes under water, maybe it's because my bladder usually starts to bother me after about an hour under water. Anyway, I dive a Suunto and have never - at least not after getting my nitrox cert - had to surface too early because my comp told me I was getting too close to the NDL.

 

[packrat12]

Let us see if we can break this down into simple terms:

1. A shorter bottom time for a given profile will result in less inert gas in your system. This is regardless of liberal or conservative algorithm.
   
   
2. The time it takes to get from the deepest part of the dive to the surface will affect how much inert gas is in your system (as any change in times and depths will).
   
   
3. All dives are deco dives. Period. For NDL diving, the first stop is the surface. Decompression as defined from Webster: “Reduction in air pressure” and from Wikipedia: “Reduction in ambient pressure experienced during ascent from depth”. If you do not believe this then change the ‘surface pressure’ by taking an unpressurized airplane ride to 10,000’ immediately after surfacing from a NDL limited dive without a ‘safety stop’ and see what happens.
   
   
4. The ‘safety stop’ is a form of a decompression stop. You are given time to off-gas while under higher pressure. On an NDL dive, this will not change the depth of the first stop though, which is the surface. This will make any profile more conservative and thus safer. This is in addition to other benefits such as slowing the ascent before hitting the surface.
   
   
5. The closer you are to the theoretical M-Value line (using Buhlmann terms), the more risk you have. This also applies to any other model limits. How close the M-Value line is to your ‘true’ line is the main concern here. You may still be far away or you may have already crossed it. We can use any other example here such as filling a scuba tank nearer its burst limits is more risky. Under some conditions, it may fail. I doubt anyone will counter that. I have known 2 people that got DCS within computer NDL limits. One was thought that mowing the lawn, alcohol and a hot tub caused it, the other was exercise after diving (lifting the gear for several people). A longer bottom time will put you closer to the ‘true’ line regardless of the model used.
   
   
6. Deserved DCS hits are where someone crossed the M-Value line. Undeserved DCS hit is where someone did not cross the line. We see both. Other factors can be included in that assessment.
   

Since a ‘liberal’ computer will have a longer bottom time without a method of off-gassing like we use in a decompression dive, you will surface with a higher inert gas content then you would when using a ‘conservative’ computer. This alone indicates that there is a higher risk from using a ‘liberal’ computer. I think it is Pete that uses the term FIGJAM to define a belief like many of are presenting. Just because empirical data is not there (DCS incidence is very low in recreational diving), does not make ‘liberal’ computers as safe as ‘conservative’ computers. The fact that you are nearer the theoretical limit of a model is riskier especially as it gets closer to the ‘true’ limit. How much closer to the ‘true’ limit of the person that theoretical limit is at that moment is what makes this riskier. Because of no other method of reducing inert gas exists in the NDL dive, the ‘liberal computer’ will simply be closer to the ‘true’ limit and hence not be as safe as a ‘conservative’ computer. Once we start talking a variation in our ascent on a decompression dive, this will no longer hold true as each stop and time will alter the amount of inert gas in our system.