DIR and computers

[MikeFerrara]

DA Aquamaster once bubbled...
The 120 rule makes sense as a backup if your computer takes a dump on you but I have to agree it lacks precision and, the way MHK has desribed it, assumes NDL's are linear. They aren't and averaging depth is effective at best to maybe 20'-30' spreads in depths. This in my opinion limits it usefulness in comparison witha computer.

I thought so too at one time. After running many profiles with different softwares and comparing the results when using average depth to a calculated multilevel profile I find that it does work. Some day I'll go through the math to find out exactly why.

The whole point here though is that Mike explained why they do it the way they do. It is working for them and without the expense of a computer. I don't think his goal was to convince any one to change the way they do things.

Foe myself I can't get a computer that'll come close to doing what I need for less than $1000 and even that would be a compromise at best so I use a $120 bottom timmer. I get all the bottom time I want and so far I'm not getting bent. I could do the same thing while wearing a computer but this way I get to keep the thousand bucks.

 

[DSJ]

I too think that MHK has done a good job of explaining why DIR doesn't use computers. I think we've also determined that DIR isn't telling us non-DIR folks to stick with square profile plans on the tables.

However, I would have to play with a deco program like Mike F has done to get comfortable with the idea that depth averages work out the same as a continous algorithm in a computer. That might work out if you always start deeper and work shallower, but I'd have to see what happens if you start moderately deep, work down to a deeper level on a wreck or canyon, then start back up. I know the latter is not a prefered profile, but such has been known to happen and any method used has to be able to handle all cases.

Using the fact that different brands of tables or computers have a 40% swing in allowed bottom time to argue for a less exacting calculation works if you start with one of the more conservative tables. But the 120' rule uses the most permissive set, and adding imprecision to that has to be a touch risky. It's not so bad if you treat it as a square profile and really run a multi-level dive as that adds a cushion, but if you try to ride the Navy NDL limits with fuzzy averages how is that different than riding a computer's NDL bar graph? As stated above, the DIR folk are supposed to be more fit than the average rec diver, but are they as fit as the Navy divers in the original research?

I like the idea of having a good idea of what the computer should be telling you just to be sure it hasn't gone haywire, but there are likely to be other indications it's not right. I can't see the algorithm suddenly changing, so any problem would most likely be due to a loss of accurate depth information caused by a mechanical problem. If you're monitoring your depth it should be apparent if the computer is significantly off. It's also a good idea to be able to estimate your remaining air time, and I'd like to get the calculation for that so I could practice with it. That's all good situational awareness for recreational diving, and imperative in the more technical diving a DIR diver is likely to do.

At least one post above mentioned treating a dive as a square profile and doing "all the necessary stops" if a computer dies during the dive. That would not be required as if you have been monitoring the dive and the computer you would catch the failure well before you hit the NDL limit. You could then surface and calculate the dive as a square profile, but if it exceeds the limits you could just treat yourself as a "Z" group diver knowing you were inside your NDL.

Any of the method's mentioned so far are likely to work if you know how they work and pay attention to them and your dive. A computer offloads some of the work and I believe that it has a place in recreational diving for the masses. A computer would help most divers stick close to which ever decompression model they have chosen (most folks know which computers "give more bottom time").

David

 

[MASS-Diver]

MHK,

Thanks for starting this thread. It seems, to some degree and for some obvious reasons, that there has been some resistance to spreading "DIR" secrets outside of GUE courses to the uneducated masses. To see a thread like this is a good thing.

Although, the 120 rule is not a GUE idea (as you said) it's good see you bringing it up here and it really adds alot to SB.

When I started diving my buddy had been using the rule of 120 for over 20 years so we went right that and saved some cash on computers (instead we have watches and D3s). Now that we dive EANx it adds even more saftey, espcially for mulitple dives.

We also run so called NDL dives through V-planner and right down the stops on a slate.

I can see why vacation divers doing 5 dives a day in the tropics want to shut down their brain and rent a computer for $10 a day, but, I think all serious divers should consider whether their computers is adding to their saftey or not. For example, NAUI now has the rules of halves (deep stops) for all dives over 60', but, from what I've heard if you scew up and run into deco time with most current computers they are going to send you up real shallow for one big long stop at say 10' - that's scary.

Thanks again

 

[MikeFerrara]

DSJ once bubbled...
I too think that MHK has done a good job of explaining why DIR doesn't use computers. I think we've also determined that DIR isn't telling us non-DIR folks to stick with square profile plans on the tables.

However, I would have to play with a deco program like Mike F has done to get comfortable with the idea that depth averages work out the same as a continous algorithm in a computer. That might work out if you always start deeper and work shallower, but I'd have to see what happens if you start moderately deep, work down to a deeper level on a wreck or canyon, then start back up. I know the latter is not a prefered profile, but such has been known to happen and any method used has to be able to handle all cases.

Don't make the mistake of thinking there is anything about a computer that can compensate for a "bad profile". Haldanian models really don't give you any edge with reverse profiles. Check out some of BRW's papers.

Using the fact that different brands of tables or computers have a 40% swing in allowed bottom time to argue for a less exacting calculation works if you start with one of the more conservative tables. But the 120' rule uses the most permissive set, and adding imprecision to that has to be a touch risky. It's not so bad if you treat it as a square profile and really run a multi-level dive as that adds a cushion, but if you try to ride the Navy NDL limits with fuzzy averages how is that different than riding a computer's NDL bar graph? As stated above, the DIR folk are supposed to be more fit than the average rec diver, but are they as fit as the Navy divers in the original research?

You might want to rethink the concept of NDL. Riding any NDL whether from a table or a computer is risky business and I know a guy with a cane that'll tell you that first hand. If I'm comming anywhere near what I would consider an NDL I'll do some minimum decompression.

Another thing you might want to do for an excersize is to take some minimum deco say...1 minute at 30, 20 and 1-3 minutes at 10 or the same thing at 40, 30 and 20 (whichever you prefer) and look at the range of depths and times this will work for.

If there is anything about a profile that makes you think it's a good idea you are always free to use a depth deeper than average or max depth. Of course you can always to a more diver freindly ascent.

IMO, the worst thing about the way we teach this whether using computers or tables is that we reinforce the idea of NDL. Many divers seem to think that if the computer is in the green or they are withinn the NDL that they are ok regardless of profile shape. that just isn't true.

By all means play with some deco software. the time to learn about decompression is when you start getting compressed which is on your first dive, IMO

 

[DSJ]

MASS-Diver once bubbled...


...from what I've heard if you scew up and run into deco time with most current computers they are going to send you up real shallow for one big long stop at say 10' - that's scary.

That would indeed be scary. How many computers would do that? Fortunately the Cobra I use is much better if you mess up and incur deco in that it calculates multiple deco depths and allows for continuing offgassing as you ascend. I played with it in simulation mode when I first got it and if I recall correctly it gave me a first stop around 70' on one trial. That's just one part of knowing your chosen tool.

Computers per-say are not the problem, but rather the tendency of most classes turning out almost-divers who are not comfortable in the water and rely heavily on other people (dive masters) or things (computers) to do their thinking for them.

There is likely room for improvement in computers, and they could come down in price as well, but I've already seen marked improvement in both from the first time I looked at them five years ago. I suspect we will see them getting better and cheaper as time goes on.

David

 

[DSJ]

MikeFerrara once bubbled...


Don't make the mistake of thinking there is anything about a computer that can compensate for a "bad profile". Haldanian models really don't give you any edge with reverse profiles. Check out some of BRW's papers.

<snip>

IMO, the worst thing about the way we teach this whether using computers or tables is that we reinforce the idea of NDL. Many divers seem to think that if the computer is in the green or they are withinn the NDL that they are ok regardless of profile shape. that just isn't true.

By all means play with some deco software. the time to learn about decompression is when you start getting compressed which is on your first dive, IMO

Very good points. Where would I find these BRW papers?
Where can I find a good deco software program to play with? Any suggestions for a program that would work best for this investigation?

David

 

[GreenDiverDown]

Would someone explain this, please?

SA

 

[Uncle Pug]

... for some folks.

For others... well... they just need to stick to their computers.

Mike has done an excellent job explaining the concept... but some folks would rather argue than learn. Oh, well.

Personally... I started without a computer... and in the years between then and now I've had a few of them... and they are OK if you like them... and necessary for some of the folks I've read posting here (those who seem to lack comprehension.)

But for me... I don't like or need them. I like keeping a mental picture of where I am in my dive profile and adjusting the amount of time I spend shallow and ascending based on that.

In other words I actively control my profile...none of this:
up/down/up/down/gladmycomputeriskeepingtrackofthisup/down/up/down/up...

The old 120 rule works too. And figuring the depth - 20% if using EAN32 works too... not using EAD and just using EAN32 for a safety factor and reducing the SI works too.

BTW: For ascent rate computers absolutely are horribly coarse. I like super slow ascents from 20' up. Much better to use the *stuff*.

 

[MHK]

Taking a look at the posts that appeared over night I thought I'd add a few more thougths. You need to understand that one of the many reasons that we continue to insist that DIR is a holistic system is because we have numerous tricks such as this incorporated within the system. In most cases by doing so, and standardizing our mixes and gear, our goal in some respect is to take the variables out of the equation and simplify the issue(s). Each and every aspect of the systems plays off of another part of the system so once you start changing one part, it effects another.. For certain divers could calculate the "best mix" formula, for certain divers could use different mixes but at some point you need to do an analysis on whether there is any upside to changing. We've done countless analysis' and we've tried to figure out how or why divers would alter the components, notwithstanding the countless man hours of dives that have gone into the thinking...

Of course we've addressed the issues relating to repetitive dives, multi-day trips and so on, but in all honesty I simply can not teach the whole class over the internet, nor would discussing some of these more advanced techniques be appropriate on scuba forums.

As to two questions that I noticed that seemed to continue to pop up let me address these points 1) the "average" depth and 2) 120..

Depth averaging isn't as complicated as you would think particularly if you aren't doing sawtooth profiles, which isn't recommended anyway. The idea is to break your dives into 5 minute snap shots.. Do this for a few dives while you are still using your computer and you'll see how easy it is once you commit to paying attention to your dive.

Secondly, some have suggested that the 120 only applies to 18 year old physically fit US Navy divers, except that when I take out my PADI tables and look at a 100' dive you see the NDL is 19 minutes so the point being is that the point isn't to fixate on the number "120", if someone feels uncomfortable as if they are pushing some kind of upper Navy diver limit then use 115, it doesn't matter if you want to be conservative and lower the set point, the point is that all you need to do is add to some number 120 or less.. I'm comfortable using 120 and it's worked for me for years, but if someone thinks they are out of shape, dehydrated or what have you then just simply lower the set point.

My point is that I'm trying to open your minds and get you guys to think outside the box a little and by fixating on 120 tells me that some may have missed the concept and fixated on numbers.. The concept is what is important, the numbers are just a means to an end..

Finally, I respect any divers rights to dive as they see fit, and my intention isn't to try to get anyone to change anything they are doing. My express purpose was to explain how we do it and then I'll allow others to judge for themselves.

 

[Uncle Pug]

Stephen Ash once bubbled...
Would someone explain this, please?

So this one doesn't get hijacked and so that the gradient factor thread will have a chance of being found by posters who are tired of this thread.