DIR and computers

[MHK]

DSJ once bubbled...


Actually what I was asking is what you would suggest folks use if they don't use the DIR 120 20% rule.

From what I have gathered so far, this is taught in the DIR-F class and the details are not taught in an open forum to make sure that whoever hears it really "gets it" and doesn't hurt themselves with an imperfect understanding.


When folks hear that DIR doesn't like computers they rarely hear about the 120 20% rule and assume the alternative is the tables. This causes a bit of unintended backlash as folks say, "What? Take away my multi-level diving capablility because some fool doesn't know what's behind the computer and rides the no-deco display? No Way!"

From what I hear you feel the best method is the 120 20% rule, but absent that, what do you say to everyone?

A) Stick with the dive tables
or
B) Feel free to use a computer, but learn the tables, read the manual, and think

David

David,

It's such a tough question to answer because it's sorta like asking the same type of hypothetical, that if you're diving solo how would you do it DIR.. In other words, it compromises the foundation of our core principles, and I'm not sure how to answer that. I'm not trying to be evasive, but I simply don't know how to answer it because in my view it's almost criminal that other agencies don't teach the 120' and/or the 20% rule, but they make you buy silly slates or expensive computers in the absence of teaching what really is basic information.

What has been most rewarding to me in the last couple of years as we've watched the DIR-F program take off has been leading the grass roots movement wherein students leave our class mystified wondering why this stuff wasn't ever taught to them previously. It's particularly rewarding when we have instrutors take our class as we then hope they take what we teach them and apply it to their classes. Our hope is that eventually divers will once again take back the power of information and apply pressure to their agencies to try harder, do more and stop watering down classes.. The 120 and 20% rule is but one glaring example of the indsutry selling devices rather then giving you knowledge..

Hope that helps..

 

[DSJ]

Ok, Mike I'll stop asking because I think my point is made. DIR isn't telling everyone to chuck their computers and dive the tables, rather DIR advocates a third approach that very few people are aware of.

So I'd love to move this to a discussion of your method vs. computers if it won't break any rules.

My first thought was that the 120 20% rule must be less accurate than a computer due to the very small sample set. If you take measurements only every five minutes, your estimate of the average depth could be a fair bit off the actual. Say I'm running along a wall at 80', then I ascend to 40' somewhere between the last measurement and my next one. Do I assume it was right at 2:30 past my last measurement, compensate if I think it was closer to 4:00? What happens if I was gazing at a frog fish and discovered it was six or seven minutes since my last computation?

On the other hand, a computer takes many more samples and does it regular as clock work.

David

 

[MHK]

DSJ once bubbled...
Ok, Mike I'll stop asking because I think my point is made. DIR isn't telling everyone to chuck their computers and dive the tables, rather DIR advocates a third approach that very few people are aware of.

So I'd love to move this to a discussion of your method vs. computers if it won't break any rules.

My first thought was that the 120 20% rule must be less accurate than a computer due to the very small sample set. If you take measurements only every five minutes, your estimate of the average depth could be a fair bit off the actual. Say I'm running along a wall at 80', then I ascend to 40' somewhere between the last measurement and my next one. Do I assume it was right at 2:30 past my last measurement, compensate if I think it was closer to 4:00? What happens if I was gazing at a frog fish and discovered it was six or seven minutes since my last computation?

On the other hand, a computer takes many more samples and does it regular as clock work.

David

David,

With all due respect I suspect that you are giving way, way too much credit for decompression being that exact of a science. Take for example, you are doing the second dive of the day and you've set out 43 minutes after a 90 dive.. Well you'd be the same pressure group if you only sat out 37 minutes and because of 1 minute you'd be in the same pressure group as if you sat out 51 minutes.. In other words the spread is basically between 37 minutes and 51 minutes, or a 14 minute differential.. So I'm not too worried that a computer, with God knows what algorithm or gradient factor may take a sample slightly more frequently then I do.. You need to understand that algorithms are mathematical models that are designed to mirror the shape of the deco curve that we are looking to utilize, the mechanism by which they "shape the curve" is plotted in depth and time, but the curve is the penultimate issue..

Does that make sense or do you wan me to explain it differently???

 

[DSJ]

A second question is what is the theory behind the 120' 20% rule? As a computer user I am told about the model used and feel confident that a number of knowledgeble people spent some time researching and thinking about it before they took the liability of selling it.

What is your method based on? Does it start with the original Haldane (sp?) research or come from some place else?

Folks shouldn't trust a new method without knowing what's behind it. Otherwise it's just as bad as watching the deco bars on your computer.

David

 

[ERP]

I play this little game with my buddy whenever I finish a dive, we estimate the average depth of the dive.

We then check it against our computers, I don't think I've ever been more than 5ft off, and I usually estimate deeper than the computer.

Give it a try sometime you'll probably be surprised how accurate you are.

 

[DeepTechScuba]

I just taught my sweetie how to use her diving compass for scuba. She struggles with it. I try to get her to navigate straight out and back. She prefers to follow the seals instead.

Next I will teach her how to use my Suunto. That will be a struggle as well. She prefers to concentrate on the colours of the bat stars and sun stars instead.

She needs to learn how to use the dive tables strictly because our instructor agency still requires it. I predict that someday dive computers will eliminate dive tables altogether, and then only tech trimix divers will ever see a dive table or deco table. The tables will be relegated to the same status of double hose open circuit scuba regulators.

The beauty of diving with her is that I have a diving buddy wherever I go, whenever she is in the mood to scuba dive. She loves her new white Faber 72. That has taken the place of my pony bottle. Now that I am with her, I no longer solo dive.

Now that I am with her, I am a better diver. She tests my limits in the water, and she has taught me patience. She has also taught me my own flaws as an instructor.

There is no way on this earth I could ever teach her artistic multi-lingual non-mathematical mind how to run an EAN32 sum-120 20% declining residual algorithm in her head. She likes to follow fish and search for seals instead.

Not everyone on this earth is mathematically minded. And even though they are not, they still deserve to experience God's undersea world. And that's why dive computers are taking over.

 

[MHK]

DSJ once bubbled...
A second question is what is the theory behind the 120' 20% rule? As a computer user I am told about the model used and feel confident that a number of knowledgeble people spent some time researching and thinking about it before they took the liability of selling it.

What is your method based on? Does it start with the original Haldane (sp?) research or come from some place else?

Folks shouldn't trust a new method without knowing what's behind it. Otherwise it's just as bad as watching the deco bars on your computer.

David

Haldane used a 2:1 ratio Buhlman went to 16:1.. The 120 rule is actually based on the Workman model developed in 1958 for the US Navy.. The Navy has been using it since 1958 and I give them credit for being "knowledgeble" people.. Besides I've been using it for years and I have no complaints about it..

 

[DSJ]

MHK once bubbled...


With all due respect I suspect that you are giving way, way too much credit for decompression being that exact of a science.

Probably true. If the error averages out I'm fine. But then if I round and err on the tight side too many times I can get into trouble.

You need to understand that algorithms are mathematical models that are designed to mirror the shape of the deco curve that we are looking to utilize, the mechanism by which they "shape the curve" is plotted in depth and time, but the curve is the penultimate issue..

Does that make sense or do you wan me to explain it differently???

A diagram might be nice

When you start talking about deco curves I think it would be helpful to at least wave your hands in air. I suspect that the 120' 20% rule is adequately explained in some text within a few pages and that is what I would have to read to get a good feel for it.

Likewise I'm pretty sure I'd get a better idea of how easy/difficult it is to put into practice if I actually tried it. But absent that I'm left with a vauge feeling that it's a bunch of work to remember to do a calculation every five minutes. I know I should be monitoring my environment including my gauges that frequently, but with a computer I don't have to make sure I hit the five minute mark every time.

David

 

[MHK]

DSJ once bubbled...


Probably true. If the error averages out I'm fine. But then if I round and err on the tight side too many times I can get into trouble.




A diagram might be nice

When you start talking about deco curves I think it would be helpful to at least wave your hands in air. I suspect that the 120' 20% rule is adequately explained in some text within a few pages and that is what I would have to read to get a good feel for it.

Likewise I'm pretty sure I'd get a better idea of how easy/difficult it is to put into practice if I actually tried it. But absent that I'm left with a vauge feeling that it's a bunch of work to remember to do a calculation every five minutes. I know I should be monitoring my environment including my gauges that frequently, but with a computer I don't have to make sure I hit the five minute mark every time.

David

Using deco plan it graphs your profile for you so you start to see the shape of your curve.

But given that this stuff isn't rocket science let me demonstrate how you calcualte "on the fly" the 120 and 20%, it's pretty basic stuff..

120, based upon Navy tables, means that depth and time always add to 120. In other words an 80' dive means 40 minutes NDL, a 90' means 30 minutes NDL a 100' dive means 20 minutes NDL and so on.. So all one needs to do is take their depth and add to 120.. If they can't do that then they shouldn't be diving. 20% means that if you standardize you mix [ie; 32%] rather then go through the silly formulas that you spent all that money in your nitrox class to learn, all you need to know is that your EAD advantage is 20%.. So if you do a 100' dive your EAD is 80'.. So an 80' dive using the 120 rule means you have 40 minutes NDL.. This stuff is simple and doesn't require a degree in math and can be done in seconds "on the fly".. Those that tell you it's too complicated more then likely don't know it themselves or are trying to sell you a computer or a class.. Now if you wanted to be conservative you could use your max depth for the 120', but if you're turning your brain on underwater you can take 5 minute snap shots so for example if I did 10 minutes at 100' and 10 minutes at 80, I could simply average it as a 90' dive, therefore 120 - 90 equals 30 minutes NDL.. If you want to be more conservative use 115, the number doesn't matter to me, the idea is that it's simple and isn't impossible as many in the dive industry will tell you and it saves you $400 for your computer.. BTW, you can use that same money to take a DIR-F class because this is but one nugget of information we teach in the class ;-).. The shape is what is important, but mathematical models can only plot in depth and time...

Hope that helps..

 

[String]

Tend to go along with that.

When i dive i have a watch (with timer), analogue SPG and depth gauge (which records max depth) and a computer.

I also carry a small slate of level 1 tables (work slightly differently to padis) in my pocket.

I dont do deep technical/mixed gas diving - if i did i think the most sensible method is to cut your own tables for those.

As others have stated tables assume the dive is square profile - great if its a certain type of wreck or known ground, not great if its a reef with gullies or a large wreck and so on.

Before the dive i get the max depth off the sounder, add on a bit for safety and look at the tables to see what my bottom time would be square profile. I can then compare to what the computer says - these normally agree fairly well.

I then do the dive using the computer. Nearly all my dives are multilevel and impossible to plan beforehand as i dont know the ground or where the current will take me.

If the computer dies during a dive, no problem, i have a watch which is telling me dive time, i have a depth gauge which has logged maximum depth and i have the tables. I'll take the last resort and go to the tables, treat it as a square profile dive to the max depth and do the required stops to get out of it.

That gives me much more flexibility and more bottom time on a lot of the dives than id otherwise get.

Taking a real life example from last weekend.

First dive, drift dive in moderate current. Maximum depth was 17.7m however the average depth was 12.8m according to the computer.

To remain within NDLs on the tables 51 minutes was my maximum time. Id be into deco for 61.

OK so that gave me 10 minutes longer. Not a lot...

Now comes the 2nd dive of the day where it really bites.

Another dive to a similar depth for 40 minutes after 2.5 hours surface interval.

That would give me 11 minutes bottom time no deco and the 40 minutes on the tables would have given me 21 minutes worth of stops !

Thats a big improvment whilst all the time remaining within NDLs.

Deco diving is the same thing.

If the computer had died on the dive i could have done the required deco (more than enough air) and still surfaced safely.

That really highlights the limitation of tables for non square profile dives, although the max depths were both around 18m the average depths for the dives were in the region of 12-13m.

Just my view on a computer for rec diving extending bottom times. Provided you know where you are regarding backup i cant see a problem with them.
True its not deep technical mixed gas diving that requires a lot more planning and discipline but for a lot of divers this is more reflective of normal sport diving.