Why do some agencies recommend using a bottom timer instead of a computer

[barth]

They made errors that resulted in DCS. Without a computer in gauge enabling them to check the log, they would have sworn they had made no errors. These two had had a couple years of training prior to this. One of them was a few months shy of his Ph.D in computer science. They evidently did not have your Godlike diving skills, but how much training and mathematical education should they have had before attempting a decompression dive without a computer?

I guess they did it right during the training. But a couple of years later they did both make a lot of mistakes and both didn’t notice that.

It would be interesting to know that rootcause. Maybe they didn’t dive for a long time.

Maybe other skills are also not good anymore.

If you are doing wrong a gasswitch and you make mistakes and you and your buddy didn’t notice the mistakes, the solution would be training. It doesn’t matter that you do it right 2 years ago during a course.

There are not much divesites here where I can do tech dives. At the end of the month I will do techdiving in Croatia. Tomorrow I will practice skills. I will also check my equipment and tech 1 notes before I arrive in Croatia. Before the project in Croatia will start I will do some diving in Croatia. That would be better than start diving with a lot of taskloading.

 

[Diver0001]

Incidentally... as a bit of a side bar... I think SOME people probably can use ratio deco very accurately (apart from the inadequacies of the algorithm). I've done 3 years of university math and what I know from that is that I'm definitely not one of them.

One of by best friends, however, is a math savant. He can literally do mental calculations faster than I can even type them into a calculator. You can give him a list of 50 non-trivial numbers spoken as fast as you can speak and he has it added up as fast as you can say it. If you then ask him to multiply, divide, average or even median that number then he has the answer faster than you can type it into a calculator. Outwardly he's perfectly normal but there is something in that brain that is definitely NOT normal.... He says that "correct" answers appear in his minds-eye in the colour 'white'.... I'm not even sure he actually does any calculations. The way he describes it, it sounds to me like he just "looks up" the right (white) answer in some kind of visualization. It's thoroughly astounding to see.

... and yes, he studied computer science and math at university, so he did actually discover that talent in time to do something productive with it.

That said, I'm sure his accuracy isn't 100% either, as a computer would be, but if I were to use ratio deco on a dive these days then he is the ONE person -- and maybe the only person I know -- that I would want to have along on that dive. All of my other friends use computers because... frankly... we need them for the dives we do.

R..

 

[barth]

"Barth", the point John is making here is that some ascent strategies are more prone to errors than others. Ratio deco has a large dependency on diver alertness, which is defacto more prone to errors than the calculations made by a computer. There have been many bends using RD due to "brain farts". One Scubaboard user, in fact, could have killed himself using ratio deco because of such a brain fart. His story is illustrative of the risks of "user error", as are the examples John mentioned in his blog.

Personally I can't even count the number of times I've been at the Albert Heijn and made a mental calculation of how much my groceries would cost only to be surprised at the cash register by how big the error in my mental math was. Granted, ratio deco doesn't require adding up long lists of numbers in your head but the principle is the same. A computer can quite simply do these things BETTER than we can, and not by a small margin. There is no getting around that.

Again I repeat what I said before, which is that ratio deco had a function in the time between -- let's say for the sake of argument 1990 and 2005 -- because in that period of time technical diving was becoming more popular but there were literally no adequate technical computers on the market. Since 2005, however, that has changed. The truly "faithful" RD users, have, in that time, not kept up with developments and are now arguing to apply a paradigm that is +/- 20 years out of date to a context in which it is no longer applicable.

R..

Not everybody who is using gauge mode is using ratiodeco.

What is better, computer/gauge mode? Can you guarantee that I will not get bent when I’m using a computer instead of gauge mode? If you can guarantee I will stop using gauge mode next dive.

Sometimes it is more complicated than just follow a computer. When my heating is not working at the end of the dive I would be more conservatism than the original diving plan. I do understand that I can also do that with a computer. But because I plan my own dive it would be much easier to modify the plan I think.

But at this moment it is really simple to do the diveplanning by myself. Maybe some day I will use a computer. That would be when I’m not sure anymore if I can do the dive planning by myself without mistakes.

I have seen people stupid things. Some people do not plan a 30 meter dive, they told me that they never make a diveplanning for rec dives when diving with a bottomtimer.

But if somebody is using a rebreather and is doing stupid things and he will die. The rootcause are not rebreathers. It would be his attitude.

But maybe I should use my computer next dive in computer mode and just give it a try. I will try it tomorrow.

 

[PfcAJ]

Things I don't understand about ratio deco:

• Why bother?
  • There are better more effective tools
  • I don't dive to babysit my deco schedule
  • Too, too many hits
  • I've got better things to do with my life, like watch grass grow
• Why expose yourself to needless risk?
  • Too many cases of DCS among RD divers.
  • There is no study that condones RD
  • RD is based on anecdotal inferences and not science
• Why you feel the need to delude yourself and others?
  • Ratio Deco is a protocol that's based on inference not science
  • Ratio Deco presupposes that you can't make a mistake and get bent
    • ...and that computers are unreliable
  • Blindly following this protocol does not make you a better diver
  • Using more effective tools is not a bad thing
  • Hubris is not a good thing.

Here's what I do understand about Ratio Deco:

• It's defective
• It's dangerous
• It was born in arrogance
• There's absolutely no science behind it
• People get hurt using it
• It merits no further study when there are far, far superior protocols that work well.
  • No, I'm not going to drink your Koolaid with you!

Confirmed. You don’t know anything about ratio deco.

 

[Diver0001]

I guess they did it right during the training. But a couple of years later they did both make a lot of mistakes and both didn’t notice that.

It would be interesting to know that rootcause. Maybe they didn’t dive for a long time.

Maybe other skills are also not good anymore.

If you are doing wrong a gasswitch and you make mistakes and you and your buddy didn’t notice the mistakes, the solution would be training. It doesn’t matter that you do it right 2 years ago during a course.

There are not much divesites here where I can do tech dives. At the end of the month I will do techdiving in Croatia. Tomorrow I will practice skills. I will also check my equipment and tech 1 notes before I arrive in Croatia. Before the project in Croatia will start I will do some diving in Croatia. That would be better than start diving with a lot of taskloading.

Big side-bar here //

We do almost weekly dives to +/-50m in de Vinkeveenseplassen. We do this mostly to just keep skills sharp. We usually do them on air but if you want to use Trimix then that's fine too. We also make a lot of dives in the Oostvoornsemeer. Oostvoorne was the opening of the Rotterdam harbor starting in the middle ages and it is pretty jaw dropping how many interesting wrecks are in that lake...authentic wrecks from the 18c. You can swim to them from shore but it usually makes for a non-trivial dive.

It's true that if you want to dive deeper than 50m in the Netherlands that you need a shovel, however, technical diving is not only about "deep", it's also about "long". If you dive technical nitrox then you might be surprised how many options you have

// end of big side-bar

 

[Blasto]

He mentioned that as exposure times progress, the ratios fall apart.
Is this a valid concern? So far the depths and exposure times at which ratios deviates from algorithms to the point where they would go from slightly less than optimum to "HOLY $H!T" are so long that you will run out of air before that becomes a concern.

And the reason for that is that RD is not a deco algorithm.

All it does is parrot the same plan for every dive at a particular level, just stretching it with BT. That's also the reason it can be calculated so much quicker than the math behind it, because it's the same plan.

While it's not how it originated, using RD today instead of computers and counting on tank capacity to keep it within its limits is essentially a tec equivalent of the "swim around till you hit 50 bar" rec philosophy. RD adds a table of levels and a bit of math as a matter of necessity, which provide the minimum amount of safety that makes typical deco dives survivable, but it's still overly dependent on external limits.

 

[Diver0001]

Hoi "Barth". Jammer genoeg moet ik antwoord blijven geven in het Engels omdat de site nou eenmaal internationaal is. Ik zie in je berichten dat het jou niet zomaar soepel afgaat. Als je liever tegen mij in het Nederlands wilt klankborden aarzel dan niet om een PM te sturen.

Not everybody who is using gauge mode is using ratiodeco.

This is true. Divers who use tables also (could) use gauge mode. In addition, divers who "cut" tables by copying schedules from a PC tool to -- say -- a slate, may also use gauge mode.

That said, divers have been using something akin to ratio deco since the late 1970's. I was certified in 1984 and we learned the "120 rule". JJ was also certified in that same period of time and would have learned it as well..... PC software made it possible to play "what if" and some intelligent divers discovered that certain ratios of bottom time to decompression time were pretty constant, particularly if one assumes a standard set of gasses.

This is one of the underpinnings of the DIR system. The other is that "standard gasses" can be mixed from buffers using a common EAN mix together with helium. The standard trimix mixes are not always "optimal" but they are indeed "practical". These "practical" mixes also correspond to the RD standard mixes.

Ratio deco, therefore is not just a calculation. It is a calculation in a very specific context that includes a limited number standard gasses. A computer, of course, doesn't care about that. You enter the mix and the computer accounts for that. IN RD, for example, if you get a "hot" or "lean" mix then you may (will) need to skip the dive. Using a computer this doesn't happen to nearly the same degree.

What is better, computer/gauge mode? Can you guarantee that I will not get bent when I’m using a computer instead of gauge mode? If you can guarantee I will stop using gauge mode next dive.

Can you guarantee that gauge mode will protect you from DCS? This argument is ... well ... absurd.

I have seen people stupid things. Some people do not plan a 30 meter dive, they told me that they never make a diveplanning for rec dives when diving with a bottomtimer.

Is this an issue of tools? If someone is going to be a screaming idiot then the tools they use are not the problem.

R..

 

[Diver0001]

And the reason for that is that RD is not a deco algorithm.

In one sense this is true and in another sense it is not. Blasto's post should be seen as "misleading".

Strictly speaking, RD is an ascent strategy, not a decompression algorithm. In this definition, Blasto is correct.

What he fails to tell the causal reader is that an ascent strategy can be applied in the context of many decompression algorithms provided that the ascent does not violate the limits of the algorithm.

In another sense, ratio deco was designed to fit the parameters of a PARTICULAR decompression algorithm. In theory it can be broadly applied. In practice it is applied in a very narrow bandwidth.

RD is essentially an extension of the "swim around till you hit 50 bar" rec philosophy into tech, adding a table of levels and a bit of math as a matter of necessity, to provide the minimum amount of safety that makes typical deco dives survivable.

This is completely false. As anyone can read from this thread I am not a fan of ratio deco (not even a little bit) but ratio deco, despite all of its shortcomings and inadequacies, is not "reactive" in nature, as Blasto suggests.

R..

 

[Jack Hammer]

I wish the folks arguing would be more clear of their position they are so strongly arguing for/against.

There is UTD/AG style RD which can be used to solely plan and execute an entire dive and/or make changes on the fly. This has more rigid parameters and is designed around using standard gasses and it gives a specific deco schedule.

Then there is recognizing a ratio between time and depth that happens with most deco algorithms (on any GF setting) that some people use to predict what their computer will tell them so they can weigh the options of making any changes on the fly beforehand. This can work with a wide variety of gasses, algorithms, settings, etc and gives a rough idea to the change in the deco schedule.

 

[Blasto]

...

All right, I didn't put the latter part well, sorry for that. It certainly isn't reactive. My poorly-made point was that the common defense that RD's narrow limits in which it's applicable aren't an issue due to the limits of typical gas supply and RMV is somewhat similar to the idea occasionally heard in rec that AL80 capacity will prevent deco obligations anyway.