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

[The Chairman]

To me, Ratio Deco helps drive and develop the thinking diver.
That is a motivation for it's contemporary use.

This is nothing but hubris. You somehow have deluded yourself into believing that only "thinking" divers will use RD. It appears that it's only closed minded divers bent (lit&fig) on proving that they must be right who latch onto RD. You're POV warriors on this and no amount of facts or logic will deter you from following this bend-a-matic approach. Using RD is not a rational choice. Too many injuries to suggest it to be a rational option. You have to blindly follow the hubris and ignore the science in order to accept Ratio Deco.

 

[TrimixToo]

I thought the same thing was possible for a VR3. I thought that is was possible for a VR3 to give a message "use tables". I think that computer was designed for techdiving.

How do you know something like that can't happen by Shearwaters,Ratio, Divesoft, OSTC, etc. ? ;-)

I'm not sure why you are trying to lay the burden of proof at tbone1004's door, since all the manuals are most likely available online. But as it happens, a VR3 doesn't abandon you. Here's what the manual says:

"Should a microbubble stop be missed, a “Use tables” message will appear. The
decompression schedule displayed thereafter, although very close to the requiredNo
duration, may be in error. If this occurs consult backup tables and add additional stops
or stop time as required. While continued diving with the VR is possible, the
decompression displayed may not be exact. “Use tables” will show for 24 hours."

A "microbubble stop" is a deep stop imposed atop Buhlmann ZHL16. So when this message is displayed, you are essentially falling back to Buhlmann. Things could be worse (grin).

A Shearwater Petrel or Perdix will not lock out, either.

I happen to own all three, but if you're curious about others I'm sure you can find out about them.

Being bits of electronic kit, computers can of course fail. When that happens all bets are off. This is why a lot of thinking divers have two of them. (In fact, I have not lately dived with anyone who happens to use only one.) If you have one and it malfunctions invisibly it's certainly a good thing if you have some clue about what it ought to be saying. If you have two and one fails, it would be a good thing if you knew which it was. For these cases, some alternative strategy that allows you to figure out somewhat reasonable stops knowing only depth and time (assuming you do) is worthwhile. That can be +/- printed tables, RD, or a buddy who's been next to you the whole time showing you a working one, or handing you a backup.

 

[barth]

- If divers make 3 mistakes in their planning when using their computer in gauge mode and don’t notice it. Then it is the fault of the methode they are using.

- if a diver makes 3 mistakes and his computer goes in error than it is the diver his fault.

Off coarse it is the diver his fault. I agree with that. But come on guys, if you use a method for planning and you and your buddy make 3 errors or mistakes it is also the fault of the divers! I may hope they did it right when they did training or the course. Otherwise they shouldn’t pass the training/course.

I think you should use a method/computer which works for you. It is your safety and your responsibility.i think both methods works fine if you know how to use it and don’t Do stupid things.

 

[PfcAJ]

This is nothing but hubris. You somehow have deluded yourself into believing that only "thinking" divers will use RD. It appears that it's only closed minded divers bent (lit&fig) on proving that they must be right who latch onto RD. You're POV warriors on this and no amount of facts or logic will deter you from following this bend-a-matic approach. Using RD is not a rational choice. Too many injuries to suggest it to be a rational option. You have to blindly follow the hubris and ignore the science in order to accept Ratio Deco.

All this coming from someone who’s proud to state that they don’t know anything about ratio deco.

Right.

 

[CAPTAIN SINBAD]

There is a serious gap between the available science and any claim that RD is "dangerous", or similar - hence I put that any such claim would be unscientific.

I believe that what Dan_p wrote above has also been expressed by Dr. Simon Mitchell to me in the following words. I am posting my question and his answer to see if they are really different.

Captain Sinbad: So far my understanding is that the initial stops in Ratio Deco 2.0 are a bit too deep to allow shallowest stops to be more effective. Does this make R.D 2.0 dangerous in your view? Or is it less than optimal? In other words, how much risk would someone be taking if they insisted on doing those deeper stops which are still deep.

Dr. Simon Mitchell: That is a difficult question to answer. Just eyeballing some of the profiles, the deep stops still seem very deep to me, and probably too deep for optimally efficient decompression. In other words, if you did the same decompression time, but distributed your stop time shallower, then you would almost certainly have less risk, The actual difference in risk might be relatively small, and perhaps not worth arguing about. Nevertheless, if people seek the truth on the path to least risk for the same decompression time I very much doubt that RD2 is the answer!

Dr. Simon Mitchell, am I still misquoting you sir? Can we not take what Dan_p wrote up there and what you wrote to me to mean that you and him are on the same page? I see that it is a dis-functional marriage but it is not a divorce yet is it? I also underlined the word "probably" in your quote up there in sincere hopes that you will expand on its meaning in that particular sentence just like you expanded on the meaning of "might" and "perhaps" in a different one.

The bigger apology that I have yet to offer is the one I present below.

If I approached Galileo in person for a private conversation, then he may honestly believe that I am not part of the mob. He may sit down with me in a private setting and open up on helio-centerism and astronomy and he will not be worried about political correctness since it is a private one-on-one conversation. Does that give me the right to make notes from that personal communication, walk out in public and yell them out in front of the whole mob later? What kind of a person would I be if I did that? Galileo would go on an all-out, damage control mode after that. He will start to point words like "might" and "perhaps" in his notes where they support the mob and he will skip entire sentences where he himself has used the same words to oppose the mob.

What does that make me?

To Dr. Simon Mitchell, one of the greatest minds of our time ....

I am sorry.

 

[The Chairman]

Dr. Simon Mitchell, am I still misquoting you sir?

You're not asking the right questions. He's an ethical researcher. He doesn't deal in absolutes in regard to "is this safe for you?". I'm not a researcher and have no issues being blunt about how problematic I find Ratio Deco to be. Here, let me ask the right questions for you... @Dr Simon Mitchell:

• Would you use Ratio Deco?
• Would you let your wife/daughter/son use Ratio Deco?
• Would you teach other divers to use Ratio Deco?
• Do you encourage others to use Ratio Deco?

Feel free to explain your answers, but please don't feel obliged.

 

[Dr Simon Mitchell]

Hello Captain Sinbad and The Chairman.

The chairman is right, you can spin it in all directions depending on how you ask the questions.... any lawyer accustomed to courtroom practice knows this.

I believe it would be challenging to justify characterizing ratio deco as "dangerous" because we have not accurately measured the absolute risk associated with its use and compared that risk with accurately measured alternatives. To deem one approach to deco dangerous in comparison to another you would have to do that and show a difference considered significant. I suppose you can interpret that as agreeing with Dan P's very technical point.

However, at the same time, the answer to essentially all The Chairman's questions is "no": I would not choose to use it myself, recommend it to my family, teach it to other divers, or encourage others to use it. This is because there is a reasonable body of evidence that there are other approaches that almost certainly carry less risk.

Captain Sinbad, with the greatest of appreciation for the compliment, I must point out that the most wildly inaccurate thing on this thread so far is this:

To Dr. Simon Mitchell, one of the greatest minds of our time ....

Simon M

 

[The Chairman]

However, at the same time, the answer to essentially all The Chairman's questions is "no": I would not choose to use it myself, recommend it to my family, teach it to other divers, or encourage others to use it. This is because there is a reasonable body of evidence that there are other approaches that almost certainly carry less risk.

Bingo. Yahtzee. We have a winner. This is as definitive an answer as I would ever want or need. Thank you for your patience and your candor.

Captain Sinbad, with the greatest of appreciation for the compliment, I must point out that the most wildly inaccurate thing on this thread so far is this:

To Dr. Simon Mitchell, one of the greatest minds of our time ....

You are a great mind as well as a humble one. Again, thanks.

 

[Dan_P]

Given a courtroom comparison, I would find the lack of evidence particularly troubling when on part of the litigators.

you mean following the teachings of the great AG who can do no wrong with his decompression approach.

Your play to a logic that if someone (here, myself) is doing A, then it must be because Y, is infinitely less correct than the answer I have given as to my motivation, on many an occassion.

Do you have a MD or PhD in a relevant field and are actively studying decompression stress?

Do you?

The guys that do say that the theory that you base your decompression on is not the best form of decompression, yet for some reason you think AG and by association, you are more qualified to determine decompression profiles than guys with terminal degrees in relevant fields who are actively studying the science and art of decompression? Please explain that one

You're still bending what I'm saying to fit a picture that deals only in absolutes, and only on one singular plane.

Here is an exchange with a similar dynamic where the reply in the second quote is of a similar nature:

I believe it would be challenging to justify characterizing ratio deco as "dangerous" because we have not accurately measured the absolute risk associated with its use and compared that risk with accurately measured alternatives. To deem one approach to deco dangerous in comparison to another you would have to do that and show a difference considered significant. I suppose you can interpret that as agreeing with Dan P's very technical point.

However, at the same time, the answer to essentially all The Chairman's questions is "no": I would not choose to use it myself, recommend it to my family, teach it to other divers, or encourage others to use it. This is because there is a reasonable body of evidence that there are other approaches that almost certainly carry less risk.

Bingo. Yahtzee. We have a winner. This is as definitive an answer as I would ever want or need. Thank you for your patience and your candor.

I believe the first quote was intentionally employing an indefinitive rhethoric.

Besides, if you believe that shallow is better, you can use RD and adapt shallower.
"How much shallower?", you may ask, to which I'd say:

You don't know.

Key points

1) The discussion on deep stop emphasis is not identical to the one on why one might use RD.
Firstly because RD evolves gradually, just like the scientific basis we have available does, and secondly because you can use RD and adapt it as you develop.
It's the intended purpose.

2) There is nothing in science to support the litigating parties in this discussion in their apparent view that RD is dangerous.

 

[rjack321]

You're not asking the right questions. He's an ethical researcher. He doesn't deal in absolutes in regard to "is this safe for you?". I'm not a researcher and have no issues being blunt about how problematic I find Ratio Deco to be. Here, let me ask the right questions for you... @Dr Simon Mitchell:

• Would you use Ratio Deco?
• Would you let your wife/daughter/son use Ratio Deco?
• Would you teach other divers to use Ratio Deco?
• Do you encourage others to use Ratio Deco?

Feel free to explain your answers, but please don't feel obliged.

so would that be GUE's version, UTD's version, Steve Lewis' version or something else?

Doing linear extrapolation of a curve is really not hard at all - despite what Boulderjohn says about it. And it can be as conservative or as liberal as you chose in part depending on your starting algorithm. You also need to know where it breaks down, 15-30min bottom times in the 30m to 70m range (ie most wreck dives) it works great.