The Chairman
Chairman of the Board
Bingo, Yahtzee, we have a winner. Add the inherent errors that comes from relying on your compromised brain and you have only guesswork to go on.
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I was trying to think about how to respond to Dan's reply this morning and decided to give it up.
In order to really compare RD to other ascent strategies then we don't need anecdotes and we don't need "faith". What we need is controlled scientific experiment. We've never had one and the population of RD users is so small that I expect we never will.
So where does that leave us?
Without facts to discuss then all we can do is exchange ideas about what we're comfortable doing. Dan is clear in his opinion. He has said a number of things that I believe can be summarized by saying "he has faith"..... he's a believer. In some sense this is no different than what a computer user does. I have "faith" in my computer. I cannot and I will not deny this.
There are so few of us who have actual proof that their ascent strategies are scientifically sound that we all -- every one of us -- eventually settle on a style in which they have increasing "faith" over time because of a mounting body of success. This is what Dan is mentioning in his context and what I would argue in mine as well.
RD is perhaps demonstrably more prone to errors (brain farts) and we've all hear the stories about divers who have been bent because their mental math was off. The computer can be off too as someone above pointed out when they paraphrased a quote that we used in the early 1980's, which is that computers are very good at rapidly making mistakes.
Of course, in the 1980's we didn't have the kinds of software certification processes for safety critical applications that we have now... but the basic idea that a computer is not infallible is something that I don't think anyone can deny.
That said, we now have "safety integrity levels". I don't know to what degree computers are tested but it wouldn't surprise me, given the risks that they are certified to SIL-3 or SIL-4. I work in the (light) rail industry and all of the safety critical software is certified to SIL-4.
Of course that doesn't say anything to most people but what it means in concrete terms is that a computer certified to that level (SIL-4 for example) has a potential failure rate of 1x10^-4 to 1x10^-5 (about 1 failure in 100,000 for incidental use). For continuous operation (like in my work) this rating means a failure rate of 1 in a billion.
So while the logic doesn't hold 100% what I want to suggest is that a typical certified computer will make an incorrect average depth calculation 1 time in 100,000 dives on average.
How many times have you been at the supermarket keeping track of what your groceries should cost in your head and then been surprised at the check out how big your mental error was? Was it 1 time in 100,000? For me, it's every time! I thought I was good at math. I studied a LOT of math and I can make mental calculations faster than most of my friends, which is a great party trick. But I never get it exactly right at the supermarket.
This ^^ is is one of the big reasons why I won't use RD. The other reason is because there is a total lack of solid scientific testing. That's two big strikes against RD and two big check marks on the side of computers. I personally don't need more than that.
R..
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I doubt there's much if any SIL cert here. It likely falls under consumer electronics and a very loose regulation.
At best, one should read the euro norm to which they certify. Does en250 cover dive computers?
At best, one should read the euro norm to which they certify. Does en250 cover dive computers?
There is a difference between using computers and using the brain that was not mentioned in the last few posts.
In most cases, if you make a mental error, you won't know it. I have frequently cited the case of two friends who got bent using RD. One of them had a computer in gauge mode being used as a bottom timer, so he was able to go into the log and find out that they had miscalculated their average bottom depth, they had taken much too long to reach their first deep stop, and they had miscounted the number of minutes they needed on their final stop and ascended early. (One of these divers had a Ph.D in computer science--no dummy.) The only reason they knew about these errors was because they got bent and checked; they may have made similar errors on every dive they had ever done and not known it because they had had no reason to check.
If you make those kinds of mistakes while using a computer, the computer knows it and tells you how deep you really were, it adjusts decompression needs to your ascent speed, and it will not show that you have cleared deco until you have actually cleared deco. If your computer makes some sort of an error in those calculations, you will see it immediately because of the discrepancy with your backup or with your buddy's. When I am doing deco dives with a buddy these days, we have 4 computers with the same settings.
That does not count the most common error in non-computer diving. With a long deco dive, you are spending a very long time doing deco stops. You are supposed to stop at a specific depth--but do you? Not many people will stop precisely at each depth for the entire time of that stop--most will be a foot or two below that for much of the time. The non-computer diver will compute as if 100% of that time was at the proper depth, but a computer will compute the deco for the actual depth.
In most cases, if you make a mental error, you won't know it. I have frequently cited the case of two friends who got bent using RD. One of them had a computer in gauge mode being used as a bottom timer, so he was able to go into the log and find out that they had miscalculated their average bottom depth, they had taken much too long to reach their first deep stop, and they had miscounted the number of minutes they needed on their final stop and ascended early. (One of these divers had a Ph.D in computer science--no dummy.) The only reason they knew about these errors was because they got bent and checked; they may have made similar errors on every dive they had ever done and not known it because they had had no reason to check.
If you make those kinds of mistakes while using a computer, the computer knows it and tells you how deep you really were, it adjusts decompression needs to your ascent speed, and it will not show that you have cleared deco until you have actually cleared deco. If your computer makes some sort of an error in those calculations, you will see it immediately because of the discrepancy with your backup or with your buddy's. When I am doing deco dives with a buddy these days, we have 4 computers with the same settings.
That does not count the most common error in non-computer diving. With a long deco dive, you are spending a very long time doing deco stops. You are supposed to stop at a specific depth--but do you? Not many people will stop precisely at each depth for the entire time of that stop--most will be a foot or two below that for much of the time. The non-computer diver will compute as if 100% of that time was at the proper depth, but a computer will compute the deco for the actual depth.
Interesting point. That argues in favor of using at least two different brands of dive computer. It is less likely that the same obscure software bug will be present in computers from different manufacturers, even if they use the same algorithm. That would exclude essentially the same computer that is OEM'd to different manufacturers. Four computers in your dive team protects against failure (dead battery, floods, electronic component failures), but not against software bugs.
Some/many electronic components tend to fail in "batches": side-effect of the manufacturing processes. AFAIK it's common in hard drives, electrolytic capacitors, there's probably others. At a guess, sensors would be the dive computer's hardware components to watch. (Edit: point being, it's not just software. Buy two same-brand computers at the same time, wear them together on every dive, the chances of hardware component X failing at the same time in both are much higher than you'd think.)
I was taught this as the SOP for really critical systems: odd number (otherwise you'll have a tie) of different ones. One could argue that having a plan written on a slate (+ depth gauge and watch), a set of rules for working out a reasonable ascent strategy on the fly, and a dive computer, would fit the bill.
(comments relating to quote, not debating)
It is extremely rare that an electronic failure results in inaccurate data, such as depth and time (the primary inputs to decompression calculations). That is different than "out of tolerance" display and data collection when it leaves the factory -- but that type or manufacturing omission should be checked when divers get a new computer in the water.
Electronic and sensor failures are almost always unmistakable. Obscure software bugs are more insidious if you are unlucky enough that it doesn't crash the computer. They are fantastically rare unless you are diving unusual profiles that are on the frontiers of established limits in technical diving.
I have read that the vast majority of failures across the spectrum of electronic products occurs in the first three months of use. However, that could relate to the first several years of use in the case of dive computers that spends most of its time turned off in a gear bag. Trust but verify.
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