"Accidental" deco with 1-day group, what to learn?

[rjack321]

I don't think there are any computers on the market right now that have a reputation for bending divers. Outlier or not, the Suunto computers have a good reputation and a lot of people will be much more concerned with how much it costs than what it does.

I don't know of any either.

Except when they're going into deco when the rest of the boat keeps on diving.

 

[doctormike]

Just drop in the actual EANx used into some planning software and start doing repetitive dives with realistic surface intervals. Do you get into hypothetical deco using Buhlmann or VPM? Yes, no, and how much deco gets accumulated? @fmerkel is only trying to "undo" the strong biases (compared to other algorithms) in the Suunto algorithm, its not hard to do that by gaming the EANx percentage.

As he pointed out, you can have 10mins of NDL left on the majority of computers in current use paired with 10mins of deco on a Suunto - which is a ridiculous spread. Bumping up the EANx percentage is a plausible tactic to bring the Suunto in line with what other computers are saying.

I'm sorry, we will just have to agree to disagree. You seem to be assuming that decompression stress is a simple metric that accumulates in a linear fashion, and therefore you can approximate risk by just comparing an NDL. I'm not a hyperbaric doc, most of these academic discussions are beyond me, but I do know that when you have a complex model and you simply enter one erroneous input, you can't rely on the fact that the output will be universally and reproducibly adjusted proportionally to what you entered.

Your thought experiment assumes that your risk of DCS is in linear proportion to the amount of deco you would be blowing off (compared to entering the actual gas that you are breathing). I don't know everything, but I do know that it's just not that simple.

The way to accommodate your own personal acceptable risk level (and lengthen your NDL) is to manipulate parameters that ARE accounted for in the model - like gradient factors, conservatism factors, etc... Putting in fake mix values isn't the same thing. It's no more appropriate than tying your dive computer to an SMB at some point and sending it to the surface to compensate for it being too conservative. That's another way to keep it from going into deco, right? How is that different than lying to it about your back gas?

 

[fmerkel]

The tweak to 26% was a direct result of mx day/mx dive with a boat full of pretty experienced club members up at Gods Pocket. Up until then the Suunto computer had pretty much been subject to single dives, some pushing the edge of deco on my UWATEC Aladin and the Suunto generally showing a tad more conservative. Single dives, probably important.

On the trip, on a particularly good dive, JUST when things were shaping up nicely my wife indicates we should ascend and she already had a deco obligation. We were at 75' with a half tank of air, not likely to go deeper. By the time we reached 15 feet (lousy viz and boring spot) she had 12" of deco, I had 12" of no deco. So, which one is right? There are numerous studies of various computers doing exactly the same dive (pressure chamber) and getting significantly different no deco times. Once again, who's right?

It wasn't an extensive 'experiment' and the ....trial.....continues, but we are back down to single dives for awhile so I'm not sure if that's going to make a whole lot of difference. As the Suunto is set up right now (2 of the available Suunto tweaks to liberalize it, AND the 26% setting) we get VERY similar results comparing graphs and NDL times through the dives we've done. We've been diving 20 years 2500+ dives between us, and I don't think complete idiots. But, neither do I spend my nights pouring over the academic level articles that are being brought to bear as evidence for points being made. Honest, I tried one, 44 pages and enough math to make Stephen Hawking happy is not my idea of a good time.
You folks that do that are amazing, and very very much in a minority. I'm glad you are around, and I'm really glad you try to make your points in a way that generally is understandable. I've learned some in this thread. But, most divers are simply not going to dot it, and mostly don't care.

The Suunto WAS researched, actually quite a bit before purchased. Unfortunately one of the main criteria was reliability. Over 20 years of diving a LOT of computers have failed (none our fault), They have been various models from various manufacturers and the only similarity to the lot of them was high cost and lousy customer service. Suunto....seemed.....to be more reliable. We hoped that it would not be so conservative as to be a problem......wrong.

We could dump it and eat the loss. Sure, it's an option. For now we are working with it to see what we can do with it. We don't want to be unsafe, hell no. OTOH, having your $$$ dive vacation dives cut short when other people doing the same dive, the same number, on the same mixture with different computers and are not having this problem gives one pause. And so far, no one has ever been bent on any trip I've ever been on.

 

[RandomGuy1]

The tweak to 26% was a direct result of mx day/mx dive with a boat full of pretty experienced club members up at Gods Pocket. Up until then the Suunto computer had pretty much been subject to single dives, some pushing the edge of deco on my UWATEC Aladin and the Suunto generally showing a tad more conservative. Single dives, probably important.

On the trip, on a particularly good dive, JUST when things were shaping up nicely my wife indicates we should ascend and she already had a deco obligation. We were at 75' with a half tank of air, not likely to go deeper. By the time we reached 15 feet (lousy viz and boring spot) she had 12" of deco, I had 12" of no deco. So, which one is right? There are numerous studies of various computers doing exactly the same dive (pressure chamber) and getting significantly different no deco times. Once again, who's right?

It wasn't an extensive 'experiment' and the ....trial.....continues, but we are back down to single dives for awhile so I'm not sure if that's going to make a whole lot of difference. As the Suunto is set up right now (2 of the available Suunto tweaks to liberalize it, AND the 26% setting) we get VERY similar results comparing graphs and NDL times through the dives we've done. We've been diving 20 years 2500+ dives between us, and I don't think complete idiots. But, neither do I spend my nights pouring over the academic level articles that are being brought to bear as evidence for points being made. Honest, I tried one, 44 pages and enough math to make Stephen Hawking happy is not my idea of a good time.
You folks that do that are amazing, and very very much in a minority. I'm glad you are around, and I'm really glad you try to make your points in a way that generally is understandable. I've learned some in this thread. But, most divers are simply not going to dot it, and mostly don't care.

The Suunto WAS researched, actually quite a bit before purchased. Unfortunately one of the main criteria was reliability. Over 20 years of diving a LOT of computers have failed (none our fault), They have been various models from various manufacturers and the only similarity to the lot of them was high cost and lousy customer service. Suunto....seemed.....to be more reliable. We hoped that it would not be so conservative as to be a problem......wrong.

We could dump it and eat the loss. Sure, it's an option. For now we are working with it to see what we can do with it. We don't want to be unsafe, hell no. OTOH, having your $$$ dive vacation dives cut short when other people doing the same dive, the same number, on the same mixture with different computers and are not having this problem gives one pause. And so far, no one has ever been bent on any trip I've ever been on.

Sounds like you should just dive without a computer or depth gauge or timer since you are going to ignore your computer, or purposely lie to it, and assume that other people computers would have the same reading as yours. For me, I know for a fact that on my dives I may be 10-15 ft deeper or shallower that the other people who are diving "the same dive" and will continue to listen to my computer.

 

[doctormike]

I once saw someone on a recreational dive boat who had put their computer into deco, and had ascended back onto the boat while the computer was alarming (I guess they didn't do the deco).

Their solution? Tie the computer to a reel and let it do the deco on its own, since they "felt fine" and didn't want to risk missing dives on their vacation because of a locked up dive computer.

 

[fmerkel]

You guys are hard......

Take a look at the link that was posted earlier ScubaLab-Computer-Test-September-2014-data.pdf

On the final 'dive #4, at 30'/29", the Suunto had 47 minutes, the Cressi had 1 minute, the Aladin had 96 minutes, and the Aeris had 103 minutes.
Assuming similar algorithms between my wifes Vyper and my Aladin Tec 2G we would be 49" apart.
Between the Cressi and the Aeris is 102".

So if we just dumped the Suunto and got a Aeris, then followed that algorithm faithfully you wouldn't have any problem with that, right? None at all?
But we are way out of line because we are trying to figure out how to deal with a pretty widely known Suunto problem?

 

[doctormike]

So if we just dumped the Suunto and got a Aeris, then followed that algorithm faithfully you wouldn't have any problem with that, right? None at all?

If you did all of your dives with a single Aeris computer, no worries. If you did all of your dives with a single Suunto computer, no worries. But that's doesn't seem to be what you are recommending.

But we are way out of line because we are trying to figure out how to deal with a pretty widely known Suunto problem?

I don't know if you are out of line, you can do what you like, no scuba police. The only reason why people sometimes comment in these threads about practices that don't seem safe is because new divers read them, and they should know that it's not a standard recommendation to adjust conservatism on a computer by putting in fake data until you get the NDL you want. Maybe you are OK with the particular dives that you do because of your experience, maybe it's just the normalization of deviance. But the point is that if a new diver without your experience takes home the message that if you have a Suunto, just set it on a richer mix than what you are actually diving, that's not good. That's why I'm bothering to comment.

Once more - just because you can play with the mix to get your NDL to match your buddy's or other divers on the boat doesn't mean that it's a good idea. Because you are looking at a single number, and the algorithm is looking at multiple dive profiles and multiple surface intervals.

Again, how is this any different than sending your computer up early on a lift bag, or clipping it to a high point on a wreck? Wouldn't that also deal with a pretty widely known Suunto problem?

 

[Diver0001]

So if we just dumped the Suunto and got a Aeris, then followed that algorithm faithfully you wouldn't have any problem with that, right? None at all?
But we are way out of line because we are trying to figure out how to deal with a pretty widely known Suunto problem?

Well.... you're not dealing with it. You're fudging it, which is as much good as just ignoring it. These computers are showing differences in shallow NDL times on repetitive dives for a reason and if you don't like the numbers then you can't just change the nitrox settings until it gives you numbers you like better...... You need to change the way you're diving with it in order to account for the variations.

To draw a comparison, you have two maps of the same area at different scales (your wife's computer and yours). What you have done is to turn and shift them in until you managed to get them to line up on one point on the map. Your assumption is therefore that they will also line up at all other points on the map. I believe that this assumption is fundamentally incorrect, an opinion that would be supported by Dr. Powell because we've had this discussion about trying to artificially align differing deco models before and I remember what he said about it. This is unnecessarily risky if you ask me and what you need is a procedural solution.

Even with two different computers you can plan the dive in order to reach a happy middle ground. If your goal is to maximize bottom time on the 4th dive, for example, then you could have ascended from 30ft to 15ft after 47 minutes and then neither of you would have had any chance of the computer going into deco. Understanding *why* this is the case is the key to understanding how maximize bottom time on any dive you wish.

R..

 

[scubadada]

Buy a computer with a decompression algorithm that fits your diving. Of course, that assumes you will know enough about the available decompression algorithms to make a decision.

 

[rjack321]

I'm sorry, we will just have to agree to disagree. You seem to be assuming that decompression stress is a simple metric that accumulates in a linear fashion, and therefore you can approximate risk by just comparing an NDL. I'm not a hyperbaric doc, most of these academic discussions are beyond me, but I do know that when you have a complex model and you simply enter one erroneous input, you can't rely on the fact that the output will be universally and reproducibly adjusted proportionally to what you entered.

Your thought experiment assumes that your risk of DCS is in linear proportion to the amount of deco you would be blowing off (compared to entering the actual gas that you are breathing). I don't know everything, but I do know that it's just not that simple.

The way to accommodate your own personal acceptable risk level (and lengthen your NDL) is to manipulate parameters that ARE accounted for in the model - like gradient factors, conservatism factors, etc... Putting in fake mix values isn't the same thing. It's no more appropriate than tying your dive computer to an SMB at some point and sending it to the surface to compensate for it being too conservative. That's another way to keep it from going into deco, right? How is that different than lying to it about your back gas?

Ummm how would doing side by side comparisons between a series of dives in multideco or VPM using nitrox 32 (the actual gas) compared to side-by-side plans using a Suunto diving the same SIs, depths, and BTs diving nitrox 36 be invalid again? If they have comparable deco schedules you have just demonstrated the validity of Fritz's approach to the Suunto "problem"
I'm sure you'll argue that they can't possibly be identical so this is invalid. I'm continually baffled by the deco micrometer you guys bring to the table.

Well.... you're not dealing with it. You're fudging it, which is as much good as just ignoring it. .

No really its not. Gaming a computer has ben going on for a decade+ in tech diving circles. Using what you know about a computer (since we don't actually know what's going on in a Suunto) you can adjust it using tools other than just the L0-3 settings. @fmerkel and his wife looked at the issue intelligently - it wasn't a random change and they weren't ignoring it. They adjusted the algorithm, just not through a Suunto approved button.

I can't wait until someone reopens the ratio deco repetitive diving can of worms again.