Computer Algorithms, Objective Evidence of Whether One is "Safer"

[Lorenzoid]

I'm not sure I can agree with that. If I'm reading the original study properly, and assuming VPM-B, +2 conservatism is the "gold" standard. That dive would have 10:30 of deco time.

The computers doing the same profile gave between +20 (dive still had 20 minutes of dive before any deco obligation) to -19 (19 minutes of deco).

That's quite a lot of variation

John, I'm getting out of my comfort zone with this and will gracefully bow out. The point I was attempting to make when I remarked that it's hard to say which computer is the most liberal is that there are so many different profiles to consider, not just the ones that might have been the subject of a study. Other than some outliers that seem to be very conservative or very liberal based on the limited profiles that have been studied, it's hard to say which are "the most liberal." I mean, a computer that is seemingly liberal in one series of one or more dives over one or more days might be surprisingly conservative under a different series. I wasn't referring to this particular study. I'm sure your point about this study is valid.

 

[Mike]

But the majority of recreational dive computers clearly operate within bounds which produce a very low risk of DCS. And I'm not sure if we even know what alterations to the algorithms really result in lowering that risk, except that shorter, shallower dives are going to be safer than deeper or longer ones.

Ditto.

I'm sure DAN would not recommend a Suunto over another brand of dive computer, but they'd without a doubt advise you not to take a hot shower right after diving, which again just proves that the most liberal dive computer is more than conservative enough and a Suunto's extra conservatism does nothing but cost you bottom time which is the most expensive component of everything surround diving.

 

[tursiops]

I'm not sure I can agree with that. If I'm reading the original study properly, and assuming VPM-B, +2 conservatism is the "gold" standard. That dive would have 10:30 of deco time.

The computers doing the same profile gave between +20 (dive still had 20 minutes of dive before any deco obligation) to -19 (19 minutes of deco).

That's quite a lot of variation

A couple of points:
1) I don't thing most folks *today* would say VPM/B +2 is the "gold standard." FYI, Buhlmann GF 85/85 on that MLD2 dive would just say ascend, do your normal 3-5min safety stop, and you are fine.
2) The fact that one of the computers gave +20 mins and one gave -19 mins is not evidence that +20 is wrong and -19 is better. I agree, it is a wide spread, but if +20 is still "safe" then -19 is just more safe, whatever that means.

What I'm not sure of is why they call it the PADI RDP test; at least MLD2 broke the PADI rules for a ML dive.

 

[halocline]

How I would choose it is the entire point of the thread: the one that gives you the most bottom time and is still objectively "safe.".

I think you're oversimplifying the differences between computer models if you believe that there is a single 'most bottom time that is objectively safe.'

If you want to maximize your dive experience (what I guess you mean by 'bottom time' and feel as safe as possible from DCS, I would simply repeat my earlier post; start thinking about what is actually happening to your body when you dive, how the basics of deco theory work, and what types of dive and non-dive behavior are associated with DCS.

If your question is simply "do the settings of increased conservatism offer proven additional protection from DCS in recreational diving" the answer is no, due to lack of statistical evidence. I'm not sure what answer you are after if it's not that.

 

[JohnN]

A couple of points:
1) I don't thing most folks *today* would say VPM/B +2 is the "gold standard." FYI, Buhlmann GF 85/85 on that MLD2 dive would just say ascend, do your normal 3-5min safety stop, and you are fine.
2) The fact that one of the computers gave +20 mins and one gave -19 mins is not evidence that +20 is wrong and -19 is better. I agree, it is a wide spread, but if +20 is still "safe" then -19 is just more safe, whatever that means.

What I'm not sure of is why they call it the PADI RDP test; at least MLD2 broke the PADI rules for a ML dive.

I haven't had much previous exposure to the Buhlmann GF algorithm prior to this or the merits of ZHL-16 vs a VPM model. Somehow that much of a dive without much planned deco seems very "sporty". Certainly this also doesn't seem like a "recreational" dive.

Just for giggles, I added in the +20 one of the tested computers would have allowed:

MultiDeco 4.04 by Ross Hemingway,
ZHL code by Erik C. Baker.


Decompression model: ZHL16-B + GF


DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = GF 85/85


Dec to 60ft (1) Air 60ft/min descent.
Level 60ft 54:00 (55) Air 0.59 ppO2, 60ft ead
Asc to 40ft (55) Air -30ft/min ascent.
Level 40ft 60:00 (115) Air 0.46 ppO2, 40ft ead
Asc to 10ft (116) Air -30ft/min ascent.
Stop at 10ft 6:20 (123) Air 0.27 ppO2, 10ft ead
Surface (123) Air -30ft/min ascent.


OTU's this dive: 13
CNS Total: 7.7%


208.9 cu ft Air
208.9 cu ft TOTAL


Looks like a trip to the chamber to me. . .

(and for my SAC, that is a crapload of gas to carry. . .)

 

[String]

"Conservative" v "Liberal" is meaningless unless comparing like-for-like profiles anyway.

For example my "conservative" Suunto Vyper Air gives me roughly 5-6 mins NDL on a 25-30m first dive than my GF utilising Shearwater. However the shearwater credits me with deep stops in a way the Suunto doesnt.
For second or third dives of the day with existing N2 loading the Petrel gives me substantially more NDL than the vyper.

There are lots of algorithms out there and the problem is for most of them its not public so not open to investigation other than reverse engineering so claims that "Its RGBM" from Suunto etc are impossible to verify.

DCS rates in recreational level no-stop diving are vanishingly small. So small that it appears that all computer models out there are "safe" provided they're followed completely. The literally millions of man-dives performed would have highlighted glaring differences in safety by now.

 

[drrich2]

How I would choose it is the entire point of the thread: the one that
gives you the most bottom time and is still objectively "safe."

<br><br>This is the underlying theme in a number of forum discussions where the basic issue is, why buy a Suunto when you could buy an Oceanic, Aeris, etc..., and get more allowed NDL, which you could cut short at your discretion?<br><br>Some people site liking the Suunto interface, or that it might be easier to get serviced in some parts of the world. And some try to pass the conservatism difference off as too small to matter (but others strongly dispute that). <br><br>If memory serves, I've been on a dive with a guide where it was figured that because he was wearing a Suunto, the rest of us probably wouldn't run out of deco. before he did. Anecdotal, he could've been wrong, all divers still need to watch their own computer's NDL, etc..., but it is what it is.<br><br>

<font size="3"><font color="#000000"><span style="font-family: Calibri">So, the thread has actually not revealed much.</span></font></font>

<br><br>

I agree that this thread has not revealed much.

<br><br>I disagree to a point. Sometimes, when current knowledge is limited such that we are largely ignorant on a topic, establishing that is in itself worthwhile. <br><br>Mike put this in practical terms, with the idea that we therefore lack a solid basis to assume a Suunto meaningfully increases diver safety by enforcing a more conservative profile, yet we do not it has a reputation for cutting peoples' bottom times short (how much &amp; the significance are disputed). <br><br>That matters. I've seen a number of postings in other threads where Suunto users express feeling better about diving with a 'more conservative' computer. But where's the rational basis? Where does this stop? Wouldn't you feel even safer if you stayed on the boat and didn't dive?<br><br>Richard.

 

[Subboy]

Ditto.

I'm sure DAN would not recommend a Suunto over another brand of dive computer, but they'd without a doubt advise you not to take a hot shower right after diving, which again just proves that the most liberal dive computer is more than conservative enough and a Suunto's extra conservatism does nothing but cost you bottom time which is the most expensive component of everything surround diving.

And that is just the reason that we all buy a computer, to be honest,

 

[Lorenzoid]

. . .
For example my "conservative" Suunto Vyper Air gives me roughly 5-6 mins NDL on a 25-30m first dive than my GF utilising Shearwater. However the shearwater credits me with deep stops in a way the Suunto doesnt.
For second or third dives of the day with existing N2 loading the Petrel gives me substantially more NDL than the vyper.
. . .

Have you tried turning on the Suunto's "Deep Stop" setting? I have not experimented with it much on my D6. But I have to wonder if the people who are constantly criticizing Suunto RGBM for being too conservative have never dived one themselves and are relying on hearsay from divers who didn't know how to get the most out of their Suuntos. I'm not doubting that the Petrel gives you substantially more no-deco time than the Vyper for repetitive dives. I just suggest that Suunto may not be QUITE as heinous as some make it out to be.

 

[scubadada]

Could you look at this from a different angle?

Does DAN (or anyone else?) have data on reported DCS incidents that also includes the type of diving and the computer used?

i do not think this will answer the original question, but it may provide food for thought and discussion.

I keep hoping that DAN Project Dive Exploration might yield some useful information in this regard. I have not seen anything to date.