Shearwater Gradient Factors=DSAT?

[dmaziuk]

Ha! Disregard my question. I was sure I read something somewhere on SB about when GF Lo could come into play with NDL’s but I have no idea where else to look. My bad.

Since I just posted that: So, who is going to market an inexpensive rec computer running Buhlmann with GF?

I'm sure Stuart and Ken Gordon also posted this at least once each.

 

[scubadada]

Until last summer I used the Oceanic DSAT algorithm but then I purchased a Perdix and had the same question. On Scubadada advice I set the GF to 45/95. The first two dive trips I did I wore both computers side by side and then downloaded them and compared the NDL values at various points over multiple dives. Initially the DSAT algorithm seemed more liberal but on repetitive dives the GF 45/95 became more liberal. I posted the comparisons last fall at Algorithm question (Perdix AI vs DSAT)

My take is that on an aggressive repetitive dive schedule the GF45/95 is very liberal and I try to leave a healthy margin off the calculated NDL.

Exactly my experience also

 

[Jay]

LOL my body says no to that anyway! My body is like my old convertible car: she looks pretty good if you don’t get too close, but it's best to be gentle on her.

Go the car analogies!

If you are not diving them to the limits, then what does it matter if/how two algorithms match up? If your regular diving is within the limits of the more conservative of the two algorithms, fine, who cares. And if your buddy's scary conservative computer is cramping your style, you should ask them whether they're comfortable diving on your more aggressive schedule. Or find another buddy.

That wasn't addressed at you personally, it was a generic "you". It irks me when people quote scubalab studies like they mean something, or say an algorithm has been well "tested by time". Six simulated dives on a few off-the-wall profiles don't make the statistics, and if you don't dive an algorithm to the limits, you're not testing the algorithm. Since many of us get out of the water well within our computers' NDLs, those NDL mean even less than what they're originally supposed to mean: that the statistical chances of (a generic) you developing DCS go from two in ten thousand to three in 9,999. Or whatever the M-values programmed in a given computer translate to.

[bolding is mine]

re "what does it matter ..." - I agree with the example you've given, but when a diver is transitioning between computers, it does matter (setting as base-line as closely as possible between the two).

re ScubaLab; I mostly agree with you but they do mean something. The 'something' that they mean is that they're NDL snap-shots of a certain set of dive profiles. And for the DSAT/16-C case in question, ScubaLab's results are consistent* with what @scubadada and @gopbroek have witnessed too. I say *, because a lot of the data was on Nitrox, SL's wasn't, and I'm not sure re Scubadada's .

Sure, none of that is scientific proof. But, I would be willing to bet that relative order between the NDLs at max depth for any reasonable dive profile for any SL tested RGBM based computer versus DSAT/16-C based computer (excluding the SP computers) holds as per ScubaLab's results. If you want to take me up on the bet, let's chat offline, sort out the rules, rewards, etc then make a new OP with it.

Betting aside, how to assess the relative conservatism of a computer for a Rec diver is a Q I've had for a while and will make a new OP about that rather than deviating this thread too much.

Until last summer I used the Oceanic DSAT algorithm but then I purchased a Perdix and had the same question. On Scubadada advice I set the GF to 45/95. The first two dive trips I did I wore both computers side by side and then downloaded them and compared the NDL values at various points over multiple dives. Initially the DSAT algorithm seemed more liberal but on repetitive dives the GF 45/95 became more liberal. I posted the comparisons last fall at Algorithm question (Perdix AI vs DSAT)

My take is that on an aggressive repetitive dive schedule the GF45/95 is very liberal and I try to leave a healthy margin off the calculated NDL.

Exactly my experience also

Thanks for posting that data. It's really interesting. Do either of you have a feeling how it would be for multi-day multi-dives on air?

 

[KenGordon]

Personally, I think anyone surfacing with a GF of 95 regularly, especially for multi day repeat dives is mad.

We know that repeat Diving is an additional risk, we know that best practice says to take a day off now and again.

I think (but would need to run the numbers to be sure) that the long, slow compartments, that carry influence from one day to the next, will never figure in the limits. So effectively each day is a new day as far as the ZHL model is concerned.

 

[dmaziuk]

re ScubaLab; I mostly agree with you but they do mean something. The 'something' that they mean is that they're NDL snap-shots of a certain set of dive profiles.

No. They're snapshots of calculated NDL at random points in a certain set of dive profiles. There is a name for ScubaLab studies: spotlight fallacy. Edit: actually, for what people quoting ScubaLab studies take from them. Caveat lectorem.

I think (but would need to run the numbers to be sure) that the long, slow compartments, that carry influence from one day to the next, will never figure in the limits. So effectively each day is a new day as far as the ZHL model is concerned.

I expect Wienke's insistence on counting 36 hours are "repeated dives" (or is it 42?) is what makes recreational RGBM so scary conservative... and I'm not convinced that's a bad thing, as our vacays are usually closer to 10 days than to 6.

 

[uncfnp]

I expect Wienke's insistence on counting 36 hours are "repeated dives" (or is it 42?) is what makes recreational RGBM so scary conservative... and I'm not convinced that's a bad thing, as our vacays are usually closer to 10 days than to 6.

It seems valid when reading research such as the DAN Europe’s Flying after Diving study that found bubbles formed even after the 24 hour wait in some of the “bubble prone” group.

 

[scubadada]

...Do either of you have a feeling how it would be for multi-day multi-dives on air?

All my dives are on nitrox, generally 32-36%. I have no personal experience diving DSAT and Buhlmann on air. Buhlmann seems a bit more relatively conservative looking at 1st dive NDLs on air rather than nitrox. Still, I wouldn't be surprised in the trends for repetitive dives was similar. Perhaps someone else has this experience.

 

[Jay]

More of a general FYI FWIW

The new SurfGF function in the Teric will be really handy. Have a look at post #7 from @Macan about that versus GF99 Teric Review

It was pointed out to me in another thread, Tec divers aim for the M_Value line (line from GF Lo to GF Hi) as a target (and bounce along it in deco), in Rec, GF Hi is a limit. Tec divers using GF Hi of 95 would be nuts. Rec divers using GF Hi of 95 and a *typical* profile may not necessarily be nuts, i.e. running NDL to zero at depth doesn't necessarily mean surfacing with a GF Hi of 95; a SS, meandering to/at shallower depths prior to ascending to SS, will result in a GF Hi of less than 95 at the surface. (If you directly ascend with no SS after hitting NDL=0 then you'll 'achieve' GF Hi of 95) ... and that's where the beauty of SurfGF will come into play.

 

[KenGordon]

Personally, I think anyone surfacing with a GF of 95 regularly, especially for multi day repeat dives is mad.

We know that repeat Diving is an additional risk, we know that best practice says to take a day off now and again.

I think (but would need to run the numbers to be sure) that the long, slow compartments, that carry influence from one day to the next, will never figure in the limits. So effectively each day is a new day as far as the ZHL model is concerned.

I ran the numbers. I took the profiles from the Scubalabs computer review and ran them for 21 days. So, four dives a day for three weeks. The leading compartment at the end of the fourth dive has a GF of 58 and it remained the same. The longest two or three compartments changed very marginally but were never going to influence the limits. I might try deeper or longer profiles to see how that goes. It will also be interesting to see how these days with multiple no stop dives compare to one or two deco dives.

 

[dmaziuk]

If the model's on- and off-gassing is roughly symmetrical, for every 8 hours of sleep you have to spend 8 hours underwater just to keep the slow compartments at constant level... Of course if the model is so right, it is safe to fly 6 hours 1 minute after your last dive, too.