Gradient Factor Presets by Manufacturer/Computer

[dmaziuk]

No, I don't believe multi-level profiles are dangerous. My multi-level profiles are done within no decompression limits. Typically the decompression dive I'm referring to is a planned deep dive with a specific objective like a wreck dive. These tend to be a square profile (more or less) and then up the shot line with the main objective to decompress rather than swim around and possibly get disoriented in blue water.

So your argument is essentially that "low GF Lo" profiles are "untested" because everybody knows decompression dives are done on square profiles? The way I see it, either your computer correctly calculates gas loading at every point in the dive, including decompression stops, or it doesn't. If it does, then it'll give you a correct next deco stop regardless. If it doesn't then you can't trust it on any dive with any GF settings.

As for presets, there's two kinds of people: those who'll dive whatever's available, and those who think they know better and go "custom". It is unclear which is smarter.

 

[dmaziuk]

I have absulutely no clue what you are talking about, or in what context. something has gone on a left turn here.

You said newbies don't buy shearwaters, I said anyone who asks on this forum what computer to buy (first, upgrade, rec, AI, any kind, under any circumstances) will be told to buy a shearwater by post #3. It is a mostly irrelevant tangent, no argument there.

 

[NAM001]

You said newbies don't buy shearwaters, I said anyone who asks on this forum what computer to buy (first, upgrade, rec, AI, any kind, under any circumstances) will be told to buy a shearwater by post #3. It is a mostly irrelevant tangent, no argument there.

There are absolute pros and cons to the shearwaters. and i recommend them because of their intuitive menu system and display. I agree it is not for those that like to tinker with it without background enough to do it. I think that most all new divers get sold what the LDS has and what that it is not in the price range of a shearwater. I also accept that there are many on the classifieds that are used and priced right for selling. I dont recommend people blow deco or safety stops but I also know that there is no difference in a 10 second window of crossing the NDL that in reality makes no difference. and the uneducated that says in this much is good then that much must be better, make logical informed decisions. you can never make a best decision on anything you dont understand. Unfortunately one thing that makes scuba easy for all is we implement a lot of one rule fits all policies. I am always wondering as we trim training more and more how much we are training ourselves into ignorant practices.

 

[CandiveOz]

I hope you are not saying to ignore the computer and skip a mandatory deco stop. Even if it is "OK" for the diver, if the computer is "in deco" and you ignore it, MOST computers will lock you out for 24-48 h.

example low conservative setting ndl is 18 and high is 10 you are set at high and you hit 11 minutes. the only reason you are in deco is because of the computer setting not the physiology of the diver.

I don't think @KWS is advocating skipping a manditory deco stop. There are some computers like my Galileo where it allows you to select a more conservative setting (known as microbubble levels; MB L1 - L5) which reduce your NDL and prescribe additional stops to minimise microbubble formation. If you ignore these stops or go past the microbubble NDL, the computer doesn't penalise you. It just cascades to the next MB level until it hits the the default level (MB L0). At this level, the stops are manditory decompression stops.

On other computers, if a diver selects a more conservative setting reducing the NDL from say 18 min to 10, (as per @KWS example), if you do exceed the 10 min NDL and do not follow the precribed deco profile, then you are absolutely correct, the computer will lock you out even though you are still within the 18 min NDL default setting. This is how my Tusa IQ 750 works running PZ+.

 

[CandiveOz]

Maybe one ought to distinguish between a "preset" and a "default. " In Tec mode you do need a default, otherwise the computer is not usable. The 30/70 used by Shearwater is fine....as a default. At least the computer will work. Feel free to change it.
So I see no problem with a manufacturer supplying a computer with a default setting, even in Rec mode, like the 40/85 (Medium Conservatism) Shearwater uses. If the new user just uses the computer without changing anything. he/she will probably be OK. Perhaps it would be even better to supply it with the default being High Conservatism (35/70).

I completely agree with you, preset vs default was an oversight on my part. With regards to Shearwater's presets and defaults, since GF-lo has no impact on NDL diving and if a diver exceeds the NDL and does light deco, it probably wouldn't make a big difference in terms of deco obligation, then having any GF-lo setting isn't really a big deal although I still wouldn't advocate a deep GF-lo. And taking it a step further, as I have previously stated, I would exclude the GF-lo setting in Rec mode in keeping with the KISS principle.

As for the, 30/70 default in Tech mode, this has a GF spread of 50 and would fall into the RED block of the table. I know the diver can change it but I would suggest a shallower default such as 40/70 or even 50/70.

Is my reasoning justified? Well, Bruce Partidge, the founder of Shearwater, has indicated that he has moved to shallower GF-lo settings. Review of Deep Stops in Technical Diving - Shearwater Research

And if you watch @Dr Simon Mitchel presentation Decompression Controversies on youtube, at approximately minute 44:30 he says, he is now using 50/70 or 50/75. So if leaders in the dive industry are making their GF-lo's shallower, I suggest it would be prudent for defaults and presets of dive computers to reflect the same.

While Shearwater uses gradient factors in their computers, I wouldn't say they were stong advocates of the system. The following is taken from the Perdix user manual.

Stating that gradient theory may be completely false, doesn't inspire a lot of confidence. Compare that to this quote taken from the Suunto site regarding their Fused algorithm.

Of course Wienke is tooting his own horn, but I don't see Suunto suggesting that his work in decompression theory may be completely false!

 

[NAM001]

I don't think @KWS is advocating skipping a manditory deco stop. There are some computers like my Galileo where it allows you to select a more conservative setting (known as microbubble levels; MB L1 - L5) which reduce your NDL and prescribe additional stops to minimise microbubble formation. If you ignore these stops or go past the microbubble NDL, the computer doesn't penalise you. It just cascades to the next MB level until it hits the the default level (MB L0). At this level, the stops are manditory decompression stops.

On other computers, if a diver selects a more conservative setting reducing the NDL from say 18 min to 10, (as per @KWS example), if you do exceed the 10 min NDL and do not follow the precribed deco profile, then you are absolutely correct, the computer will lock you out even though you are still within the 18 min NDL default setting. This is how my Tusa IQ 750 works running PZ+.

I agree CavediveOZ. My more specific point is that I could have 3 identical computers on,,, each with a different setting for conservatism lo med and hi and do a dive. now we know that the high will trigger deco first and low last. If low is the proper stetting for you. then the med and high will call deco too soon and when called it will be a over protective call for deco. The key to the setting is setting it right for you. now if I were to use one computer and set it to hi and it hits deco. am I (my body) really in deco, because if I had set it for low I would still be with in NDL parameters. Under that case I would not have a problem skipping a mandatory stop and go directly to the surface with out worry. Had that happen with a setting of low one should probably think twice about taking that course of action. In other words the margin of safety or buffer by the setting of GF hi is being cashed in on. So the question . which gf hi setting has the highest demand for NDL compliance???? 100 or 70. My answer would be 100 because it allows the body to use all the available 2:1 ratio haldene provided. In another wording you have a 2:1 physiological red line. The gf's allow you to modify this to,,, in the case of x/70 using a 1.4:1 line instead of 2:1 . If you are less than 1.4 you are in a solid green dive condition if over 2:1 then you are red and hard in deco, between is yellow and for all phisiological purposes an area of negotiation. Only the use of a computer and its locking out function forces you to actually stop if you want to continue diving later. if given a choice of skipping a stop if needed I would skip a 40/70 stop before i would ever skip a 20/95 when heading up at the time the computer went from a ndl dive to a deco dive. This yellow area of negotiation I refer to is similar to what ratio deco divers use to shape their ascent curve, so it is a common practice but not in the rec world. Yes RD divers aree in and have deco to do but they decide how much time at what depths to do that deco. they have flexibility regarding how they do it. Unlike the computer that gives you one option. In reality isnt pushing a 15 min ndl to 15 minutes as 10 seconds just what a safety stop is there to cure when you use a timer and not a dive computer in normal mode.

 

[CandiveOz]

So your argument is essentially that "low GF Lo" profiles are "untested" because everybody knows decompression dives are done on square profiles?

No, that is not my argument.

As for presets, there's two kinds of people: those who'll dive whatever's available, and those who think they know better and go "custom". It is unclear which is smarter.

Well if there are only two kinds of people and you don't think either of them are smart; which camp do you belong to?

The way I see it, either your computer correctly calculates gas loading at every point in the dive, including decompression stops, or it doesn't. If it does, then it'll give you a correct next deco stop regardless. If it doesn't then you can't trust it on any dive with any GF settings.

Ah, the answer is not as black and white as you suggest. We are dealing here with a decompression model. The key word is MODEL. It calculates gas loading and decompression stops but how accurate it is for each diver is debatable. Actually its not debatable, we know its good, but its not perfect. If it was truly accurate; no one would suffer the bends. All models have limitations but in general, by following the model, your risk of DCS is reduced.

So Erik Baker takes Albert Buhlmann's model and creates the Gradient Factor model. The purpose of this model was to artificially create deep stops. Sure, mathematically correct, but probably not in line with what Buhlmann was thinking. For all we know Albert may be rolling in his grave! Now why can't we program inverse GF's (eg. 95/70) on our dive computers? It can be done mathematically but it wasn't part of Baker's model or objective to generate deep stops so its not allowed.

Now just to hammer the point home with an example. Baker, in his paper, uses a trimix dive down to 90 meters. You could plan the dive with GF 10/99 using available software. It will mathematically generate appropriate deco stops according to the model. Would you be happy to do this deco profile because it is mathematically accurate? Or would you sit back and think maybe, just maybe, you are pushing the limit of the decompression model.

I'll leave you with that thought. And if you disagree, go back and read what Shearwater says about of gradient theory.

 

[dmaziuk]

Well if there are only two kinds of people and you don't think either of them are smart; which camp do you belong to?

I believe in freedom from choice: my computer has one button that I press when saving dives to PC, it turns on in the water and shows me numbers. And I don't have to fill my pretty little head with all this nonsense, I'm free to enjoy the dive and colourful fishies.

Ah, the answer is not as black and white as you suggest. We are dealing here with a decompression model. The key word is MODEL. It calculates gas loading and decompression stops but how accurate it is for each diver is debatable. Actually its not debatable, we know its good, but its not perfect. If it was truly accurate; no one would suffer the bends. All models have limitations but in general, by following the model, your risk of DCS is reduced.

So Erik Baker takes Albert Buhlmann's model and creates the Gradient Factor model. The purpose of this model was to artificially create deep stops. Sure, mathematically correct, but probably not in line with what Buhlmann was thinking. For all we know Albert may be rolling in his grave! Now why can't we program inverse GF's (eg. 95/70) on our dive computers? It can be done mathematically but it wasn't part of Baker's model or objective to generate deep stops so its not allowed.

Now just to hammer the point home with an example. Baker, in his paper, uses a trimix dive down to 90 meters. You could plan the dive with GF 10/99 using available software. It will mathematically generate appropriate deco stops according to the model. Would you be happy to do this deco profile because it is mathematically accurate? Or would you sit back and think maybe, just maybe, you are pushing the limit of the decompression model.

No. The purpose was to add conservatism and specifically add it to protect fast tissues more than the base model does. I wouldn't be "happy" with 10/99 because current thinking is fast tissues don't need quite that much protection after all, so all that extra deco time and gas would be wasted. But not because it's "pushing the limit of the model" -- it doesn't. It counts in all the extra gas loading accrued during stops, it calculates the next deco stop by Buhlmann formula with your added conservatism factor.

It does that even on 95/70 and no, off the top of my head I don't know why some (all?) vendors won't let you set GF Lo above GF Hi. I don't see what's wrong with spending more time at shallow stops, in fact that's what they all tell us recreational divers to do with longer safety stops and RGBM computers that penalize for short SSs. But I only fix computers, you'd have to ask a meat doctor: maybe they know why it's good for rec and no good for tech.

 

[NAM001]

I was just dont some digging on Gf's and I now don't know how i got there, but along with tatcics like doing a 100 ft dive with a 20 min NDL,,,,, safety tactics that have been used are processing it as a deeper dive the more conservative the deeper you process it as. so a 100 ft dive would be calculated as a 120 ft dive. that generates a shorter NDL. Another is adding x amount of minutes to the dive time and making you reach a table NDL earlier. Using GF Hi to determine an altered NDL and then this. Gf hi has no effect on NDL. Gf low comes into play to generate a deep stop that allows off gassing so that there is less at the surface to off gass. This latter case is using a deep stop similar to using a required safety stop to create a bit of buffer so that the tissue ambient presure ratio is less at the surface. Makes sense. I have tried to identify some computers that do this but have not been successful. This technique may no longer be used. No matter what if you use absolute minimum skill levels and doing a NDL dive with out exceeding NDL, you can not depend on all divers having good depth control ,,,so any maintained stop can not be anticipated as a reasonable technique for a new diver with minimal depth control skills.. Accepting that last thought as a given and that a NDL dive , by definition ,, allows direct ascent to the surface, no stops can be required and that should make any use of Gf Lo a non player.

 

[dmaziuk]

I was just dont some digging on Gf's and I now don't know how i got there, but along with tatcics like doing a 100 ft dive with a 20 min NDL,,,,, safety tactics that have been used are processing it as a deeper dive the more conservative the deeper you process it as. so a 100 ft dive would be calculated as a 120 ft dive. that generates a shorter NDL. Another is adding x amount of minutes to the dive time and making you reach a table NDL earlier.

It's in Baker's "understanding M-values". As well as using higher-that-actual inert gas fraction (as in diving nitrox w/ computer set to air) and modifying half-times for asymmetrical: slower off-gassing. As I understand it, Thalmann slows down off-gassing at some point, and bubble models do too when they think the bubble's "too much", i.e. that last one amounts to ditching the pure dissolved-phase model" not Buhlmann aymore.