not a shearwater fan ?

[KenGordon]

I think the point is you calculate your NDL off GFHi X but then at the safety stop you wait for your GF99 to drop to Y. I expect with carefully selected Y < X you can get this to keep you exactly for 3 minutes at the stop.

Now why you'd want to do that instead of just stopping for 3 minutes -- or lowering your GFHi to produce a "deco" dive with a 3-minute deco stop at 3 msw -- is another question, and I always thought GF99/SurfGF were just glitzy eye candy, but it doesn't have to eliminate the stop.

Suppose I decide that I will dive x/85 but only surface at 75. I do a deepish multilevel dive that would be a no stop dive for 30/70 (or whatever high conservatism is) and arrive at 6m at gf99 of 70, so I get to go directly to the surface.

I think the catch all 3 minute rule makes better sense.

 

[scubadada]

Suppose I decide that I will dive x/85 but only surface at 75. I do a deepish multilevel dive that would be a no stop dive for 30/70 (or whatever high conservatism is) and arrive at 6m at gf99 of 70, so I get to go directly to the surface.

I think the catch all 3 minute rule makes better sense.

I believe GF99 does not equal SurfGF until you surface. I use SurfGF to reduce my surfacing GF to the mid to high 80s when it initially exceeds that value. I do a routine 3 min SS when my SurfGF is already below that.

 

[dmaziuk]

Suppose I decide that I will dive x/85 but only surface at 75. I do a deepish multilevel dive that would be a no stop dive for 30/70 (or whatever high conservatism is) and arrive at 6m at gf99 of 70, so I get to go directly to the surface.

Yeah, I suppose you have to make you fg99 a fraction of what you have at 3m stop, not of of your planned gfhi, for this to work. But by the same token you don't have to make that 3-min stop either, that's why it's not "decompression" safety stop. So it is, in fact, perfectly fine for you to go directly to the surface.

I think the catch all 3 minute rule makes better sense.

It's boring, we wouldn't be having a dozen 20-page threads on that. And you couldn't show the user a shiny infographic with their compartments and heat map gradients during the simple 3 minute stop.

 

[KenGordon]

I believe GF99 does not equal SurfGF until you surface. I use SurfGF to reduce my surfacing GF to the mid to high 80s when it initially exceeds that value. I do a routine 3 min SS when my SurfGF is already below that.

SurfGF, GF, GF99 what’s the difference?

 

[KenGordon]

It's boring, we wouldn't be having a dozen 20-page threads on that. And you couldn't show the user a shiny infographic with their compartments and heat map gradients during the simple 3 minute stop.

Computers are not really very interesting, the key thing is they track depth and time. After that it is pretty much a done deal.

 

[NAM001]

Part of not being a Shearwater fan is not thinking that GF is a great scheme.

Following your plan hardly anyone would do a safety stop unless they had just done a pretty quick ascent or an actual deco dive.

The idea of a safety stop is to add APPARENTLY UNNECESSARY time which on average reduces bends.

not true. using the GF surface will tell you when you have met the necessary min required portion of the safety stop rule. It will allow you to ascend the surface in the min time if needed based on your ACTUAL profile. instead of using the global catch all 3-5 minutes which are based on nothing related to YOUR dive. In other words ,,,, using measured metrics and not guessing and doing over kill because it is the only option you have is an advantage IF YOU CHOOSE TO USE IT.

As far as hardly anyone doing a safety stop..... if the metrics support it let it be so. The rule of thump supports that. UNDER 60 FT NO STOP REQUIRED OR RECOMMENDED. OVER 60 FT 3 MIN. RECOMMENDED, WITH IN 10% OF NDL ADD 2 MINUTES TO THE NOW REQUIRED 3 MIN SAFETY STOP. I strongly suspect that different agencies can not agree with aspects of that.

the part of ,,,,,,,,,,,, Part of not being a Shearwater fan is not thinking that GF is a great scheme. ,,,,,,,,,, is singling out one attribute of the computer and basing its value on it. If you dont like GF then the computer does not force you to use it. Configure it to not display it. Do you remember when using a dive table you used the max depth and dive time to determine your repeditive dive allowed. how much error was in that process. it too was over kill and generated so much conservatism in its results they were ignored. the computer voided those problems. It appears that you position on GF's is may be akin to not using a depth gage and instead counting the times you clear your ears because that is all we had before depth gages were made. Why do that when technology has provided you a more precise device. I fully understand those that are sick of hearing shearwater over and over again as being the king of diving computers. Whether they are or not I cant say. but there seems to be a lot of competition to make other brands that not only look like a shearwater but include their functions and simplistic menu structure that I think is the real attraction of shearwater. Yes they were probably in the fore front of the pack making a usable computer and that caught on and now so many are playing catch up. That is not a bad thing. Nor is it derogratory to what you are using if not a shearwater. I always tell people to look at the features. simplistic menu system and other attributes because if you cant find it in the menu you really dont have the function. That specific attribute convinced me to buy one. not till after did I start looking into not just all that was in it, but how it could be used. And as far as using GF's the conservative settings in them were once by GF's now they are low med and hi to be more compatible to other brands.

Yes shearwater and other like tech computers are not made for the weak in understanding mind. But is it better to ignore the technology to hold onto age old methods developed when computers did not exist. Which by the way neither did BCD's and al tanks that had working PSI's of around 2500 or less. I have to say that when it comes to technology i have much less issues with a digital compass than an analog one. Technology again at work.


.

 

[KenGordon]

not true. using the GF surface will tell you when you have met the necessary min required portion of the safety stop rule. It will allow you to ascend the surface in the min time if needed based on your ACTUAL profile. instead of using the global catch all 3-5 minutes which are based on nothing related to YOUR dive. In other words ,,,, using measured metrics and not guessing and doing over kill because it is the only option you have is an advantage IF YOU CHOOSE TO USE IT.

Don’t forget the computer is running a simplistic model of the body. The body is what counts, not the model.

The point of a safety stop is to cover the low probability edge cases with an extra bit of deco, plus force divers to actually stop and have a controlled final ascent. This is deliberately regardless of the actual profile. It is extra ‘just in case’ deco that improves outcomes on average.

the part of ,,,,,,,,,,,, Part of not being a Shearwater fan is not thinking that GF is a great scheme. ,,,,,,,,,, is singling out one attribute of the computer and basing its value on it. If you dont like GF then the computer does not force you to use it. Configure it to not display it. Do you remember when using a dive table you used the max depth and dive time to determine your repeditive dive allowed. how much error was in that process. it too was over kill and generated so much conservatism in its results they were ignored. the computer voided those problems. It appears that you position on GF's is may be akin to not using a depth gage and instead counting the times you clear your ears because that is all we had before depth gages were made. Why do that when technology has provided you a more precise device. I

I think you have my objection backwards. I think that the very simplistic dissolved gas model plus Baker’s gradient factor hacks (ie the GF scheme used by Shearwater and others) ignores a bunch of stuff we know about the risks so of a bend. A dive computer, as opposed to planning software or tables, can evaluate those risks and present them to the user. To a minor extent SW do that by optionally extending the safety stop from 3 to 5 minutes in some circumstances.

It could look at a dive like this and tell the diver not to be so stupid:

Unfortunately pure dissolved gas models think that profile is pretty much the same as an hour at 3m.

This means watching the SurfGF/GF99 numbers is only a tiny part of the story.

 

[Dr Simon Mitchell]

Hi Ken,

You raised a couple of points in your post that piqued my curiosity.

First:

I think that the very simplistic dissolved gas model plus Baker’s gradient factor hacks (ie the GF scheme used by Shearwater and others) ignores a bunch of stuff we know about the risks so of a bend.

What "other stuff" ignored by dissolved gas / GF models are you thinking about, and what alternative models are there that don't ignore it?

Second, (and in a related vein):

Unfortunately pure dissolved gas models think that profile is pretty much the same as an hour at 3m.

How do other models interpret the dive?

Thanks,

Simon M

 

[NAM001]

Don’t forget the computer is running a simplistic model of the body. The body is what counts, not the model.

The point of a safety stop is to cover the low probability edge cases with an extra bit of deco, plus force divers to actually stop and have a controlled final ascent. This is deliberately regardless of the actual profile. It is extra ‘just in case’ deco that improves outcomes on average.



I think you have my objection backwards. I think that the very simplistic dissolved gas model plus Baker’s gradient factor hacks (ie the GF scheme used by Shearwater and others) ignores a bunch of stuff we know about the risks so of a bend. A dive computer, as opposed to planning software or tables, can evaluate those risks and present them to the user. To a minor extent SW do that by optionally extending the safety stop from 3 to 5 minutes in some circumstances.

It could look at a dive like this and tell the diver not to be so stupid:


View attachment 590571

Unfortunately pure dissolved gas models think that profile is pretty much the same as an hour at 3m.

This means watching the SurfGF/GF99 numbers is only a tiny part of the story.

I agree with you to a point ,,,,but the example you give is not what I would refer to as a standard dive profile. and In this example it looks like one in a 25 ft lake where one can not navigate or you are engaging in a training evolution etc. I dont think this would be reflective of a normal 60+ ft dive. In addition to that I think anyone (assuming) that would do such a profile ,,probably would not think of the resuiltant necessity of a SS. In regards to GF's they would think that such a profile would be reflected. I am not suggesting that if using GF's you can abandon SS's. I am suggesting that if you go to do a 3 min SS you have GF's to provide the metrics to say you need to do more than 3 min or that in 1 min you have cleared reason for continuing further SS time. That the excess N2 load where it is safe to proceed to the surface with some measured knowledge of SS time rather than pushing a chamber ride. I really dont know anything about you and attempt to not sling names at persons but have no problem labeling what I think is a failing thought process. Let me offer a few more aspects in GF's behalf. IN my early days we had no SPG's on our tanks. A tank new was 50$ a reg was 50$ etc. Granted they were 40 cheaper on the ark. But we gaged our air on how hard the reg breathed and then pulled the rod on the J valve. The J valve like many other aspects had its problems. Then came K valves with SPG's on the regs. It would be foolish to tape over the spg and continue to determine gas level by how the reg breathed. As mentioned before we had no BCD's we did things differently until that technology came along. Same goes with using tables and dynamic / multilevel profiles ( created bby rec divers), when computers came along. At some point we have to review our standard practices in the light of new technologies. As gf's go they have been around for a long time under one name or another. they are the root of of deco computation. conservatism settings, and more. If I configure my puter for medium conservatism i am selecting a 35/80 or a 30/85 GF pair to govern the dive even if the selectio says medium. Computers that do not have GF displayed use GF. IN the days of the 3 minute stop we had no levels of conservatism other than use of the tables saying ndl was 15 minutes so we cut the dive at 10. When doing that 2 things happened. We for most purposes negated the need for the SS because we cut short the exposure. the SS is a no metric means to extend the overall ascent times to prevent a chamber ride because all we had was a depth gage adn a watch. It would have been just as well done by slowing from 30 fpm to 20. The problem with that was that tables was based on a set ascent rate and to slow to 20 demolished any validity to the table data. Even then the ascent rate was 60.. Im not by far any kind of deco expert but I know the basics within the confines of rec diving. Much like If i hit a tree in the car , I will do less damage at 5 mph than 15 mph. We have more information on our wrists today than there was available in the 50's to the highest experts in diving. Lastly to put too much emphasis on SS for the WRONG reasons makes a SS a requirement and not a recommendation for some circumstances. Our training has removed discussions of DECO and how it works, and how it is mitigated at the most basic level. Rules of thumb are same as law for most because of lack of knowlwdge or access. Much like the stop sign in the middle of a desert on an abandoned road where a town use to be. Now Would I tell people not to do safety stops. No but I would have a meaningful discussion on deco and how it happens and what promotes the offload of N2 along with how much you have to be off gassed to to surface. The last part is the important part. either you look at your N2 gage (or GF level) or do the 3 minute stop if you dont have one. This is just my opinion but LIke using nitrox in its beginnings GF's are viewed as technical data and rec diving is viewed as non technical diving. As such the 2 worlds should not mix and stay on their respective side of the tracks. GF's are out opf hte pandoras box lets learn to use them. Lets train on them in say the AOW phase of training.

 

[KenGordon]

The trouble with the SurfGF/GF99 numbers shown by dive computers is they are a very narrow view. It comes from just the leading compartment. Other compartments are hidden, thus the first dive of the first day on x/95 and the last dive of the day a week later will have different risks for the same Indicated GF. This is the same mechanism that has been used to show excessive deep stop profiles are bad. The extra loading on slow tissues becomes an issue with repetitive dives too, but the one leading tissue is used to generate the single number which is supposed to represent risk.

I am absolutely not saying tables and silly rules are better than a live computer. I am saying that using an indication like SurfGF instead of safety stops is a mistake.

Ps of course, not forgetting that the dissolved gas models like ZHL16C ignore a lot of things, so even if the measure was over more compartments it is still terribly simplistic.