Padi Advanced OW - Deep stops??

[scubadada]

What the OP is concerned with is only part of the story. In the stuff said to and by the OP there are things I disagree with and which I have an opinion on. The main one is the use of NDL times as a worthwhile indicator of how a computer will perform.

Also, all dives are deco dives, it is just that some have ascent rates slow enough to avoid stops below the surface. There is no line where suddenly a dive is a technical. If you want to make that line be compulsory stops then apparently all the Suunto users will suddenly be technical divers while the brand X buddies are still recreational divers. My reference to the NEDU study is not about technical diving but the comparison of algorithms use by the navy with comercial ones to argue against a single point you made.

It really is pertinent to the the OP, some of these algorithms will give lots of stops once past the NDL. That is a foreseable contingency, like needing an octopus, and should not be ignored. If you do not have the gas for those stops then some degree of concern will be due. But of course it depends on the dives.

I suggest that anyone wanting to research a computer gets in the water with one, or sees what their buddies dive and how they behave.

Again, you completely missed the point. This is no stop diving with deep stops. Revision 6 of the US Navy Diving Manual was the first revision of the diving tables since 1959. The new tables utilized the Thalmann exponential-linear decompression model parametized with VVal 18M, a modified version of VVal 18 with updated maximal permissible tissue tensions. A few of the results were manually edited to produce the final tables. As I posted in in post # 85 these are still the most liberal NDLs for a single air dive. Single dives do not define computer performance, that's why I have previously posted the ScubaLab data. The 2016 data is now available as an Excel file off their website, no surprises.

Technical, decompression dive is another topic, let's try to keep them separate.

Best, Craig

 

[DevonDiver]

Single dives do no define computer performance,..

A critical factor that all too often seems to get overlooked.

 

[KenGordon]

Again, you completely missed the point. This is no stop diving with deep stops. Revision 6 of the US Navy Diving Manual was the first revision of the diving tables since 1959. The new tables utilized the Thalmann exponential-linear decompression model parametized with VVal 18M, a modified version of VVal 18 with updated maximal permissible tissue tensions. A few of the results were manually edited to produce the final tables. As I posted in in post # 85 these are still the most liberal NDLs for a single air dive. Single dives do not define computer performance, that's why I have previously posted the ScubaLab data. The 2016 data is now available as an Excel file off their website, no surprises.

Technical, decompression dive is another topic, let's try to keep them separate.

Best, Craig

I have been off diving so could not continue to bicker over minutiae.

Your point seems to be that NDL times are an indication of the general behaviour of a computer, and that at a push the four dives in the Scubalab review are an indication of their behaviour.

I am saying that that is too simplistic. Depending on the profile different compartments will come into play. I have been diving a Suunto Helo2 and a petrel extensively the last week and seen NDL times cross, with one longer at one point in the dive and the other later. The same can go for stop depths on dives with stops.

These numbers are not a good guide.

There is known good practice for diving to reduce the chances of a bend, picking a computer with the longest NDL times is not one.

My point about the navy tables is that in one setting, single dive NDL, they may seem aggressive, while in others they may not be. Have you tried using those tables to do the four scubalab profiles? Are they still more aggressive?

Deco diving and NDL diving are the same thing if the stops only depends on the computer you chose. Keeping them separate is part of a comercial choice made to minimise the time taken to qualify holiday divers.

 

[scubadada]

I have been off diving so could not continue to bicker over minutiae.

Your point seems to be that NDL times are an indication of the general behaviour of a computer, and that at a push the four dives in the Scubalab review are an indication of their behaviour.

I am saying that that is too simplistic. Depending on the profile different compartments will come into play. I have been diving a Suunto Helo2 and a petrel extensively the last week and seen NDL times cross, with one longer at one point in the dive and the other later. The same can go for stop depths on dives with stops.

These numbers are not a good guide.

There is known good practice for diving to reduce the chances of a bend, picking a computer with the longest NDL times is not one.

My point about the navy tables is that in one setting, single dive NDL, they may seem aggressive, while in others they may not be. Have you tried using those tables to do the four scubalab profiles? Are they still more aggressive?

Deco diving and NDL diving are the same thing if the stops only depends on the computer you chose. Keeping them separate is part of a comercial choice made to minimise the time taken to qualify holiday divers.

Hi Ken,

Of course, comparing 1st dive NDLs and multilple dive simulations is a simplification. Short of making multiple dives with the computers, how is one able to make any comparison of the many decompression algorithms available? Discrepancies in the ranking of algorithms by 1st dive NDLs are often revealed by the multiple dive simulations, this information is very valuable. Lately, I have been diving my primary computer, running DSAT, and a backup computer running Buhlmann ZHL-16C with GF. Even when relatively well matched for NDL by the chosen GF hi, the two algorithms behave somewhat differently based on depth and nitrox mix. Though I've only made about 50 dives with both computers, I've learned a considerable amount, much that would not be revealed by the type of objective data that is available.

Do you have data to support your premise that the rate of DCS is higher when diving decompression algorithms generating longer NDLs?

I would be very glad to see the numbers you generate using the Navy tables to evaluate the 4 ScubaLab dive profiles. the multilevel dives will require a fair amount of manual labor on your part.

No stop and deco diving are on a continuum, the differences in decompression algorithms draws the line between the two in slightly different places. I'm not at all sure we really disagree about anything, maybe we are just bickering over minutiae

Good diving, Craig

 

[KenGordon]

Do you have data to support your premise that the rate of DCS is higher when diving decompression algorithms generating longer NDLs?

Consider two algorithms with the same ascent and decent rates. If one (A) gives an NDL of 10 minutes and another (B) 18 minutes then you would have a higher over saturation when diving the second one to the NDL as it is longer and so there is greater exposure. Doing the 18 minute dive on algorithm A would lead to stops and so a reduced over saturation. I assume greater over saturation leads to a higher probability of DCS.

BTW you should find that the GF lo number controls the NDL. The instantaneous GF (and so maximum over saturation allowed) is interpolated between the first and last stop. Thus at the first stop you would expect to be at the GF lo and so exceeding GF lo will lead to a stop. Thus for an NDL dive the surfacing GF ought to be below GF lo or a stop would have been inserted. It is easy to imagine bugs which might lead to GF high being the limit though.

Going back to Thalmann, since the off gassing is linear rather than exponential, you might expect it to be slower than the usual exponential decay models. Thus repetitive diving will require longer surface intervals. I have not tested this as I am never quite that bored.

 

[stuartv]

BTW you should find that the GF lo number controls the NDL.

I thought GF Lo wasn't even used for NDL dives. It's only used once you have a deco obligation.

 

[KenGordon]

I thought GF Lo wasn't even used for NDL dives. It's only used once you have a deco obligation.

When do you start to have a deco obligation? When you reach the GF lo.

 

[scubadada]

I thought GF Lo wasn't even used for NDL dives. It's only used once you have a deco obligation.

That is my belief also and borne out by planning. I believe Ken has misspoken on this topic. It is difficult for many to discuss both no-stop diving and deco diving without confusing the two. I think we all agree about one thing, most recreational divers would not have a clue to what we are discussing.

 

[scubadada]

When do you start to have a deco obligation? When you reach the GF lo.

No, you do not have a deco obligation until you exceed the GF hi with a normal ascent. Come on now, this is wrong.

 

[stuartv]

When do you start to have a deco obligation? When you reach the GF lo.

BTW, you do realize, don't you, that your GF is always 0 until you start to ascend, so you will never reach your GF Lo ... until you begin your ascent.