Algorithm

[Diver0001]

I liked the suggestion that @bergersau made. Conservatism is not only about the algorithm but it's between your ears. It's about habits, context, diving style, conditions, physical health etc etc. His idea of backing off the NDL in order to build in a buffer is a good one.

Here are some other suggestions in the same line.

- as a general rule when making multiple dives on a day, make the deepest dive first. The potential risk factor in reverse profiles isn't particularly well established but this is a good habit none the less because it will give you the longest bottom time during the deepest dive. Reverse profiles of 12m or less don't seem to be problematic.

- If you are fatigued, back off the NDL

- If you are dehydrated then drink more water and back off the NDL

- are you getting older? back off the NDL

- are you less then fit? back off the NDL

- do you smoke? back off the NDL

- are you making multiple dives in a day or multi-day-multi-diving? Back off the NDL

- If the water is cold, back off the NDL

- If you have to work hard on the bottom -- for example, swimming into a current -- back off the NDL

- never skip safety stops unless there is a very compelling reason to

- extend safety stops whenever doing so is practical. 3min is the MINIMUM duration for a safety stop but they can be extended indefinitely. Make use of that fact if you still have air.

- avoid heavy exercise after a dive for at least 6 hours.

That's the crash course in the most common potential risk factors that could turn a marginal dive into a dive where the diver develops DCS symptoms.

R..

 

[scubadada]

Hi @RTC'83

You have 4 choices with your dual algorithm, Oceanic computer. From most liberal, to most conservative, DSAT, PZ+, DSAT with conservatism, PZ+ with conservatism. The 1st dive NDLs for DSAT and PZ+ are listed in the back of your owner's manual. There is little information regarding the performance of dive computers for repetitive dives. ScubaLabs simulated repetitive dives in a hyperbaric chamber will give you an idea how many of the most commonly used decompression algorithms perform over a series of 4 no stop dives, including DSAT and PZ+
http://ads.bonniercorp.com/scuba/PDF/ScubaLab-Computer-Test-September-2014-data.pdf

Good diving, Craig

 

[KeithG]

I liked the suggestion that @bergersau made. Conservatism is not only about the algorithm but it's between your ears. It's about habits, context, diving style, conditions, physical health etc etc. His idea of backing off the NDL in order to build in a buffer is a good one.

Here are some other suggestions in the same line.

- as a general rule when making multiple dives on a day, make the deepest dive first. The potential risk factor in reverse profiles isn't particularly well established but this is a good habit none the less because it will give you the longest bottom time during the deepest dive. Reverse profiles of 12m or less don't seem to be problematic.

- If you are fatigued, back off the NDL

- If you are dehydrated then drink more water and back off the NDL

- are you getting older? back off the NDL

- are you less then fit? back off the NDL

- do you smoke? back off the NDL

- are you making multiple dives in a day or multi-day-multi-diving? Back off the NDL

- If the water is cold, back off the NDL

- If you have to work hard on the bottom -- for example, swimming into a current -- back off the NDL

- never skip safety stops unless there is a very compelling reason to

- extend safety stops whenever doing so is practical. 3min is the MINIMUM duration for a safety stop but they can be extended indefinitely. Make use of that fact if you still have air.

- avoid heavy exercise after a dive for at least 6 hours.

That's the crash course in the most common potential risk factors that could turn a marginal dive into a dive where the diver develops DCS symptoms.

R..

Does anyone have any ideas / recommendations / scientific info on "how much" and "which part of the dive" to back off NDL?

As vacation divers we do repetitive multi level dives. Generally a max of 4 dives a day, generally no deeper than 70 feet, with average depth above 50 feet. I personally never see an NDL of less than 10 minutes. Is that conservative?

How far away from NDL is considered conservative? I would claim 1 minute for the majority of the dive means you are aggressively riding your NDL. Is 5 minutes NDL at the deepest part conservative? How about 10 or 20 minutes?

What about multi level dives? Is 5 minutes NDL at 80 feet at the start of the dive any different than getting to 5 minutes NDL at 45 feet near the end of the dive?

 

[scubadada]

Does anyone have any ideas / recommendations / scientific info on "how much" and "which part of the dive" to back off NDL?

As vacation divers we do repetitive multi level dives. Generally a max of 4 dives a day, generally no deeper than 70 feet, with average depth above 50 feet. I personally never see an NDL of less than 10 minutes. Is that conservative?

How far away from NDL is considered conservative? I would claim 1 minute for the majority of the dive means you are aggressively riding your NDL. Is 5 minutes NDL at the deepest part conservative? How about 10 or 20 minutes?

What about multi level dives? Is 5 minutes NDL at 80 feet at the start of the dive any different than getting to 5 minutes NDL at 45 feet near the end of the dive?

Significant difference between approaching the NDL when diving a Cochran computer, or one running DSAT, than when diving one of the conservative RGBM based algorithms

 

[Diver0001]

Does anyone have any ideas / recommendations / scientific info on "how much" and "which part of the dive" to back off NDL?

Unfortunately divers are put in the position of having to "best guess" this. Current algorithms cannot and do not account for physiological factors. In layman's terms, the computer has no idea how you are feeling.....

R..

 

[KeithG]

Unfortunately divers are put in the position of having to "best guess" this. Current algorithms cannot and do not account for physiological factors. In layman's terms, the computer has no idea how you are feeling.....

R..

Understood - you have suggested to back off NDL, I am asking if anyone knows "how much"? If I change from 10 minutes away from NDL to 15 minutes way from NDL have I actually done anything material or significant (according to the scientific algorithm). Or is 10 minutes away already incredibly conservative?


I recently ran across this scientific single dive square profile test of 43 dive computers from 2014 http://www.sams.ac.uk/martin-sayer/pdfs/decompression-management-43-computers It was interesting that the NDLs for 30, 40 and 50msw were all within a few minutes. In my mind this makes it very hard to claim one algorithm (computer) is significantly more conservative or liberal than others. Maybe it becomes apparent once you start doing multiple dives?

The link A sense of algorithm in post #2 provides data for a pair of "extreme leisure" dives where the computers were taken well into deco on purpose. I am not sure how I would extrapolate the data into non-deco dives.


FYI: Sayer tested the following 43 computers:
Brand Model
Apeks Pulse
Apeks Quantum
Beauchat Voyager
Cressi Sub Archimede 2
Cressi Sub Edy II
Mares Icon HD
Mares Nemo Sport
Mares Nemo
Mares Nemo Excel
Mares Nemo Air
Mares Nemo Wide
Mares Puck Wrist
Mares Puck Air
Oceanic Atom 2
Oceanic Datamask Hud
Oceanic Pro Plus 2
Oceanic Veo 250
Oceanic VT 3
Scubapro Xtender
Seeman XP 5
Suunto Cobra 2
Suunto Cobra 3
Suunto D9
Suunto D6
Suunto D4
Suunto Stinger
Suunto Spyder
Suunto Vyper
Suunto Vyper 2
Suunto Vyper Air
Suunto Vytec DS black
Suunto Vytec Silver
Tusa DC Hunter
Tusa DC Sapience
Uemis SDA
Uwatec Aladin One
Uwatec Aladin Prime
Uwatec Aladin Tec 2 G
Uwatec Galileo Luna
Uwatec Galileo Sol
Uwatec Galileo Terra
Uwatec Smart Tec
Uwatec Smart Com

 

[scubadada]

...I recently ran across this scientific single dive square profile test of 43 dive computers from 2014 http://www.sams.ac.uk/martin-sayer/pdfs/decompression-management-43-computers It was interesting that the NDLs for 30, 40 and 50msw were all within a few minutes. In my mind this makes it very hard to claim one algorithm (computer) is significantly more conservative or liberal than others. Maybe it becomes apparent once you start doing multiple dives?...

I see it differently. The 4 Oceanic computers running DSAT at the bottom of table 3 have NDLs that are, on average, 23-28% longer than the 2 Cressi computers running their version of RGBM near the top of the table for dives of 20, 30, and 40 meters. As pointed out, these are single, square profile dives and do not give insight into computer performance for repetitive dives. They do point out significant differences between the more conservative and the more liberal computers of the bunch.

 

[Dive Cochran]

Unfortunately divers are put in the position of having to "best guess" this. Current algorithms cannot and do not account for physiological factors. In layman's terms, the computer has no idea how you are feeling.....

R..

Really...............

 

[RTC'83]

Well, I appreciate all of the responses. I think all of them were very educational. Got a lot to learn.
I am by nature a conservative, I am in my Mid 50's in good health and lead a healthy life style.
My upcoming trip will consist of maybe 2 - 3s dives a day for about 6 days.
I will be diving with a well known outfitter and have gone over my newness to diving with the DM.
My intention with asking my question was to learn from you guys,(experience divers) of were to start.
Some of your answers have given me a place to start.
Thanks,

 

[DevonDiver]

Does anyone have any ideas / recommendations / scientific info on "how much" and "which part of the dive" to back off NDL?

As vacation divers we do repetitive multi level dives. Generally a max of 4 dives a day, generally no deeper than 70 feet, with average depth above 50 feet. I personally never see an NDL of less than 10 minutes. Is that conservative?

How far away from NDL is considered conservative? I would claim 1 minute for the majority of the dive means you are aggressively riding your NDL. Is 5 minutes NDL at the deepest part conservative? How about 10 or 20 minutes?

What about multi level dives? Is 5 minutes NDL at 80 feet at the start of the dive any different than getting to 5 minutes NDL at 45 feet near the end of the dive?

It's important to understand WHAT an NDL represents.

Simplistically, it's the permissible time allowed by the LEADING tissue compartment; which must arrive at the surface at, or below, a maximum pressure gradient between dissolved inert gas and ambient pressure. (for dissolved gas models)

What's relevant to your questions is this issue of the LEADING tissue compartment.

In single dives, or very light diving, the LEADING tissue compartment is usually a FAST tissue (defined by the half-time speed of the modelled tissue).

Fast tissues are forgiving... hypothetically imagine the arterial and venous vascular network .. and well oxygenated muscle tissues. These on-gas quickly, but off-gassing just as rapidly. If bubbles did form, they're handled quite well by the physiology.

With fast tissue NDLs.... you can be sure that it'll offgas effectively very soon after you leave the bottom. The NDL will leap up substantially. There isn't a lot of inert gas diffused into slower tissues... the overall loading is light.

Now consider repetitive deeper dives.. and/or multi-day diving scenarios.

Repeated exposures allow the saturation of slower tissues to creep up. The volume of dissolved inert gas becomes higher.

Whilst the NDL still means you're clear for a direct ascent at/or below a designated speed the maximum surfacing gradient now rests with a slower tissue.

The fast tissues will clear quickly on ascent... but the slower tissues wont.

Indication of this is where you ascend from bottom and the NDL extends minimally as depth gets shallower. This is common when people 'ride the NDL' up to the surface..

I personally think that this behaviour (riding up the NDL) is particularly malignant from a DCS perspective.

It's my gut feeling that 'riding up the NDL' causes a preponderance of these 'undeserved hits'.... where divers get DCS even when never exceeding an NDL.

Riding up an ascent on the limit of an NDL means that tissues are switching roles as the 'leading compartment'..moving ever slower. It's like a relay race.

What's happening physiologically (as it's modelled) is that you are still ON-GASSING into many slower tissues, even as you ascend.

Naturally, it's not clever to on-gas more nitrogen as you head to the surface... and an increasing volume of nitrogen dissolved your body equates to an increasingly risk of DCS.

In short... you need to understand what the algorithm is trying to model and predict physiologically in your body....before you can start to appreciate what is... and what is not.. a conservative NDL at a given time in the dive.

A good tip is to do very distinct multi-level profiles. Don't just meander up to the surface without a plan. It's better to ascend, stop, ascend... than it is to crawl up a slope.

Dont ride up your NDL. If your NDL doesn't increase significantly as you ascend a moderate distance... then take the hint that you're not efficiently off-gassing... and get the heck outta Dodge! i.e. stop dawdling and get yourself up to a depth that gives you a very extensive NDL again.