As a rec diver, what to do if I breach my computer's NDL???

[Rhone Man]

An "algorithm" sounds very scientific, but don't confuse precision with accuracy. They are all compiled by reverse engineering incidence of DCS into mathematical formulae.

But as everyone knows there are huge random elements you cannot account for. Anecdotally people tend to get "skin bends" before they get "joint bends". But there are any number of incidents where people have had joint bends without any skin presentation. Similarly "the chokes" is pretty rare, but again, has been known to occur without other symptoms presenting. If the rate at which tissue absorbed and off-gassed was linear, that should never happen - symptoms would always present in order. But they don't. There is simply a randomness to incidents of DCS that we can't fully account for. Hence algorithms which are designed to be a "best guess".

 

[uncfnp]

That's regurgitating manufacturer blurb. What can you actually tell us about the algorithm?

...and what on God's great earth is a "smoother deco process"?!?

I wondered what you would make of it. Just more blurb at this point. Trying to Learn how to run a simulated deco dive plan with it but no luck yet...

In the manual it says:

"CRESSI RGBM algorithm. New algorithm created through a collaboration between Cressi and Bruce Wienke, based on the Haldane model, integrated with RGBM factors for safe decompression calculations in repetitive multi-day dives.
Tissues: 9 with saturation half-times between 2.5 and 480 minutes;
Dive program: Full processing of dive data, including decompression if applicable, for every dive performed with Air or Nitrox."



Here is the info given for dives beyond NDL:

"DIVING OUTSIDE THE NO DECOMPRESSION LIMITS
DANGER: do not use this device to dive beyond the no decompression limits! Cressi discourages the use
of this device for decompression dives.
However, if during the dive, due to negligence or emergency, you have exceeded the no decompression limits, Giotto will provide specific information related to a correct ascent and to the decompression information.
Upon violating the limits, the computer will issue an audible alarm and the screen on the display will change to look as indicated in the following picture providing the diver with the following information:
1) Stop icon with the word DECO showing that the no decompression limits have been violated and that decompression must be performed. The arrow indicating UP will flash.
2) Depth of first decompression stage (the deepest), indicated in metres (m) or feet (ft). This can vary from a maximum of 24 m to a minimum of 3 m by steps of 3 m in 3 minutes.
3) Duration in minutes for the first decompression stage (the deepest).
4) TOTAL icon, indicating the total ascent time, that is the time required to ascend to the deepest stage, respecting the ascent rate, PLUS the stop time at that depth and any other subsequent stop (indicating the deep stop if necessary), PLUS the safety stop time, PLUS the time required to reach the surface after completing the decompression stages.
5) “DIVE.T” icon giving the total time spent while diving.
REMARK: In this case, the maximum depth can be displayed by pressing the right button.
REMARK: In case the deepest stage is a Deep Stop, by pressing the right key you will see, always on the right top corner, the depth and duration of the first stage, not Deep Stop."

I have yet to dive with it so no personal experience. Sounds as if it will give me multilevel ascent for deco though...

 

[DevonDiver]

In the manual it says:

"CRESSI RGBM algorithm. New algorithm created through a collaboration between Cressi and Bruce Wienke, based on the Haldane model, integrated with RGBM factors for safe decompression calculations in repetitive multi-day dives.
Tissues: 9 with saturation half-times between 2.5 and 480 minutes;
Dive program: Full processing of dive data, including decompression if applicable, for every dive performed with Air or Nitrox."

It's a proprietary RGBM. As others have mentioned in the thread already (for Suunto RGBM) - they'll keep the tweaks secret, if there are any tweaks at all. I'd expect the model to get you shallow and hang you there.... unless it's tracking substantial residual nitrogen over a multi-day profile. That's where the RGBM would kick in - to account for the longer compartment half-times.

It's not "full processing of dive data...for every dive..." because it later says: " a maximum of 24 m to a minimum of 3 m by steps of 3 m in 3 minutes."

If they've limited the deepest stop to 24m, then there are obviously some significant limitations to the model. With VPM-B or ZHL16, on a 50m dive with any meaningful bottom-time (say 20min+) I'd be expecting my deco stops to start from 27m.

So... it's a good illustration of the difference between a recreational and technical model/computer. This algorithm wouldn't do the job for a typical Tec50/Extended Range level dive.

Here's a Tec50 dive (20min@50m) using VPM-B:

Stop at 27m 0:27 (23) Air 0.77 ppO2, 27m ead
Stop at 24m 2:00 (25) Air 0.71 ppO2, 24m ead
Stop at 21m 2:00 (27) Air 0.65 ppO2, 21m ead
Stop at 18m 3:00 (30) Air 0.59 ppO2, 18m ead
Stop at 15m 4:00 (34) Air 0.52 ppO2, 15m ead
Stop at 12m 5:00 (39) Air 0.46 ppO2, 12m ead
Stop at 9m 8:00 (47) Air 0.40 ppO2, 9m ead
Stop at 6m 11:00 (58) Air 0.34 ppO2, 6m ead
Stop at 3m 20:00 (78) Air 0.27 ppO2, 3m ead
Off gassing starts at 34.6m


The same dive planned with Buhlman ZHL16-B +GF (20/80):

Stop at 27m 0:27 (23) Air 0.77 ppO2, 27m ead
Stop at 24m 1:00 (24) Air 0.71 ppO2, 24m ead
Stop at 21m 1:00 (25) Air 0.65 ppO2, 21m ead
Stop at 18m 1:00 (26) Air 0.59 ppO2, 18m ead
Stop at 15m 2:00 (28) Air 0.52 ppO2, 15m ead
Stop at 12m 4:00 (32) Air 0.46 ppO2, 12m ead
Stop at 9m 5:00 (37) Air 0.40 ppO2, 9m ead
Stop at 6m 8:00 (45) Air 0.34 ppO2, 6m ead
Stop at 3m 18:00 (63) Air 0.27 ppO2, 3m ead


Here is the same dive with neo-haldanian and RGBM (+2 conservatism)... it's a close as I could get using laptop software (which, in itself, illustrates one problem with 'rec' deco machines that use proprietary and secretive algorithms...):

Stop at 27m 1.00 (24) AIR
Stop at 24m 1.00 (25) AIR
Stop at 21m 1.00 (26) AIR
Stop at 18m 2.00 (29) AIR
Stop at 15m 3.00 (32) AIR
Stop at 12m 4.00 (36) AIR
Stop at 9m 4.00 (41) AIR
Stop at 6m 8.00 (49) AIR
Stop at 3m 9.00 (58) AIR

 

[uncfnp]

Great analysis. Thanks Andy.

 

[AfterDark]

I use a PDC however I keep track of time with a watch and my trusty old analog depth gauge has a max depth indicator. The back of my old console has old air navy tables on it. I would simply take my max depth and total time refer to the navy tables and add a 3-5 minutes to each stop. In most cases 10-15 minutes at 20-10 feet is going to more than cover a few minutes over the NDL at any rec depth. I carry the air to cover that contingency, do you?

 

[victorzamora]

I carry the air to cover that contingency, do you?

I do. The question I've got for you is this: Do you have enough redundancy that with an inattentive/lost buddy and a blown o-ring you could still surface safely?

 

[AfterDark]

I do. The question I've got for you is this: Do you have enough redundancy that with an inattentive/lost buddy and a blown o-ring you could still surface safely?

I dive ID's and sling a pony if I'm not diving solo so the answer is depending on the stage beginning, middle or end of the dive; who's O-ring mine or my lost buddies? Probably yes. I change my O-rings once per year sometimes twice so unlikely it's mine.

 

[drbill]

I have no formal deco training but have been doing deco dives for decades. Not suggesting that for anyone else of course. Once I started diving with computers (I believe in redundancy), I generally doubled or even tripled the deco obligation indicated on the more conservative unit whenever I had sufficient gas to do so. So far, so good. I think for a decent diver, going into minor deco is more of a psychological issue than anything else. I can't comment on it for indecent divers though.

 

[Rhone Man]

I can't comment on it for indecent divers though.

If you know any indecent divers, send 'em my way!

 

[PsyWulf]

I think for a decent diver, going into minor deco is more of a psychological issue than anything else

Admittedly when I started the idea of going into even 1 second of Deco was drilled into us as cardinal sin nr1. Never really told us what to do _in case_ since they trained us to avoid it like our lives depended on it. Only after my deep speciality did I actually come to learn it isn't the end of the world,and certainly not hard to resolve