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

[DevonDiver]

... when a rec dives goes into "light deco" is he/she adequately covered by their DC when they follow the computers response?

That'd depend upon how the deco was accrued (depth/time), relative to how it was calculated by the algorithm.

Going slightly beyond an NDL though over-staying at the bottom on a single 25m dive....is very different 'lite' deco from a bounce dive to 50m (which some might classify as 'rec')... is very different to slightly exceeding NDL after a series of multi-day deep no-stop dives...

The variation would come from numerous factors; including the number of tissue compartments used in calculation, the half-times of those tissue compartments, considerations for bubble growth on ascent, special emphasis given to repetitive dives/residual nitrogen and surface interval credits etc etc etc..

A rec diver, on a rec computer, on a typical rec-profile dive (accidental overstay) is probably "adequately covered" simply because the normal parameters of their dives are so forgiving... and the computer models are supported through large and lengthy sample sizes (statistical proofs). Deep 'bouncers' and aggressive deep-rec multi-day series divers would be pushing the statistical curve and boundaries of the model somewhat more...

At the end of the dive, is that diver on an equal footing as a diver that did the same dive with a tech computer?

It'd depend how the tech diver configured their tech computer; algorithm choice, gradient factors, conservatism level etc etc

I use VPM-B +2. That'd typically give me longer deco for a given dive compared to Suunto proprietary RGBM. It'd also give me more stops on the way up... a better 'curve'. I can't give a statistical answer for 'equal footing' (assuming you meant people who get bent), but I can say I feel much better...more vital, more fresh, after a properly shaped curve on ascent.

Here's a Buhlman ZHL-16B +GF (Gradient Factors) 20/80 running a very simple, single 30m/100ft dive for 25 minutes (just 5 minutes past the NDL on the PADI RDP). You can see what it gives you:

Stop at 15m 0:20 (27) Air 0.52 ppO2, 15m ead
Stop at 12m 1:00 (28) Air 0.46 ppO2, 12m ead
Stop at 9m 1:00 (29) Air 0.40 ppO2, 9m ead
Stop at 6m 3:00 (32) Air 0.34 ppO2, 6m ead
Stop at 3m 6:00 (38) Air 0.27 ppO2, 3m ead

Total ascent time is 9 minutes, with stops at 12, 9, 6 and 3m.

I'd hazard a guess that most recreational dive computers would simply drag you straight to 3m and sit you there for a few minutes... compare on your own computer/s.

Here's an even more obvious example, using VPM-B +2 (my normal preference) on the same parameters (30m/100ft for 25 min):

Stop at 12m 1:00 (28) Air 0.46 ppO2, 12m ead
Stop at 9m 4:00 (32) Air 0.40 ppO2, 9m ead
Stop at 6m 6:00 (38) Air 0.34 ppO2, 6m ead
Stop at 3m 10:00 (48) Air 0.27 ppO2, 3m ead

Total ascent time is extended to 23 minutes, with stops at 12, 9, 6 and 3m. Longer stops from deeper. Again, cross-reference that against your computer - chances are it'd bringing you straight to 3m and hold you there. No curve whatsoever, under those dive parameters. Something a lot more like this:

The same is true, of course, with the RDP table mandated 'emergency deco'. It'd look just like the above profile (except, obviously, the ascent line would appear near vertical, if you were doing an 18m/min ascent, rather than the 9m/min these tables show...scary). For that 5 minute over-stay, there'd simply be a 15 min hang at 5m.

Using VPM-B I would (and do) multiple deco dives in a day. I'd feel fresh and vital afterwards. Using a 'rec' computer for that same dive, I'd seriously consider the prudence of diving again that day... and I wouldn't make plans for a big night out afterwards. Using the PADI RDP rules... I'd be sitting around somewhere close to an O2 bottle for a few hours after...

Are not the underlying algorithms equal?

No. Recreational computer algorithms are generally concerned with emergency deco. That means getting you up quickly under 'reasonable' safety parameters, in the assumption that you didn't plan to be there. That'd mean an assumption that you hadn't planned gas supply sufficiency, were not using optimal ascent gases and didn't have equipment to deal with any problems that might arise. As boulderjohn rightly mentioned, they are "not intended for planned decompression and should be used for decompression only in cases when it was not intended".

Mathematical platform for studies on VPM and Buehlmann decompression algorithms

Decompression Comparison between Departure, VPM and RGBM

DECOMPRESSION MODELS: A Review of Basic Physics and Decompression Theory

Here's a useful quote, illustrating the difference between models (even variants of the same model...):

"Planning considerations in VPM
...VPM-B and VPM-B/E models for planning dives. The VPM-B is the current model in wide spread use today. VPM-B/E is an extension made for divers undertaking very deep and long dives. It generates plans that simulate an overlapped bubble model and a Haldane plan, for time times when maximum safety is required. For most divers, the B and B/E plans are the same, because B/E only begins to deviate after large gas loads (90-100 mins deco required)." Reference: Multi-Deco Frequently Asked Questions

 

[Tigerman]

I wonder what people would call an Aladin 2g computer with its buhlman zh-l8 adaptive algorithm, gauge mode, 2 gas switch, Profile Dependent Intermediate stops, 120m max depth, no trimix options (guess gauge mode would be a good plan there ) and a VERY recreational pricetag..

 

[uncfnp]

I wonder what people would call an Aladin 2g computer with its buhlman zh-l8 adaptive algorithm, gauge mode, 2 gas switch, Profile Dependent Intermediate stops, 120m max depth, no trimix options (guess gauge mode would be a good plan there ) and a VERY recreational pricetag..

Or my yet untried Giotto...

From dive-computer-wizard.com.
Benefits of using Cressi Giotto dive computer:

-you will get dive computer with one of the best price-functionality ratio;
-you will have instrument with 2 Nitrox gas mixes - smoother DECO process and intro into the technical diving;
-you will easily be able to replace battery yourself in Cressi Giotto;
-you will definitely be able to save money on this double Nitrox gas mix dive computer!

Bold added by me.

 

[Lorenzoid]

Holy cow, Andy, the partial differential equations in those articles make me want to give Mark Powell a medal. Those are the sort of articles people formerly had to wade through before Deco For Divers?

 

[boulderjohn]

Holy cow, Andy, the partial differential equations in those articles make me want to give Mark Powell a medal. Those are the sort of articles people formerly had to wade through before Deco For Divers?

I agree.

Have you read anything by Bruce Weinke, the creator of RGBM? I have tried. I recently told someone that I think no one disagrees with him because no one really understands what he said.

 

[Mr Carcharodon]

]

I'm not sure exactly how you distinguish recreational computers from others?

A strong indication is when the recommeded deco is decoupled from deco theory and physics. Arbitrary penalties for going a bit into deco make no sense for a computer and suggest the programmer wasn't trying or did not know what they were doing. Tables do that but that is just a result of the limitations of fitting a plan on the limited space of a card.

 

[DevonDiver]

Holy cow, Andy, the partial differential equations in those articles make me want to give Mark Powell a medal. Those are the sort of articles people formerly had to wade through before Deco For Divers?

Yes, that's what was available. Mark deserves a medal.

 

[dmoore19]

1. Before entering the water, know what your computer is going to display and what you are looking at.

2. Follow the information given and surface in the manner the computer suggests, stopping as directed and completing as much of the deco obligation as possible with the gas you have.

3. If unable to complete the required stop, let the crew know of your problem and ask for oxygen. Be very aware of the symptoms of DCS a do not deny their existence.

4. This should be number 1, Pay attention and do not get yourself into that position to begin with.

 

[DevonDiver]

From dive-computer-wizard.com.
Benefits of using Cressi Giotto dive computer:

-you will get dive computer with one of the best price-functionality ratio;
-you will have instrument with 2 Nitrox gas mixes - smoother DECO process and intro into the technical diving;
-you will easily be able to replace battery yourself in Cressi Giotto;
-you will definitely be able to save money on this double Nitrox gas mix dive computer!

Bold added by me.

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"?!?

 

[dmoore19]

I have read Deco for divers and have a slight understanding of Deco. Nothing as in depth as what is being discussed here. While I know very little I have tried to understand the algorithms. Sorry for my less than advanced additions to the thread.