Past NDL. And then this???
- Thread starter stepfen
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Those are reasonable questions, as long as you understand the bigger picture. It sounds like you are getting there. The most important thing to avoid is the normalization of deviance.
I'm no subsurface expert but did you notice the post by giffenk? That to make subsurface mirror your dive computer you might need to tweak subsurface?
As to why, subsurface just has a more aggressive or less conservative than your dive computer algorithm. Both your computer and subsurface are simultaneously right. Your computer was correct you incurred a 35 min+ deco obligation, and subsurface is correct that you incurred no/or met you deco obligation (provided you loaded all previous dives into it). However, since you only dive with your computer and it is still correct, you need to honor it. If you want to use subsurface as a dive planner, then you'll end up with square dive profiles.
Just because the outcomes are different, it doesn't mean the way each device arrived there is inherently wrong. If you feel your dive computer's algorithm is too conservative, then unfortunately your only choice is to sell it and get something different. As you could take 5 different computers (at least using 4 different algorithms) and get 4 different NDLs and Deco times, each one just looks at the data differently.
Here is some reading on the difference's in computer models:
Algorithms And The Modern Diver | Simply Scuba Blog
A sense of algorithm
That second one I think is very valuable for you to look at and comprehend. They took 9 different computers and logged their outputs, notice how they are very very different. So if you carried just one, do you see how your computer saying you are now in deco but your buddies computer says he is fine are both simultaneously true? Each computer will give you different amount of credit for things like surface interval, nitrox, ascent rates and penalize you slightly differently for time at depth, lack of surface interval, days of repetitive dives.
So why didn't the other dives get into deco on your dive? Without a direct dive profile comparison a few factors are potentially at play. Their dive computer models are more aggressive than yours, conversely yours is more conservative then theirs. Their profiles were ultimately slightly shallower than yours (ie, your ultimately were beneath them by a few feet/m on average, which adds up). Where you always the first one down/last one up? That would give you more of a penalty then your cohorts. Did all the people do all the same dives as you, if not skipping a dive would greatly reduce their NDL comparatively.
Last edited:
DevonDiver
N/A
PM me with your email address and I'll send you some resources to read.
A basic foundation in decompression science will help you understand the answers people will give to your question.
Someone's already mentioned Mark Powell's "Deco For Divers". It's an invaluable read and you'd find it interesting.
The OP's profile said his training is both PADI and IANTD. I assume the PADI certification is OW. With the PADI OW course, you have the choice of taking the table version of the course or the computer version. Can you tell me. @stepfen, which version it was?
In the table version of the course, you do not learn much about computers; you learn how to use the tables, and you learn to use procedures in case you unintentionally go into deco, procedures that would have been completely inappropriate to this incident and that were not mentioned. In the computer version of the course, you learn to use the ascent rate prescribed by the computer, and you learn to follow the directives of the computer in case of an unintentional deco obligation.
In the table version of the course, you do not learn much about computers; you learn how to use the tables, and you learn to use procedures in case you unintentionally go into deco, procedures that would have been completely inappropriate to this incident and that were not mentioned. In the computer version of the course, you learn to use the ascent rate prescribed by the computer, and you learn to follow the directives of the computer in case of an unintentional deco obligation.
I think others have already hit upon this here, but again, it's really important to understand how decompression and algorithms work. I agree that 35 min is a big difference, but given two different algorithms and different profiles underwater, getting a difference like this is not difficult. Here's one example, my dive buddy and I dive really tight, and by that, I mean we are rarely more than 1-2 meters apart. On a recent dive, her computer put her into deco and mine did not. Granted, our deco differences were only a few minutes apart. We run identical computers, with identical algorithms (gradient factors), and we were gas matched that day. When I went back and looked at our profiles, we were right there together throughout the dive, when my depth profile went up, her's went up, when mine went down, her's went down. The only thing I noticed is that on the bottom, she was maybe 1/2 meter deeper in a few places and was about a 1/2 meter below me on ascent. You really couldn't have asked for a tighter profile between two divers and yet we got a difference. So when you add in different algorithms, different bottom profiles, and a really slow ascent rate, yes, you can quickly generate very different deco obligations from other divers doing the same dive.
What is the default setting on Subsurface for its Buhlmann algorithm? Does it use Gradient Factors, maybe 100/100 as a default? Perhaps that is why Subsurface gave the OP no significant deco but his Leonardo did. (I am assuming the OP is using the default Subsurface....)
Isn't this kind of like telling someone to get a different doctor if they don't like the advice provided by the current doctor?
If someone doesn't like the advice provided by their current computer, they don't actually need to buy another computer. They can simply ignore the current computer... right? Essentially the same philosophy, but saves a good deal of money.
Stepfen, if I can offer one bit of advice that I didn't see addressed in any of the responses yet. Get in the habit of planning your dives, every dive. In this case, if you had spent 60 seconds with your buddy before that dive and compared the projected NDLs given by your computers for the next dive, you would have likely seen in advance a noticeable difference. You could then have discussed with your buddy how the two of you would handle the dive, and both of you would have been aware of the situation (i.e. that your NDL was going to be much less than his) before getting in the water.
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