So you want to buy a new computer?

[rongoodman]

The standard answer for your question is "Deco for Divers" by Mark Powell. Most of the algorithms mentioned above are proprietary, including Suunto's version of RGBM. The two that are more or less standard and implemented on a number of platforms are VPM and Buhlmann GF, making it easier to coordinated with a buddy's planning software or computer.

 

[Goodtime Adventures]

Awesome information with regards to computer choices, conservatism etc. I shall be bookmarking this to show students. Thanks for taking the time to put this together, its excellent

 

[jacktrner]

Thanks for the info.

 

[dylan48]

Great information! Thank you so much!

 

[KevinA]

Thanks for putting all this information together. I'm new to SCUBA and doing research for my first equipment purchases. I've been considering the Atomic Cobalt 2. It looks amazing (I just wish it were wrist-mounted!) Does anyone know how the Atomic algorithms compare with the other brands? conservative, liberal, middle-of-the-pack?

 

[stuartv]

I sent this out (somewhat paraphrased) in an email to some friends earlier today. I thought it might be worth also posting here, for posterity.

Caveat: Some of this is really only relevant to Rec diving to NDLs, so take away what is useful to you and ignore the rest.

On algorithms:

Here is a link to a post from RonR, who IIRC (I *think*) from other posts is an engineer at Atomic Aquatics. The summary is that different computers can implement their algorithms differently, even if they are “the same” algorithm. So for example, one computer that implements Buhlmann ZHL-16C might still give you a different dive profile than another computer that also implements Buhlmann ZHL-16C. Needless to say, computers with different algorithms (Buhlmann, RGBM, VPM, Haldanean/DSAT, etc), can give very different dive profiles.

Read RonR’s post for the full explanation:

http://www.scubaboard.com/forums/co...-leonardo-too-conservative-4.html#post7342293

I think it’s also worth noting that many computers use a proprietary implementation of their algorithm. So, attempting to predict what the computer will do by modeling your plan on your desktop computer may or may not produce exactly the same results, for the same reason above. The best way to be assured of the best match between desktop computer and dive computer is by using the desktop software that is provided by the OEM of your dive computer (IF they do in fact provide any). Aftermarket software, for example V-Planner, may use the same algorithm as your dive computer, but it could still be implemented differently, thus giving you a different dive profile.

On conservatism:

The first post in this thread has a simple chart comparing 1st dive NDLs for a variety of different types of computers. One example from the chart: At a depth of 100’, USN and NOAA tables allow 25 minutes, PADI RDP allows 20, the most liberal dive computer allows 19, and the most conservative dive computer allows 8 minutes. Of course, pretty much any dive computer will let you adjust it to make it more conservative, so you CAN use the most liberal computer and still have it limit you to not come so close to the PADI 20 minute limit (as one example), if you want.

However, my research convinced me that looking at what the computers do on repetitive dives in the same day is at least as important to look at. For that, Scuba Magazine did a couple of good articles where they put all the computers they were testing into a hyperbaric chamber and replicated doing 4 dives of different profiles, with surface intervals in between. They logged the NDL shown on each computer for various simulated depths.

Their 2014 test results are here:

http://ads.bonniercorp.com/scuba/PDF/ScubaLab-Computer-Test-September-2014-data.pdf

which is from this article: SCUBALAB 2014: Dive Computer Review - Page 2 | Scuba Diving

As one example, this test shows a 1st dive to 100’, then a 60 min surface interval, then a second dive to 70’. After EDT (Elapsed Dive Time, on the 2nd dive) of 18 minutes, at 70’, the most liberal DC shows an NDL of 20 minutes and the most conservative shows 12 minutes. The differences are even bigger when the diver starts to ascend some and spend time at shallower depths.

The differences get even more dramatic on the 3rd dive, which is after a 120 minute SI, to 80’, then gradually working up to 40’. After EDT of 36 minutes, at a depth of 40’, the most liberal computer allows 56 minutes of remaining time before the NDL, and the most conservative allows 2 minutes.

56 minutes versus 2 minutes is a pretty big difference!

Scuba Magazine’s test results from 2007 (I think) are here:

http://www.scubadiving.com/files/old/images/pdf/scubalab_200706_10_new_comps.pdf

which is from this article: dive computers: scuba gear reviews | Scuba Diving

This one is kind of more of the same, but with some different computers, as the first article.

I hope this helps someone as much as finding it all helped me when I was shopping for my computer.

 

[stuartv]

RonR just posted another very insightful post regarding the implementation of algorithms in dive computers and how they can vary based on choices the implementer makes, while still complying with the actual algorithm. I think it's worth also adding a link here, for posterity.

http://www.scubaboard.com/forums/co...uter-divemaster-internship-4.html#post7388301

 

[stuartv]

As a newbie diver, I want to add another post to this thread, addressed to other newbies who may come along after me and bother to read this thread.

I have been diving for 9 months and have just completed my 35th dive. I actually did 5 dives off the Outer Banks of North Carolina this weekend, so dive #s 31 - 35.

In the 9 months since I got certified, I have had a number of discussions on this board with older, more experienced members about dive computers. Specifically, about dive computers for new Recreational divers and the importance (or non-importance) of the algorithm the computer uses. I have been told time and again that the relative conservatism or liberalism of the computer does not matter - especially to new divers buying their first computer. Many of the "old hands" on here insisted that one reason it doesn't matter is that I (and other new divers) won't be hitting my "No Decompression Limits" anyway because I'll run out of air first. Thus, liberal or conservative doesn't matter because I'll be coming up for air before even a conservative computer tells me to anyway.

This post is to say that, despite all the words of wisdom that liberal or conservative does not matter, I did a lot of research before I bought my first dive computer and I bought one that, based on my research, is among the most liberal computers you can get - an Oceanic Atom 3.0. And I dive with it set to the more liberal of the two algorithms it supports, and with no "conservatism factor" set.

With that (most liberal) computer, I did 5 dives that past weekend, with a shallowest dive of 64 feet (the wreck of the Suloide), and 3 dives over 110 feet (the U-352, the Spar, and the Aeolus). And on all 5 dives, I went right up to within 1 to 3 minutes of my NDL before beginning my ascent, diving Nitrox mixes from 28 to 31 percent, depending on the dive. The least amount of gas I got back on the boat with was 920 psi. The most was 1440 psi (I might have had a little more NDL than 3 minutes left on that one when I began my ascent - but it was still a 28 minute dive with an average depth of 75 feet and max of 111). This data is all according to my data downloaded from my Atom to SubSurface, which includes gas data from the wireless Air Integration transmitter.

Now, I'm not some Superman of diving gas efficiency. I simply chose to find some good deals and buy HP120 tanks for myself. But, I could have used 100s and still had more than 500psi left on even the dive where I used the most gas (78 cu ft).

The point:

I feel vindicated in saying that even a brand new diver should consider the algorithm used by any computer they are contemplating to purchase. They should educate themselves on how THAT computer performs on calculating NDLs for initial dives and repetitive dives (i.e. second and subsequent dives in the same 24 hour period). It's not the black magic that some preach. it's math and science, and the data is out there. And potential computer buyers should be aware of the ramifications if they choose to buy a computer that is "more conservative."

I don't think I'm anything special. Therefore, I think that there will be plenty of other new divers like me that will come along and who would actually benefit (by getting more bottom time) from choosing a computer that uses an algorithm that will allow them more No Deco Limit time. They may not get that benefit in their first 10 dives. Maybe not in the first 20 or 30 or 40. But, why spend money on a computer that you'll outgrow after even 50 dives? Or at least, why do that unwittingly? Based on the research and lab testing, which I posted links to before, I am really glad that, when I did my second dive to 112' yesterday, after a 1:30 surface interval, I was able to get an enjoyable amount of time on the Aeolus - which I do not believe I would have gotten if I had had certain other "more conservative" recreational dive computers.

To be extra clear, before people start piling on saying I'm an ignorant idiot, I am NOT saying that people should only buy the most liberal computer. I am saying that people should have all the info available to them, so that they can make THEIR OWN INFORMED DECISION about what computer to buy and why. Telling people "don't worry about that. It won't matter to you for a long time" is peddling pablum. It seems to me that scuba diving should absolutely be about people learning about, making, and taking responsibility for their own decisions - not "trusting" the advice of more experienced people JUST because that person has more pages filled out in their log book. People whose advice is "trust me. This is what you want. That difference there doesn't matter for you," with no actual explanation of why - or worse, explanations that don't actually survive contact with reality (e.g. "that doesn't matter because you won't be hitting your NDLs any time soon"), are fostering a culture of ignorance and "trust me" divers. New divers should "trust" the old hands because those old hands give them logical, fact-based explanations for why they endorse a certain way of doing something. Not because "I'm more experienced and I say so."

I may be one, but I do not believe all new divers are stupid idiots. And I believe that talking to them as if they are - which seems to be the default mode for a number of people on here (presumed an idiot until proven otherwise) - does a disservice to new divers, to ScubaBoard, and to the entire scuba community.

And that is all just the opinion and experience of one newbie diver. So, take it for what it's worth. Which is to say, DON'T take my word for it. Do your own research and make your own decisions about what's important to you. Don't just blindly accept anyone else's word (including mine) about what is important. I think it's ALL important - right up until YOU decide that it's not important for YOU.

 

[Dr. Lecter]

I think that there will be plenty of other new divers like me that will come along and who would actually benefit (by getting more bottom time) from choosing a computer that uses an algorithm that will allow them more No Deco Limit time. They may not get that benefit in their first 10 dives. Maybe not in the first 20 or 30 or 40. But, why spend money on a computer that you'll outgrow after even 50 dives? Or at least, why do that unwittingly? Based on the research and lab testing, which I posted links to before, I am really glad that, when I did my second dive to 112' yesterday, after a 1:30 surface interval, I was able to get an enjoyable amount of time on the Aeolus - which I do not believe I would have gotten if I had had certain other "more conservative" recreational dive computers.

Fair enough, but dive computers don't change physics. When my Petrel running VPM without any conservatism, or Buhlmann with 60/85 gradient factors, puts me into deco before some of the more liberal computers out there say I've run out of NDL (and I know it does next to a Cobalt), I say screw it and do the stops. If I wasn't prepared for performing deco, I'd prefer to end that dive before what most experienced divers would call aggressive algorithms/settings indicate I'm out of NDL time. That doesn't mean you're wrong, but it does raise the question of whether one has "outgrown" a DC just because they have oodles of gas left when they're out of NDL.

Even when the standard tank for many people is an underfilled AL80, though, I agree with you that the whole 'you'll run out of gas long before you hit NDLs' concept is a bit overhyped.

 

[stuartv]

it does raise the question of whether one has "outgrown" a DC just because they have oodles of gas left when they're out of NDL.

Totally fair question. But, my point is that, among recreational dive computers, the reports I have looked at show a wide disparity in NDLs - especially on repetitive dives. People have to make their own decisions. But, when you're talking about a computer that gives you 8 minutes of NDL, where another computer would give you 50 minutes of NDL, I think it's totally legit to think you may have "outgrown" the computer that only gives you 8 minutes.

I do not mean to suggest AT ALL that using up all the NDL on one of the liberal computers means that you've outgrown the computer. I'm just saying it's not so hard (apparently) to be at the point that you CAN do that. In which case, many people might feel like, if they had a more conservative computer, they have outgrown it and start to have regrets about their purchase.

As I have said before, I'm not going to dive past the limits my computer prescribes. So I personally would much rather have a computer that gives me the maximum "safe" (obviously "safe" being a very relative term) NDL and lets me turn up the Conservatism Factor than have a computer that is going to limit me and tempt me to think something like "my buddy's computer is giving him another 10 minutes, we've been together the whole time, and I have a lot of air left..." And, of course, that's all based on an assumption that the computers in question have an established record of not getting people bent and/or using known algorithms that have been shown to be reliable at not getting people bent.