Variation in NDL values: Comparing different dive tables and dive computers

[BDSC]

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Computers do, in fact, vary widely;

Just curious here. Can anyone give an example of how two different computers taken down on the same dive would "vary widely"? In other words, are there two different computers out there that if I were to take them down on the same dive I'd get a big difference in bottom time or whatever?

I ask not to challenge what is being said but because I really have no idea and I'm not really directing my question to Thal but to anyone who might have an example.

Thanks

 

[Blackwood]

Just curious here. Can anyone give an example of how two different computers taken down on the same dive would "vary widely"? In other words, are there two different computers out there that if I were to take them down on the same dive I'd get a big difference in bottom time or whatever?

I ask not to challenge what is being said but because I really have no idea and I'm not really directing my question to Thal but to anyone who might have an example.

Thanks

If you have a Buhlman and a VPM computer and run a dive including deep stops, you may see distinctly different results.

I can't quantify it for you, only report having seen it. I did a dive with a buddy once (first of the week for both of us) that my old Oceanic considered 'within NDL' for which his VR3 wanted something like 20 minutes of deco (don't recall the exact amount, just that it seemed inordinate). Shrug.

RGBM will probably vary as well.

 

[Quero]

Just curious here. Can anyone give an example of how two different computers taken down on the same dive would "vary widely"? In other words, are there two different computers out there that if I were to take them down on the same dive I'd get a big difference in bottom time or whatever?

I ask not to challenge what is being said but because I really have no idea and I'm not really directing my question to Thal but to anyone who might have an example.

Thanks

Manufacturers use different algorithms for their calculations, but there aren't very many mathematical models out there, so the differences are not as huge as might be imagined. As an example, the manufacturers of my dive computers are Suunto and Uwatec. Suunto uses an RGBM model and Uwatec uses a Buhlman ZHL-8 model. In practical terms, the Uwatec allows for longer bottom times but much slower ascent rates than the Suunto, whereas the Sunnto allows shorter bottom times but isn't as fussy on the ascents. I usually use a Uwatec, but most people I dive with, including my students, wear Suuntos. So I need to be aware that their bottom times may be a few minutes shorter than mine, according to the computer calculations. The differences are never huge, though they may be significant, so when planning the dive, it's important to do so using the computers of both dive buddies so that the more conservative profile will be the one the dive plan is based on.

 

[notivago]

Just curious here. Can anyone give an example of how two different computers taken down on the same dive would "vary widely"? In other words, are there two different computers out there that if I were to take them down on the same dive I'd get a big difference in bottom time or whatever?

I ask not to challenge what is being said but because I really have no idea and I'm not really directing my question to Thal but to anyone who might have an example.

Thanks

Although what matter for the discussion at hand is the user interface variation, the result under water also vary from computer to computer, even the same brand/model can vary due to the algorithm in use.

Have a look at Dive computer - Wikipedia, the free encyclopedia.

Mine for example can be configured with 3 different algorithms, so if you buy 2 of them, exactly the same computer and set each with a different algorithm, you will get different results for each one.

 

[BDSC]

I guess one has to define what they mean by "Varies widely". I would have thought that most computers, say for rec diving, would be within a few minutes of each other as far as bottom time goes. I was just wondering if there are computers out there that may give you say 25 minutes at some depth and then another would give maybe just 15 minutes at that same depth. To me that would vary widely.

 

[Quero]

I guess one has to define what they mean by "Varies widely". I would have thought that most computers, say for rec diving, would be within a few minutes of each other as far as bottom time goes. I was just wondering if there are computers out there that may give you say 25 minutes at some depth and then another would give maybe just 15 minutes at that same depth. To me that would vary widely.

Here's a chart comparing maximum NDL times for a single dive according to a number of tables and decompression models. You will easily notice how "conservative" the Suunto is in terms of NDL compared to other computers. (The Uwatec display doesn't go above 99 minutes, which is why the chart reads 99+ at the top.)

Now, as to the algorithms used in particular computers, each make/model has only one basic algorithm; you can see the algorithm used by each major manufacturer listed above the name of the decompression model. This can be a little confusing because some manufacturers misleadingly claim that their computers have more than one algorithm. Oceanic, for example, markets its computers as "dual algorithm," but this is merely a function allowing users to change the parameters and not one of selecting an entirely different algorithm. That is, user settings can be applied to make the computer calculations more conservative, but this does not mean that there is an entirely different algorithm--just that the fundamental algorithm is adjusted, as if it were calculating for a deeper dive than actually takes place, for example. A user might adjust the settings towards more conservative calculations for cold water, hard work, aggressive dives (or multiple dives over multiple days), risk factors such as age or fitness level, or even simple preference.

 

[Puffer Fish]

I happen to own several computers, so I just checked a Galileo, a nitek Duo and two Oceanic (what I had near me)...and my normal dive buddy dives a Suunto.

Only the Oceanic matches the chart above.. the Galileo and the nitek Duo are both 19 minutes at 90 ft, and the Suunto is 17 minutes. The Suunto has adjustments to the next dive based on accent rates and surface interval (too short incurs an extra reduction on the next dive), while the Galileo has adjustments to the dive based on water temperature, breathing rate and if using the heart monitor, heart rate.

Oceanic has none of those on the two versions I have.

It is not so much the first dive, but if you continue making dives, the differences between say an Oceanic and Suunto (particularly if you have fast accents (more than 30 fpm) and a shorter surface interval than it likes (there is an icon to warn you) become rather amazing.

As we make a lot of dives together, if the Suunto diver follows the computer, and the Galileo diver does not exert themselves, the second 80 ft dive will have a difference of around 5 minutes. To a Oceanic, this would be over 20. But make any mistake with a Suunto, and the rest of the dives that day will be really short.

I believe the Unwatec also has some sort of penalty for rapid accents, but as I do trimix accents even when not on trimix, I don't see them.

What I notice is that there are places were tables work great (or memory) and there are places where tables suck. I remember last year, with the oil spill, where we would just go around until we found a spot to dive that did not involve diving thru oil...75ft.... 105.... 85 who knows. And your next dive would involve the same searching. We would all sit with our computers in planning mode for each spot, and adjust as needed.

But I have a major issue with trying to teach both. More and more I see divers that don't get enough time to learn even the basics.

This last Monday was a perfect example. On our boat we had one diver without a computer... so the boat loaned them one. The person stated they knew how to use it, but they also had PADI tables with them. The first dive was on a ledge... that has roughly 8 ft of relief... the bottom is at 86 ft and the top is around 78 ft. One can make the whole dive easily staying above 80 ft.. or one can look for stuff under ledges and go to the 86 ft level... or some pattern of that. The computer was a Uwatec prime.. so one could expect a dive between 22 and 28 minutes.

The diver joined us during the dive and my buddy, at the 28 minute mark, saw that our diver was still with us, and indicated to me he was going to take them up. I should note that the diver did not stay up on the high side of the ledge.

I continued on for my normal 50 minute dive... and went up with a friend that joined me for the rest of the dive.

On the surface he told me that the diver ascended without a safety stop, and seemed to be going up way to fast. The computer was locked out. When I asked what happened, I was told the computer locked out because they went up too fast, and then told me they did not actually understand what it was telling them.

So, thankfully they seemed ok, and did not do the next dive. The next dive is to the same depth, but on a wreck (so if one wants, they can stay above 70 ft). While we were doing our surface interval, a small private boat arrived and as we entered, so did they. My buddy and I did a 36 minute dive with an average depth of around 78 ft, and started our ascent... to our surprise, there were two new divers on our line with us (on air). I dropped down to make sure they had enough gas (as they had entered the water just before us) and notice the one diver had a 18 deco obligation...they did a 3 minute safety stop and surfaced, where they told the crew on our boat that their computers broke and then they swam back to their boat. Don't have a clue if they had any issues later.

I believe the total number of divers involved were 9 on our boat and 4 on the small boat...with two people that were trying really hard to get a DCS hit. Pretty clear they did not understand the computer, and equally clear they did not understand tables (our other diver commented that they were only below 80 ft for a portion of the dive.. so one can use 80 ft). I know there is no cure for some things, but doing trying to get these people to understand both systems would seem to be outside the range of all the instructors I know (including me).

 

[Quero]

Cappy Happy (Puffer), it was a great idea to check the chart against the actual instruments you own. I first found that table a few years ago, so maybe it's out of date, dunno. The Galileo has probably had software updates in that time, and the Suunto number you provided is within the range listed. Even taking the variance of your numbers into account, however, the chart still shows that there aren't huge differences between one model of computer and the next, though as I said earlier, a few minutes can be significant, precisely because of the risk of getting locked out by blowing a deco stop.

It might be an interesting exercise to update the chart. If different SBers had the urge, there must certainly be enough of us here on the board with lots of different kinds of computers to get quite a lot of data. (But such a project should be its own thread.) I should have posted the original link to the developer of that table and not just the screen shot I made of it, so here it is: Dive Table and Dive Computer Comparison: Is your dive computer liberal or conservative?

 

[Puffer Fish]

You are correct, there have been updates to the Galileo, and I suspect the Nitek.. here are the numbers for these two (one the ones I have)

Feet Galileo (current software) Nitek (unknown, but computer purchased just over a year ago)
30 199 200
40 112 100
50 65 64
60 46 45
70 33 34
80 25 24
90 19 19
100 15 16
110 12 13
120 11 11
130 9 9

These are from the dive planner in each computer... but keep in mind that I know the Galileo can reduce those numbers during a dive, as well as the next dive.

Not sure what the Nitek will do.

Notice the Galileo now shows more than 99 minutes.

 

[notivago]

however, the chart still shows that there aren't huge differences between one model of computer and the next, though as I said earlier, a few minutes can be significant, precisely because of the risk of getting locked out by blowing a deco stop.

Are they not? If I understood well the table(but I could be confused as you think I am always) Oceanic to Suunto ratio of time allowed can be as high as 2:1 that seems a huge variation to me. Besides, the difference carries over to the next dives as the model is calculating N2 absorption and the gassing and degassing is not linear along the time, that means that the time gap would only diverge across repeated dives.

Not to mention that some computers allow you to go a lot deeper than others.