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

[Quero]

Unfortunately my Suunto D4 needs a battery change and doesn't want to let me get into the menus much. But I got out my rental Suuntos, switched to imperial for this exercise, and got the following:
30 feet--(unlimited/limited only by air supply)
40 feet--120 minutes
50 feet--69 minutes
60 feet--50 minutes
70 feet--36 minutes
80 feet--28 minutes
90 feet--22 minutes
100 feet--17 minutes
110 feet--13 minutes
120 feet--10 minutes
130 feet--9 minutes
140 feet--7 minutes
150 feet--6 minutes

 

[Quero]

My Uwatec
30 feet--99+ minutes
40 feet--99+ minutes
50 feet--72 minutes
60 feet--51 minutes
70 feet--36 minutes
80 feet--28 minutes
90 feet--22 minutes
100 feet--17 minutes
110 feet--14 minutes
120 feet--12 minutes
130 feet--10 minutes
140 feet--9 minutes
150 feet--8 minutes
160 feet--7 minutes
170 feet--6 minutes
180 feet--6 minutes
190 feet--5 minutes
200 feet--4 minutes

 

[halocline]

I have noticed that the big difference between the oceanic (modified Haldanean) and the more conservative suuntos in recreational diving is not predicted NDL at the beginning of the dive, but the amount of off-gassing credit as expressed as NDL towards the end of the dive. This is especially evident on repetitive dives and on less-than-ideal profiles, like reverse profiles. For example, I've seen well over an hour of NDL credit on my aeris near the end of a dive while my buddy's suunto might show 10 minutes. The N2 loading bars on each computer typically are much closer to each other, and to me that's much more useful information anyway, especially at the end of the dive.

It is pretty daunting for a new diver who's concerned about DCS risks to see such disagreement among computers that are supposed to "keep you safe," at least it was for me. I think its important to note two things that can make newish divers feel a little better: 1) that there is no-zero-correlation between computer algorithm and DCS incidence in recreational diving, and 2) that extending safety stops at 10-15 feet is remarkably efficient at lowering N2 loading from recreational levels. So, recreational divers trying to be safe would do well to not worry so much about the computer, and spend more time at 15 feet following deep and/or repetitive dives. I've also found that I feel less fatigued on long dive days when I do 10 minute plus stops.

 

[Quero]

Excellent observations. When I'm leading dives I often spend seven to ten minutes at safety stop depth, poking around the reef. My divers sometimes don't even notice we've carried out our safety stop during that last portion of the dive.

Anyway, do you have the NDL times for the Oceanic so that we can include them in an updated chart?

 

[fisheater]

By way of example, a couple of weeks ago my buddy and I dove very close together, though he usually stayed a bit "up hill" of me as we explored some pinnacles.

I thumbed the dive and shot a bag to ascend when my Oceanic VT-3 showed 5 NDL minutes left. We did a normal ascent and a 3 min. safety stop at 15 feet. I surfaced, with a "happy" computer.

He, on the other hand, was told by his Aladin that he had a 10 minute, 10 foot deco obligation. So, he stayed.

(If only he'd let me know that! It would have avoided a whole boat load of worry and emergency response. But that's another story.)


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[Bubbletrubble]

I thumbed the dive and shot a bag to ascend when my Oceanic VT-3 showed 5 NDL minutes left. We did a normal ascent and a 3 min. safety stop at 15 feet. I surfaced, with a "happy" computer.

He, on the other hand, was told by his Aladin that he had a 10 minute, 10 foot deco obligation. So, he stayed.

(If only he'd let me know that! It would have avoided a whole boat load of worry and emergency response. But that's another story.)

@fisheater: Why did you ascend to the surface without your buddy?

 

[explorer08]

If you compare computers it may be that same models give same results as long as you compare them on the surface in dive plan mode. But you should compare them under water as well and than you would wonder. It is nothing exceptional for them to differ in depth up to 2 meters. And this difference produced by the depth sensors will lead to differences in NDL's or deco times as well.

By the way, if you compare different dive tables, you will find a wide variety as well. Several years ago I did such a comparison:
1.D 24m/20min – Surface intervall 1h – 2.D 20m/30min


1. SSI------------1.D N-S-Dive-----2.D not realizable (max. 20 min)
2. Deco 2000-----1.D N-S-Dive-----2.D Deco 17min at 3m
3. DCIEM---------1.D N-S-Dive-----2.D Deco 10min at 3m
4. MN90----------1.D N-S-Dive-----2.D Deco 9min at 3m
5. NAUI-----------1.D N-S-Dive-----2.D Deco 8min at 5m
6. PADI RDP-------1.D N-S-Dive-----2.D N-S Dive recom. safety stop. 3min at 5m
7. US Navy-------1.D N-S-Dive-----2.D N-S Dive
(D=dive; N-S=no stop)

As you can see the differences are fairly good, not for the first dive, but on the second one. That may be amazing for many divers. To understand why these differences exist, you have to have a look behind the tables. And same applies for computers because they are based on the same foundation than tables. For everybody who is interested in decompression theory and what tables and computers are based on, I strongly recommend lecture of "Deco for Divers" by Mark Powell.

But in reality that all doesn't really matter, because everything all tables and computers are based on are theories. To date their exist very little proven scientific knowledge about what really happens in our bodies during compression and decompression. Most is based on empirical cognizance around which scientific explanations have been built and based on these theories algorithms have been developed which are the basics for dive tables and computers. Most confusing is even though some of these theories are contradicting others, all tables as well as all dive computers currently on the market are working. They are daily in use on thousends of dives worldwide. That is proof for the most important concept in decompression theory: What works, works!

So on one side, we shouldn't worry about a few minutes more or less while one the other side we always should be aware that it is a good idea to stay away from the limits of whatever tool we are using, because there are too many factors influencing what is going on in our body and "testing the limits" is always a poker game with our health.

 

[notivago]

Interesting read on the subject:
http://www.dive-tech.co.uk/resources/suunto-rgbm.pdf

 

[eponym]

By way of example, a couple of weeks ago my buddy and I dove very close together, though he usually stayed a bit "up hill" of me as we explored some pinnacles. I thumbed the dive and shot a bag to ascend when my Oceanic VT-3 showed 5 NDL minutes left. We did a normal ascent and a 3 min. safety stop at 15 feet. I surfaced, with a "happy" computer. He, on the other hand, was told by his Aladin that he had a 10 minute, 10 foot deco obligation.

I remember doing two long easy altitude dives a few years ago with a visitor to our region. Lake surface 5800 fasl, water temp 40F, max depth 71 feet dive 1, 45 feet dive 2. We swam slow, stayed mostly shoulder-to-shoulder, slow rambling ascents on the gentle bank, no hard work.

At the end of the second dive my Suunto Vytec suggested the usual 3-minute safety stop while my buddy's computer (sorry, can't remember the make) wanted a compulsory 10-minute stop at 10 feet, which we did (together).

Are Suuntos "more" conservative? Yes, mostly, but it depends.

-Bryan

 

[Puffer Fish]

Most likely a Uwatec.. they adjust for water temperature by reducing time as the water gets colder. It is not a feature one can turn off.

I remember doing two long easy altitude dives a few years ago with a visitor to our region. Lake surface 5800 fasl, water temp 40F, max depth 71 feet dive 1, 45 feet dive 2. We swam slow, stayed mostly shoulder-to-shoulder, slow rambling ascents on the gentle bank, no hard work.

At the end of the second dive my Suunto Vytec suggested the usual 3-minute safety stop while my buddy's computer (sorry, can't remember the make) wanted a compulsory 10-minute stop at 10 feet, which we did (together).

Are Suuntos "more" conservative? Yes, mostly, but it depends.

-Bryan