Suunto Left Bar graph

[Antarctic-Adventurer]

If the D6 bargraph is like that on other Suunto computers, it is a graphical display of NDL time remaining, NOT nitrogen SATURATION levels.

The difference is that a bargraph of N2 saturation (such as on Oceanic/Pelagic/Aeris/Sherwood computers) changes only as the loading in the controlling compartment changes. The Suunto CBT (or NDL time) bargraph display will immediately change as one changes depth -- since NDL changes immediately with depth.

The difference is most obvious during the ascent from a dive near NDL limits. The Suunto bargraph will immediately go to the green zone upon ascending to safety stop depth. A true N2 saturation bargraph will only return to green as the compartments offgas (relatively quickly for a short deep dive such as 15 minutes at 120', much more slowly for a dive such as 35 minutes to 70').

OK you are right that it is measuring Consumed Bottom Time, i.e. the percentage of your NDL that you have remaining, which is not exactly the same as saturation level but the NDL is calculated from known saturation levels surely? I.e the reason that you have say 5 minutes left on your NDL is because you are 5 minutes from entering into a decompression dive which is calculated using a mathematical model of saturation levels?

 

[Charlie99]

OK you are right that it is measuring Consumed Bottom Time, i.e. the percentage of your NDL that you have remaining, which is not exactly the same as saturation level but the NDL is calculated from known saturation levels surely? I.e the reason that you have say 5 minutes left on your NDL is because you are 5 minutes from entering into a decompression dive which is calculated using a mathematical model of saturation levels?

NDL and % of M-value are related, but not the same. While diving, I find a display of loading as a % of M-value limit is much more useful than simply repeating the NDL time graphically. It isn't just a matter of semantics.

"NDL Time Remaining" would be a better name than "Consumed Bottom Time" since the Suunto CBT bargraph is nothing more than NDL time displayed graphically. For example, in the Cobra there are 11 bars in the CBT graph. The 1st bar goes on if NDL is less than 200 minutes ... the 5th bar with less than 50 minutes, then one more bar for each 10 minute reduction, with the two yellow bars being 10 and 5 minutes of NDL. The bargraph doesn't show the fraction of bottom time consumed,but instead is set only by the number of minutes.

What this means is that if one starts off with no N2 loading and drops down to 80', most of the bars immediately are activated. This doesn't mean that you are loaded to greater than 70% of the limits. It simply is showing that NDL is less than 30 minutes.

Conversely, even if one stays down ato 80' or 100' right up to the point of going into deco and then ascend to 15', the Suunto bargraph will immediately go well into the green, with just 2 or 3 segments out of 11 being on. This is even though you are still very heavily loaded with nitrogen.

I find the CBT bargraph much less useful than a display that displays the fraction of loading in the compartment closest to the limit. Suunto does something along that line with their OLF or Oxygen Limit Fraction in that they look at both CNS and pulmunary O2 loadings and then show the higher one as a fraction of the limit.

While diving, I pay little attention to the NDL time on my Oceanic computer, and instead look at total dive time and the N2 bargraph. When I ascend, the bargraph will back off into the green only at the rate at which I am offgassing. OTOH, the CBT bargraph of a Suunto computer will instantly go back into the green, giving the diver a false indication that he is no longer has a heavy N2 loading.

 

[mason417]

NDL and % of M-value are related, but not the same. While diving, I find a display of loading as a % of M-value limit is much more useful than simply repeating the NDL time graphically. It isn't just a matter of semantics.

"NDL Time Remaining" would be a better name than "Consumed Bottom Time" since the Suunto CBT bargraph is nothing more than NDL time displayed graphically. For example, in the Cobra there are 11 bars in the CBT graph. The 1st bar goes on if NDL is less than 200 minutes ... the 5th bar with less than 50 minutes, then one more bar for each 10 minute reduction, with the two yellow bars being 10 and 5 minutes of NDL. The bargraph doesn't show the fraction of bottom time consumed,but instead is set only by the number of minutes.

What this means is that if one starts off with no N2 loading and drops down to 80', most of the bars immediately are activated. This doesn't mean that you are loaded to greater than 70% of the limits. It simply is showing that NDL is less than 30 minutes.

Conversely, even if one stays down ato 80' or 100' right up to the point of going into deco and then ascend to 15', the Suunto bargraph will immediately go well into the green, with just 2 or 3 segments out of 11 being on. This is even though you are still very heavily loaded with nitrogen.

I find the CBT bargraph much less useful than a display that displays the fraction of loading in the compartment closest to the limit. Suunto does something along that line with their OLF or Oxygen Limit Fraction in that they look at both CNS and pulmunary O2 loadings and then show the higher one as a fraction of the limit.

While diving, I pay little attention to the NDL time on my Oceanic computer, and instead look at total dive time and the N2 bargraph. When I ascend, the bargraph will back off into the green only at the rate at which I am offgassing. OTOH, the CBT bargraph of a Suunto computer will instantly go back into the green, giving the diver a false indication that he is no longer has a heavy N2 loading.

Do you like the Oceanic better? Which model do you use?

 

[Charlie99]

I have a relatively old Ocean Data Plus 2, with an even older 1 button Data Plus as backup.

Simple, straightforward non-AI computers. Rather agressive algorithm and I know that if I get out of the water right at the Oceanic NDL that there is a pretty good chance that I'll be fatigued later that day. My usual practice is to do a couple deep stops and a sliding 20' to 10' safety stop for as long as it takes to get the tissue loading bargraph back into the green or at least down to the first yellow bar. After a while, I've gotten a pretty good feel for how long that will take beyond any small deco obligations I've incurred.

There is a philosophical difference between the Oceanic and Suunto computers. The Oceanic is a very simplistic bookkeeper and it doesn't care if you do reverse ascent, yo-yo (sawtooth) profiles, 200fpm ascent rates or anything else. It is up to the diver to dive a decent profile, and to add appropriate fudge factors for how they are feeling that day or fudge factors to account for less than optimal profiles.

The Suunto computers attempt to monitor diver behavior and modify the limits accordingly. Unfortunately, this makes it so that the algorithm is no longer transparent and you don't always know why the computer is saying what it says.

It's not really a case of one being better than the other. Just different ways of handling things. For me, I prefer the Oceanic, but because I keep reasonably close track of my deco status mentally and have a pretty good idea of what the computer will say before I look at it (This is a good exercise with any instrument -- a good practice to get into is to make a guess before reading NDL or SPG and see how close you are).

The Suunto is definitely better for a diver that doesn't want to bother with paying much attention to time/depth/profile shape and just want a computer that gives good guidance. In this type of diving the Suunto is much better as it will indeed make reasonable compensation for things like short SI, reverse profile, sawtooth dives, missed safety stops, and rapid ascents. Now that the tissue loading bargraph patents of Oceanic have expired, perhaps Suunto will add that feature to their computers because IMO that bargraph is both more informative and more intuitive than the CBT bargraph Suunto now uses.

 

[mason417]

The Suunto is definitely better for a diver that doesn't want to bother with paying much attention to time/depth/profile shape and just want a computer that gives good guidance. In this type of diving the Suunto is much better as it will indeed make reasonable compensation for things like short SI, reverse profile, sawtooth dives, missed safety stops, and rapid ascents.

Thats great advice. Thank you for explaining it to a novice like me