Ceiling/Deco obligation in recreational diving

[nick73]

I am either confused or unhappy (don’t know yet) as to how my diving computer passes information to me about potential need for decompression
I downloaded all my dives to Subsurface, and according to calculations based on the deco algorithms that Subsurface includes, there was a ceiling for several of my dives.
I was surprised, as I thought that my computer, using the supposedly more conservative RGBM algorithm (vs. the VPM-B and Buhlman algorithms in Subsurface) and also programmed to a more conservative personal setting would show me that information.
I am absolutely sure that the computer did not show any such information on those specific dives. I am familiar with how my Vyper Novo shows such information, because in one dive I had a deco obligation and the computer guided me above the required floor, then just under the deco ceiling and was counting down minutes to clear me from deco.
So, I wonder what is the source of this discrepancy. Could it be that my computer was aware of that ceiling but it assumed that deco would clear as I would progressively reduce my depth on my way back? Now my confusion (or problem) is the following: was there indeed a deco ceiling in those dives that I was unaware of? Or is it that the Subsurface algorithms are even more conservative than Suunto’s? I have no tec training, and I am not familiar with the different deco algorithms, so I asking here. I bought the Suunto based on recommendations that it is probably the most conservative for recreational diving. So, what happens here?
Secondly, if indeed there was a deco ceiling, how the Suunto assumed that it would clear because a normal slow-lengthy ascent would happen in the future minutes of the dive and it didn’t bother to inform me about it? What if something happened and I needed to make a shift (but safe) ascent? I would be very unhappy to suddenly see my computer indicating that I have to stop for deco.
In my less experienced mind, I would prefer to have it show me that according to my dive profile up to this minute there is a ceiling at … say 7 meters. I understand that practically all dives are decompression dives, just that in recreational diving profiles deco obligations clear on the go. But if there was a case that a fast-safe ascent would have to be interrupted for a deco stop, I think would like to know about it. Or shouldn't bother, and that is why the computer didn't tell me?
So, after having being confused by the different approach between my supposedly conservative computer and calculated ceiling/deco time from Subsurface’s algorithms, anyone keen on helping me understand?

 

[tbone1004]

the more conservative you get on the computer, the earlier that ceiling will show up. I don't know how the Suunto's work as I don't like them, use them, or recommend them, but it is possible that even though it didn't give you a deco obligation, the safety stop could have been considered mandatory and when factored into the ascent time it didn't have a "Deco" obligation.

That is something we experience pretty regularly in actual deco diving especially in caves. You will have a mandatory 30ft stop for instance, and then by the time you get to 30ft, that stop has cleared and then you go into the rest of the stops for 20ft etc.

The Suunto algorithm is very strange and it part of why I don't like them because you don't actually know what it is doing which has been debated in another thread throughout the last few days. If you can post pictures of the dive plan from subsurface that would be helpful

 

[Jack Hammer]

It may help if you post a screenshot. I havent used Subsurface so I can't comment specifically on that software.

Even on recreational dives you may see a ceiling in your graph, this relates to ongassing. That ceiling may decrease as you go shallower. Generally speaking, the important part is that you are not crossing and staying above it.

 

[nick73]

I am attaching screenshots:

Suunto DM5 profile, completely uneventful:

For all I knew during the dive, when I was at the deepest point I could very well go all the way up to surface with no problems, even skipping the safety stop, as I had no feedback from the computer about a mandatory stop, or a deco stop or whatever. Of course not advisable to skip the safety stop, but could do it for all I knew, based on my computer info (or lack thereof).

Now look at the subsurface profile, showing ceilings and deco obligations:


For comparison, in case you are not familiar with suunto's computers, these are examples of dives with a mandatory stop:


and with a deco stop:


for all of which I was informed on my computer screen during the dives.

 

[nick73]

Another example of a different dive, completely uneventful (no mandatory stops indicated by the computer at any time during the dive):


whereas, with subsurface I see that there was a mandatory stop at 3m for some time during the dive (which cleared later).



But still, what if during the time that subsurface's algorithms say that there was a ceiling had I the need to surface?

So, is it a discrepancy due to different algorithms? bad implementations of suunto or subsurface software? Else?

The easy answer is to forget I ever installed subsurface and not bother with deco algorithms. The troubling answer is that there MAY be ceilings at some points during the dive that the suunto does not show them?

 

[victorzamora]

Nick, it looks like your Suunto's algorithm was calling you safe....but Subsurface said you had required deco according to Buhlmann 30/75 and/or VPM-B +3.

What you need to realize is that different algorithms will say different things, and none of them are completely right. I personally can't stand the Suunto algorithm (or Mares or any other RGBM-based proprietary algorithm)....but that doesn't meant it's unsafe.

If you have had success diving your Suunto computer, I wouldn't worry about what the other algorithms say. As you asked, it simply seems to be a discrepancy in the different algorithms.

 

[Blasto]

I understand that practically all dives are decompression dives, just that in recreational diving profiles deco obligations clear on the go.

Not quite. The defining rule for recreational diving is that a deco obligation is never to be incurred. So that your N2 loading at any time during the dive is acceptable for a direct ascent at the allowed rate, and no specific profile has to be followed.

If an obligation is incurred, but cleared 'on the go' (as in, in a multilevel dive?), that's no longer in the recreational range. The reason is that rec equipment includes no mandatory backup. If you plan to incur a deco obligation at any time, no matter when and how it's cleared later, you need a redundant air source and tech skills, in case your primary fails right when you have that obligation.

To extend the dives, the definition of "acceptable" is set much more aggressively than in tech diving, the resulting GF are too high for daily use. So a safety stop is added to bring some conservatism back in.

PDCs, especially Suuntos, tend to add a bit of conservatism
Note that "riding your computer up" - bringing NDL down to zero (or, worse, a bit into deco) and slowly reducing dive depth to keep NDL near zero - is risky in terms of DCS. More so than basic tech dives or an aggressive rec dive down to zero NDL with a normal ascent and an extended safety stop.

The [most likely] reason is that you keep accumulating N2 in your slower compartments, so in the end, instead of being maxed out in the one governing compartment and much lower in the rest, you're maxed out in most compartments. And since the compartments aren't really independent, it's just a model, you maximize your risk, above what happens with proper profiles that the limits are based on.

 

[scubadada]

Hi @nick73

I assume that Buhlmann 30/75 is slightly more conservative than Suunto RGBM for this dive and gave you the short deco obligation. I would imagine you could avoid this by increasing the GFs used by Subsurface in the calculations. 30/75 is quite conservative for recreational diving, the Shearwater most conservative preset in the rec mode is 35/75. You might want to change to something like the Shearwater intermediate setting of 40/85 and see how you do. I think this would probably work for you. Let us know.

 

[stuartv]

the Suunto assumed that it would clear because a normal slow-lengthy ascent would happen in the future minutes of the dive and it didn’t bother to inform me about it?

That's exactly it and nothing to be concerned about. Dive computers generally assume you will ascend at 30fpm (10 meters/min in metric mode). If the computer calculates that an ascent at the speed will get you to the surface before your tissues get to the point of bubbling, then it does not show any deco stop info (because there are no required stops).

Simple as that.

Any time you do a "fast" ascent (meaning faster than what the computer is programmed to expect), you risk incurring DCS. The longer and/or deeper you have stayed down, the more risk there is.

I don't know of any recreational computer that shows you your ceiling. A ceiling that you won't ever hit (in a 10m/m ascent) is not a deco obligation. However, if you ascend too fast, it could possibly turn into one. There's a simple answer for that: Don't make too-fast ascents and don't put yourself in a position where you would ever need to make a too-fast ascent. Plan your gas properly and keep your buddy close by.

 

[scubadada]

As per @stuartv, a "normal" ascent is part of the algorithm. A short deco obligation does sometimes "clear" before reaching the stop