Ceiling/Deco obligation in recreational diving

[DavidFL]

Subsurface has a User Forum; there are some threads in that forum that compare Subsurface profiles with Suunto profiles; even some comments from Linus Torvald from the earlier development phase of Subsurface. You might consider posting there. On a quick read of the OP, my hunch is that it relates to the planned ascent rates, which GF's you choose for Bulhmann and/or which conservatism level you choose for VPM-B. There is a profile photo in the Subsurface Dive Planner section of the Subsurface Users Manual for planning a recreational dive to not exceed NDLs, and there is a ceiling in that profile that just barely 'clears' if the dive maximizes the NDLs.

 

[Lorenzoid]

. . . 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? . . .

I believe that's what the Suunto does, yes.

 

[rhwestfall]

it likely is due to what you had the factors and algorithm set for that you imported the dive under....

it isn't the same as the one on your PDC....

 

[Diver0001]

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?

No. If your computer enters decompression mode it immediately shows you a ceiling.

Now my confusion (or problem) is the following: was there indeed a deco ceiling in those dives that I was unaware of?

Probably not. As you already deduced, not all algorithms calculate the same way. Some are more conservative than others but that conservatism is all in a "grey area". A good example of this is the NDL at 18 meters. When I first started diving the NDL at 18m was set to 60 minutes. The PADI tables changed it to 56 minutes and some computers show an NDL at 18m of under 50 minutes. IIRC the Mares Puck gives an NDL of 47 minutes at 18m. Pure unmodified Buhlmann gives an NDL of 58 minutes. Those are some big differences.

If you stay 52 minutes at 18m you would have a noticeable decompression obligation according to some computers, but not according to the table or the unmodified buhlmann algorithm.

That's very likely what you're seeing here....different results in a "grey area".

The big question is if shorter NDL's are "safer". Is additional conservatism synonymous with additional "safety"? If a decompression algorithm is giving (minor) DCS events at the rate of 1:100,000 is shortening the NDL to the point where the rate is 1:120,000 really that much safer? What if we set them to the point where the rate is 1:1,000,000? That's gotta be safe, right? but what if the consequence of that is that we could never make a dive longer than 20 minutes?

That's the background discussion in this whole thing. Higher levels of conservatism in and of itself doesn't necessarily lead to significant improvements in safety. It's a game of diminishing returns. Computer manufacturers play with this as a way to make their product seem unique and "better" than the competition but in reality the DCS rates are already very low -- assuming you dive responsibly -- and the whole conservatism discussion comes across to me as fiddling in margins.

R..

 

[stuartv]

First, let's keep in mind this is the Basic forum.

And, second, what you have said here is correct. But, what you are implying is not correct.

No. If your computer enters decompression mode it immediately shows you a ceiling.

Yes, that specific statement is exactly correct. IF your computer goes into deco mode, then yes it will show you a ceiling. But, the implication that no deco mode means no ceiling is not correct.

It is quite common to have a ceiling and not (yet) have a mandatory deco stop. Thus, you have a ceiling but your computer would not be in decompression mode. Your post (more than just the piece I quoted) implies that if you have a ceiling you must have a decompression obligation.

The ceiling is the depth where, if you ascended INSTANTLY, the computer has calculated that you would be crossing the line into DCS (being simplistic, as we are in Basic). But, the computer assumes you will not instantly go from, say 40m to 6m. So, even though your ceiling may be 6m, the computer does not go into decompression mode (yet).

To get technical, yes, when you have a ceiling, you do have a decompression obligation. But, at first, that decompression obligation will be satisfied by a no-stop ascent at 30fpm/10mpm. Thus, you do not have a decompression obligation in the sense of having a mandatory stop.

 

[Lorenzoid]

. . .
It is quite common to have a ceiling and not (yet) have a mandatory deco stop. Thus, you have a ceiling but your computer would not be in decompression mode. Your post (more than just the piece I quoted) implies that if you have a ceiling you must have a decompression obligation. . . .

That's what I was thinking with my post above. Since the Suunto displays a ceiling if you exceed NDL, I believe it always keeps track of your ceiling--it just doesn't display it unless you exceed NDL. There is always some depth that you can't pass above without exceeding the maximum ascent rate--you just don't normally need to know what it is because you're not exceeding the maximum ascent rate.

 

[nick73]

Thanks for all the answers, they helped clear my mind and formulate better the question that vaguely formed in my mind when I saw the subsurface profiles. So without getting too much into deco algorithms and such (which I suppose make part of more advanced stages), here is what struck me when I saw the subsurfuce profile:

In that particular dive, while at 39m depth and with no warnings/notifications from my computer I was assuming I am at 3.9 minutes from surface, should a reason require shift surfacing. I wonder whether that assumption of mine was really true OR my computer on my way up to the surface would ask me to stop for deco, because I cat the dive short and the return profile is now not as smooth as the computer would like.

In other words, could there by "hidden" deco stops, that I am unaware of, while diving within "recreational" limits/tables? I guess it would make some difference to the psychology of my diving knowing that I am 4 minutes from surface OR 4 minutes + eventual mandatory deco stops that my computer isn't telling me yet.

 

[Diver0001]

That's what I was thinking with my post above. Since the Suunto displays a ceiling if you exceed NDL, I believe it always keeps track of your ceiling--it just doesn't display it unless you exceed NDL. There is always some depth that you can't pass above without exceeding the maximum ascent rate--you just don't normally need to know what it is because you're not exceeding the maximum ascent rate.

Computers don't work like that.

A Buhlmann computer, to keep it simple, continuously recalculates 16 (or 17) "compartments". All of that information in in the computer at every point during the dive. There is, however, always ONE compartment that is closer to the NDL (or further over the NDL) than the rest. THIS compartment is the one that is displayed for the diver to see.... it is the "controlling" compartment.

So when your computer is showing you a ceiling it's showing you the deepest ceiling of all of the compartments at that particular moment. Some of the compartments may not have a ceiling at all...... In fact, most of them probably don't unless you are over the NDL.

Meanwhile it is recalculating your dive continuously and displaying the controlling compartment at every given point in time.

R..

 

[stuartv]

Computers don't work like that.

A Buhlmann computer, to keep it simple, continuously recalculates 16 (or 17) "compartments". All of that information in in the computer at every point during the dive. There is, however, always ONE compartment that is closer to the NDL (or further over the NDL) than the rest. THIS compartment is the one that is displayed for the diver to see.... it is the "controlling" compartment.

So when your computer is showing you a ceiling it's showing you the deepest ceiling of all of the compartments at that particular moment. Some of the compartments may not have a ceiling at all...... In fact, most of them probably don't unless you are over the NDL.

Meanwhile it is recalculating your dive continuously and displaying the controlling compartment at every given point in time.

R..

Huh? I think it works exactly like @Lorenzoid said.

And what you said is sort of unrelated. Yes, there is a controlling compartment. Yes, WHEN the computer shows you a ceiling, it is the deepest compartment.

None of that changes the fact that if a no-stop ascent (at 10mpm) won't break the ceiling (because the ceiling rises as you decompress during your no-stop ascent), the computer doesn't show you the ceiling exists. It only starts to show the ceiling once you have exceeded the point where you cannot make a no-stop ascent without hitting the ceiling before you hit the surface (i.e. you exceeded your NDL). The ceiling is there before you exceed your NDL. It is just not displayed (because the computer assumes a 10mpm ascent and, thus, that you won't hit the ceiling before you hit the surface).

 

[Lorenzoid]

Computers don't work like that.

Buhlmann, sure. But Suunto? Suunto is an odd animal. For example, the manual says:

When the ascent rate exceeds 10 m/33 ft per minute continuously for more than 5
seconds, the microbubble build-up is predicted to be more than is allowed for in the
decompression model. The Suunto RGBM calculation model responds to this by
adding a Mandatory Safety Stop to the dive. The time of this Mandatory Safety Stop
depends on the severity of the ascent rate excess.
The STOP sign appears in the display and when you reach the depth zone between
6 m and 3 m/20 ft and 10 ft, the CEILING label, ceiling depth, and the calculated Safety
Stop time also appear in the display. You should wait until the Mandatory Safety Stop
warning disappears. The total length of the Mandatory Safety Stop time depends on
the seriousness of the ascent rate violation.

This sounds like it can give you a "mandatory safety stop" that has a "ceiling" without actually violating any NDL. That's not what I understand happened in the OP's case, but I wouldn't be surprised if it tracks some sort of ceiling.