Riding deco ceiling on ascent

[inquis]

I recently had a look at a friend's Suunto D5 user manual and was very surprised to see this statement:

NOTE: It is always recommended to keep close to the decompression ceiling when ascending.​

In fact, the depth -- labeled "Stop" on the main screen -- seems to be identically the ceiling value. Their guidance is to stay within a range of [-2 +10] ft of that changing value. In their words, "providing continuous decompression with optimum ascent time."

In contrast, Shearwater computers (and I actually think my very old Suunto Mosquito did as well) round the ceiling up to the next multiple of 10 ft / 3 m for the displayed Stop depth. The continuous ceiling is available, of course, and the Perdix can even be set to automatically display it in place of the NDL field. However, the Perdix manual has this note:

Please note that there is very limited information on the effects of following a continuous ceiling instead of stopping at stops and only moving up to the next stop when the stop has cleared​

The Teric manual, in contrast, does not have such a statement.

I'm curious then...
Is the lack of such a warning in the Teric manual and flat out recommendation by Suunto's newest release indicative of any recent research?
If you use a different Suunto computer for technical diving, does it also guide you toward a continuous ascent (as the D5 does) rather than traditional discrete stop depths?
If you use a Shearwater, do you ride the ceiling up on a routine basis? (I can easily see doing this in an emergency situation, likely with more aggressive GFs as well.)
Any sense of the runtime savings by such a practice?

 

[tursiops]

I recently had a look at a friend's Suunto D5 user manual and was very surprised to see this statement:

NOTE: It is always recommended to keep close to the decompression ceiling when ascending.​

In fact, the depth -- labeled "Stop" on the main screen -- seems to be identically the ceiling value. Their guidance is to stay within a range of [-2 +10] ft of that changing value. In their words, "providing continuous decompression with optimum ascent time."

In contrast, Shearwater computers (and I actually think my very old Suunto Mosquito did as well) round the ceiling up to the next multiple of 10 ft / 3 m for the displayed Stop depth. The continuous ceiling is available, of course, and the Perdix can even be set to automatically display it in place of the NDL field. However, the Perdix manual has this note:

Please note that there is very limited information on the effects of following a continuous ceiling instead of stopping at stops and only moving up to the next stop when the stop has cleared​

The Teric manual, in contrast, does not have such a statement.

I'm curious then...
Is the lack of such a warning in the Teric manual and flat out recommendation by Suunto's newest release indicative of any recent research?
If you use a different Suunto computer for technical diving, does it also guide you toward a continuous ascent (as the D5 does) rather than traditional discrete stop depths?
If you use a Shearwater, do you ride the ceiling up on a routine basis? (I can easily see doing this in an emergency situation, likely with more aggressive GFs as well.)
Any sense of the runtime savings by such a practice?

You ask four questions.
1) No, not that I've seen. Suunto lives in an alternate universe.
2) I do not use a Suunto, nor would I.
3) No. Risky game, per the Perdix warning.
4) Not enough benefit to balance the extra risk, but that is a guess.

 

[GF99/99]

Suunto lives in an alternate universe.

Spot On!


Don't waste your money, stick with a computer that runs a Bühlmann decompression algorithm. Your dive buddies that are all running a Bühlmann computer will appreciate it too.

 

[broncobowsher]

This is the technical diving forum, we just use Shearwater. You can change the GF to be as agressive or conservative as you like.

 

[Pressurehead]

1) No, not that I've seen. Suunto lives in an alternate universe.
2) I do not use a Suunto, nor would I.

^^+1^^
Suunto make a good compass, that's about the extent of it.

 

[Angelo Farina]

The same recommendation of following a continously-varying depth during deco was contained in the manual of my SOS decompression meter (known as the bend-o-matic device in some country).
And possibly this is among the reasons which caused the device to get such a surname...

 

[rjack321]

Any sense of the runtime savings by such a practice?

None, AT MOST you are 9ft below your ceiling. You are still offgassing. In fact most everyone is even further below the 100% of Buhlmann ceiling since using something between 75 and 90% GF high covers the vast majority of tech divers.

Throw out the Suunto, nobody can even tell you how their proprietary algorithm defines a ceiling. Following their advice on how to tech dive is insane.

 

[inquis]

To be clear, I personally use Shearwater computers and Buhlmann + gradient factors to define my continuous ceiling value. I don't agree with how Suunto computes their ceiling.

However, that is a separate consideration from the concept of a continuous vs discrete ascent increments. Computers have moved us into using what is effectively a continuum of timesteps, which is universally regarded as a Good Thing. It's not a stretch to think that reducing the quantization of stop depth might also be a Good Thing.

Intuitively, there seems to be an increase in risk by the mere fact one is ascending sooner. That doesn't seem overly different, though, from just using a slightly higher GF (and discrete stops).

So are there advantages to a hard stop every 10 ft? The engineer in me says absolutely -- an "on/off" control system is simpler and therefore easier to execute. Fewer things going on at once the better from a task loading perspective.

OTOH, if tissues can support the gradient present when first arriving at a discrete stop, can they not continue to support that level of offgasing? And if they can't, I would think maintaining a lower gradient (continuously) might be better than the stress of *bend & mend".

This thread is really just me asking why is the sky blue, so to speak. And maybe the answer is simply because all the testing has been done with discrete stops, so that's where we are at.

I probably muddied the water unnecessarily by bringing up Suunto. However, I suppose we can add "making recommendations without testing" to the litany of reasons they are shunned by most technical divers.

 

[rjack321]

Decompression algorithms are not a perfect match for anyone's physiology, certainly not down to the foot increment. Not that you can practically maintain yourself an inch under a ceiling anyway - plus where do you measure? your wrist? your back? your chest? feet?

It's been a couple years, best if the topic comes up again anyway

Continuous Decompression AND Staged Decompression

*Edit* - Stressing out that this query is not in relation to formal continuous decompression methods employed in some saturation or chamber operations. I meant 'continuous' also in the context of a staged decompression, but where the ascents are calculated micro-stops themselves. Thus curving...

scubaboard.com

 

[inquis]

Thanks again, @rjack321, for that pointer. Lots of good info, including the 1 ft schedule that shows negligible benefit.