Riding deco ceiling on ascent

[Dsix36]

1 - Being in perfect horizontal trim will always be the best position for decompressing due to keeping all the tissues at basically the same pressure differential, otherwise the highest body part will be decompressing faster than the lowest body part. Does this make any significant difference in the real world of diving? Not that I have noticed.

2 - Climbing the deco ceiling in 1 foot stops (an attempt at a curve) will in theory save a bit of deco time just by simple logic but does it save enough time to make it worthwhile? I think not in almost all instances, BUT in cases of extremely long deco it does shave several minutes off the schedule. Is it something to pursue will be a matter of personal comfort levels, real need to get out of water, and overall abilities.

3 - Don't do the dive if you can't or don't want to do the deco. Trying to cut time off deco can lead to serious, or even deadly, consequences.

 

[boulderjohn]

I do a lot of decompression dives in a deep sink hole (280 feet). During the summer and into the fall, a significant thermocline develops at almost exactly 37 feet. With an algae bloom, you can see it above you as you ascend. When you are starting to shiver on the 40 foot stop, you begin to look longingly at that cloud formation 3 feet above you.

 

[Gareth J]

If I understand the Schreiner equation correctly which determines inert gas pressure in the tissues, when you first arrive at a deco stop the pressure gradient is at its greatest. As you hang at the stop the gradient decreases non-linearly according to the Schreiner equation. When the gradient has dropped off to a point you can ascend at 30 ft/min to the next stop and thus increase the gradient back to more efficient offgassing. So, the amount of time spent at maximum offgassing is a fraction of the overall offgassing rate for the entire stop.

In contrast, a continuous ascent will maintain a maximum offgassing rate because you are maintaining a fixed higher pressure gradient as the depth changes and the ambient pressure decreases. The question is: Can the body maintain a fixed high rate of (safe) offgassing for the entire trip to the surface? The ascent rate for continuous deco would probably be lower than the 33 ft (10 m) / min rate between deco stops.

The primary question is what is safest?

Apart from the practical issues of attempting a continuous ascent rate rather than the traditional stepped decompression profile.

There is an inherent safety buffer in the stepped approach. i.e. you wait until the next 3m step is within the M-Value criteria. As against riding the edge of the M-value criteria.
There have been millions of dives using the fixed stop approach, going all the way back to Haldane's original tables.
Where as, riding the maximum off gassing gradient is a relatively new approach.

It should never be forgotten, that these 'models' are just that models, they are not a true reflection of how the body responds. They are a rough mathematical solution to a problem, they are NOT A TRUE representation of what is actually happening to the body!

 

[EFX]

@EFX hang on, you posted results in that other thread that indicated continuous ascent saved a whopping 12 seconds off a 15 minute deco. Did I misunderstand your post?

No. The post I entered above was from 2-12-19 post #22. I forgot about post #27 from 8-2-20 which you referred to here. It appears that it makes no difference in total deco time between a staged and continuous deco schedule. The overriding consideration is one of safety and precedence in the diving community. This makes sense in that it is the ceiling that is calculated the same way for both schedules.

 

[boriss]

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?

While running a few calculations for my thread regarding ascent rates, I actually did a test of setting the last stop (on oxygen) at 20ft vs 10ft. Interestingly I tried it with various dives and deco strategies and the impact was minimal (<1%), HOWEVER, it had a huge impact on CNS loading and doing the same deco at 20ft would put the diver over the limit, compared to 10ft (CNS: 84% vs 131% in the 250ft dive example)

This would be even more important for people on rebreathers, who are probably more CNS constrained.

I didn't see mention of CNS in the discussion, so I thought I'd bring it up. Not sure if it helps the conversation, but it may certainly be a good reason not to discount Suunto's decisions. This would be a great question for Shearwater, actually! I'm sure they can add the option into the computer.

 

[DeepSeaExplorer]

Shearwater gives the option of setting the last stop depth at 20fsw, or not. I have mine set to 20.

 

[boriss]

Shearwater gives the option of setting the last stop depth at 20fsw, or not. I have mine set to 20.

The conversation is about riding the deco ceiling, not the setting depth for the last stop. For example, you might have a ceiling of 41ft, but Shearwater will round it up to 50ft mandatory stop.

I was just mentioning the specific example, because it's makes a point that although the profile runtimes are the same, not sticking to a ceiling means there's a significant increased risk of CNS toxicity. It's not all about runtime; that may just be a nice byproduct.

Below is an image to illustrate what we're talking about. The dark green portion is the ceiling, the light green squared off steps are stops.

Hope this helps

 

[inquis]

although the profile runtimes are the same, not sticking to a ceiling means there's a significant increased risk of CNS toxicity. It's not all about runtime; that may just be a nice byproduct.

Significant? Maybe for some dives, but reverse-engineering the one you illustrate (a non-trivial, hypoxic dive), it has a CNS loading of about 85%. (For a fairer comparison, you should not limit the last stop to 20 ft.) This risk can also be mitigated in other ways.

On the potential drawback side of riding the ceiling, there is an increased risk of overshooting the ceiling while riding it up (possibly multiple times).

On dives where there may be a significant CNS difference between the two approaches, the tissue loading is also likely to be significant. Breaking the ceiling is not something I would want to risk in such cases.

 

[Wibble]

This would be even more important for people on rebreathers, who are probably more CNS constrained.

Actually less constrained on CCR compared to OC as you can vary the PPO2 during the dive. The final 1.6ATA stop (6m/20ft) will not be 100% oxygen as you will exhale the off gassing inert gas

 

[Russjstewart]

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?

From past experience a Suunto DC will “punish” you for being deeper than the ceiling on a deco stop. I have found that being at 5m when the ceiling says 3m,results in a 5 min stop being upwards of 7 or 8 minutes, depending on the dive. Other divers I know who have acquired a deco obligation have said the same thing. I assume it is because i was not off gassing at the optimal rate for the algorithm.