Riding deco ceiling on ascent

[Gareth J]

The first question is why do we use staged (stepped) decompression.

This is a case of practicality, going back to the original tables designed by Haladyn.
It is much easier to winch a (hardhat) diver to a particular depth, (which is easy to measure and control,) leave them suspended there for a particular time. Then move them up to another predefined stop.
It was also far easier to print a decompression schedule with a set of predefined decompression stops.

Due to the practicalities, this has been the approach used ever since.
Hence millions of dives have been done using staged decompression for dives that are longer than the NDL time.

It's not really been practical to calculate decompression profiles in real time until the advent of the dive computer.
The VR3 had the option of using the 'dope on a rope', to indicate a sliding decompression schedule back in the late 90's.

I am not going to knock Suunto, millions of dives have been conducted using their computers, including what many refer to as technical.
GF's are only a relatively recent tool. First appearing on PC decompression software (replacing the conservatism setting), then being added to the top of the range (technical), dive computers.

The VR3 had the option of using the 'dope on a rope', to indicate a sliding decompression schedule back in the late 90's. (VR3's didn't have gradient factors). As @Angelo Farina mentioned, the SOS had a similar feature, there are probably other examples.
Even with modern diving improvements, it is still easier to decompress in stages (steps), than have to be continuously moving up (and watching your computer).


Also remember, decompression models are mathematical representations. They don't accurately mimic human physiology. They are a best (scientific) guess representation of how gases dissolve and condense, in tissue, using a very simplified approach (compartments/tissues). As an example, Buhlmann uses 16 compartments (or tissues), to represent the complexity of the body!

 

[broncobowsher]

Another thought, the decompression models are based on a stepped decompression studies. The historical data if a model works is based on stepped decompression.

Steps also take you to the edge (edge is very loosely defined at this point) and give time to stabilize, back away from that edge. Then go to the next step and do it again.
Riding the curve you are riding that edge the whole time. Being the edge is poorly defined, based on models, is different in everyone, can be different in the same person but on a different day, what you had for dinner last night, etc.

You can back away from being at the edge, stay closer to the models with stepped decompression.

 

[atdotde]

I wouldn't spend too much thought on it. I think Gareth's comment is the most important one: It is considered to be practically easier to hold stops at fixed depths rather than follow a continuous ceiling. That's all. There is no more theory behind this. And the fact that everybody uses stop depth of multiples of 3m is a historical coincidence of some divers using imperial units and having ten fingers so still have some preferences for the decimal system.

 

[DS79]

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.

I wished, Shearwater would give the option of a "non-discrete stop value" in 0.1m in addition or instead of the 3m/10ft steps (warnings should be adapted). This "ceiling" value should be according to the GF values chosen (not based on Bühlmann 100/100 - I think that's already the case at the moment).

Before I started Tec Diving, I owned a Suunto EON Steel. I agree with many here, that they are basically un-usable for Tec dives (especialy in high altitude which is always the case here in Switzerland) but the Compass was way more reliable compared to my Shearwaters (I own 4 different Shearwater models) and the non-discete stop values were actually a thing I like(ed) much more than the traditional 3m steps.

I would think, the reasons, why computers still stick to the 3m/ 10ft steps are:

1. In a (buddy) team it's a bit easier to stay together and to communicate (sign "one step up").
2. It's easier to teach and to establish simple team rules.
3. Technical divers tend to do what has been done for a long time and they came along with quite ok-ish (although there might be better options).
4. Technically, it may has been easier to compute with discrete depth values in the past (?) and everything started with tables as we know.

To me, none of the reasons seem to be sufficient to stick to the 3m stops without questioning them.

I think, for the shallower stops (e.g. <10m and especially the very last meters) , non-discrete values could make some difference (positive effect on safety or deco efficiency), so I would like to do it. This is indeed pure speculation but I think it's consensus that finishing the last stop at 6m (20ft) and then surfacing rapidly within 20 seconds may not be the smartest thing you can do after a serious deco dive. Going shallower incrementally could be nicely managed with a non-discrete deco-stop value on the computer in 0.1m steps.

By the way: Another thing my Shearwaters are really missing are to show the "BO TTS" value dynamically for CCR diving. I would like to always have this information available, what's my TTS in case I have to BO the next minute (you actually have to switch to BO to get to know this). I would have a few mor wishes towards Sherwater.

 

[Jimchris]

Bühlmann with gradient factors is a software update available for the eon steel. Its replacement the eon steel black comes with it from the factory. Stepped or continuous decompression profiles can be set by the user.

 

[Gareth J]

I think, for the shallower stops (e.g. <10m and especially the very last meters) , non-discrete values could make some difference (positive effect on safety or deco efficiency), so I would like to do it. This is indeed pure speculation but I think it's consensus that finishing the last stop at 6m (20ft) and then surfacing rapidly within 20 seconds may not be the smartest thing you can do after a serious deco dive. Going shallower incrementally could be nicely managed with a non-discrete deco-stop value on the computer in 0.1m steps.

By the way: Another thing my Shearwaters are really missing are to show the "BO TTS" value dynamically for CCR diving. I would like to always have this information available, what's my TTS in case I have to BO the next minute (you actually have to switch to BO to get to know this). I would have a few mor wishes towards Sherwater.

I don't think I know of a serious diver, (tech or otherwise), who would go straight to the surface after the 6m stop.
All those I know, move to 3m for another stop (after clearing all stops), then move in 1m increments to the surface. If you are in a hurry, then the maximum would be 1 minute to the surface from the last stop (or ascent check depth) - granted, that's a bit of BSAC thing.
Even those who've set the last stop to 6m, they always still move to 3m for another minute or three.

I would suggest you forward the CCR BO TTS idea to Shearwater. They see to be very good at picking up (sensible) diver request and implementing them. - At least you could have it on the bottom section of the screen as one of the scroll through options. It does seem like a very sensible idea. Not that I have it on any of my controllers.
I have only just acquired my first Shearwater, they have been very good to talk to, and like sensible feedback / requests. - I'll forward your suggest as well.

 

[scubadada]

I don't think I know of a serious diver, (tech or otherwise), who would go straight to the surface after the 6m stop.
All those I know, move to 3m for another stop (after clearing all stops), then move in 1m increments to the surface. If you are in a hurry, then the maximum would be 1 minute to the surface from the last stop (or ascent check depth) - granted, that's a bit of BSAC thing.
Even those who've set the last stop to 6m, they always still move to 3m for another minute or three...

The GF99 really jumps during the final ascent. This dive was barely into deco. The GF99 went from 36% at the end of the safety stop, to 79% upon surfacing. This was a 1 min ascent from 15 feet. I knew what to expect from my SurfGF. I was startled when I first saw this phenomenon on the Shearwater Cloud

 

[Gareth J]

The GF99 really jumps during the final ascent. This dive was barely into deco. The GF99 went from 36% at the end of the safety stop, to 79% upon surfacing. This was a 1 min ascent from 15 feet. I knew what to expect from my SurfGF. I was startled when I first saw this phenomenon on the Shearwater Cloud

I am very surprised you are surprised that the biggest effect is in the last 6m (10m).

The biggest change in pressure is in the last 10m (33ft).
Remember the volume of a ballon will double from 10m to the surface, so if you thing of a bubble, it will double in size going from 10m to the surface.

This is why the ascent rate in the final 6m is significantly slower, and most divers will force a stop at 3m, then ascend in 1m increments from 3m. This is especially important if you have an aggressive profile (i.e. you are on the edge of the NDL, or have incurred decompression stops.)

Adjusting the GF high, allows you to increase the 'safety buffer' for this significant drop in ambient pressure (from your 6m stop to the surface). This is why a high GF high is not desirable.

One of the reasons the 3m safety stop has been introduced, is that it slows peoples (recreational NS divers) ascent at the most dangerous phase of the dive, even if they can't hold the stop, they at least attempt to hold the stop.

 

[Centrals]

If diver wants to ascent straight from 6m to the surface after finishing the last deco stop. I have no problem with that but I won't do it myself. 1m increments from 6m.

Just follow what the computer tell you otherwise change it or at least set it at gauge mode.

 

[broncobowsher]

That is another reason I just cork around on the surface for a few more minutes before I work my way out of the water. Even on the surface my body is mostly a foot or more under water (and under a little pressure)