Riding deco ceiling on ascent

[Centrals]

Some divers will still breath for a while from his/her 100% O2 bottle after surfacing.

 

[Gareth J]

One of the reasons almost all UK dive boats have a lift is because the risk of inducing DCI climbing a ladder back on to the moment are so much higher, especially after dives involving extensive decompression. This goes back to how close to the M value you are once you surface.

Whilst practices did change, de-kitting in the water and subsequently hosting kit back on board with a winch was an option. Sea condition are often such that this is not practical. So, on the 'tech' boats, lift started to be introduced. Both divers, and as importantly, skippers started to see the benefits. So now you seldom find a UK dive boat that doesn't have a diver lift. Even the big boats (converted trawlers), with high freeboards have lifts.

Reducing the work load (physical exercise), directly after surfacing is of significant benefit, and reduces the risk of DCI, when you are close to the M value.

Skippers like lifts because recovering divers, even in rough seas is significantly easier. (So they can go out in slightly worse conditions.) Recovering injured divers using a lift is also significantly easier.
Divers like them because it is a lot easier to swim into the lift, standup (get your feet under you), and be lifted out of the water where you are debited and escorted to the bench, than climb a ladder in full kit (possibly with stages).
It also caters for old divers or those with injuries or physical restrictions.

 

[broncobowsher]

Some divers will still breath for a while from his/her 100% O2 bottle after surfacing.

Common for a rebreather to stay on the loop until they take a seat on the boat. That is nearly 100% O2

 

[scubadada]

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

Hi @Gareth,

I was not surprised by the phenomenon, but by the magnitude of the GF99 increase over the last15 feet (4.6 m), less than a half atm decrease in pressure. The more than doubling of my GF99 in the example was surprising. Perhaps this is all perfectly explainable by the physics, it was not obvious to me. I would be willing to bet that the magnitude of the change would be a surprise to many, if not most, divers, just not you.

Fortunately, with the modern tools available, like SurfGF, we have great control over our ascent and surfacing.

 

[Gareth J]

Hi @Gareth,

I was not surprised by the phenomenon, but by the magnitude of the GF99 increase over the last15 feet (4.6 m), less than a half atm decrease in pressure. The more than doubling of my GF99 in the example was surprising. Perhaps this is all perfectly explainable by the physics, it was not obvious to me. I would be willing to bet that the magnitude of the change would be a surprise to many, if not most, divers, just not you.

Fortunately, with the modern tools available, like SurfGF, we have great control over our ascent and surfacing.

My comment was not supposed to be a criticism.

My diving practice has certainly changed over the years, as I have learnt more.

Keep safe

 

[inquis]

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).

The Shearwater ceiling is indeed based on the GF values. If you choose to do a continuous ascent, Ceiling is your indicator, rather than Stop depth. As previously mentioned, you can set some Shearwaters to automatically switch the NDL slot to Ceiling when NDL is exceeded.

 

[GF99/99]

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.

Hi @Gareth,

I was not surprised by the phenomenon, but by the magnitude of the GF99 increase over the last15 feet (4.6 m), less than a half atm decrease in pressure. The more than doubling of my GF99 in the example was surprising. Perhaps this is all perfectly explainable by the physics, it was not obvious to me. I would be willing to bet that the magnitude of the change would be a surprise to many, if not most, divers, just not you.

Fortunately, with the modern tools available, like SurfGF, we have great control over our ascent and surfacing.

This is spot on and I think alot of people forget about this.

For deep dives sub 130m from 35m to 6m I ascend at a glacial pace to each stop and once I get to 6m and clear my 6m stop (I set computers for last stop at 3m) I will hang out at 6m until I cut my 3m stop time in half then I will literally crawl up to 5m and hang out at 5m until I cut my 3m stop time in half again, then crawl to 4m and hang out till I cut my 3m stop time in half again, then finally crawl up to my 3m stop. Once I clear my 3m stop I then make the worlds slowest accent to surface.

This is where having a big floaty SMB comes in handy. Just go a little negative and literally wind your self up inch by inch to each stop

 

[EFX]

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.

 

[inquis]

In contrast, a continuous ascent will maintain a maximum offgassing rate

This came up earlier in the discussion, and the result was the difference in deco time wasn't actually significant.

Task loading considerations alone tip the balance toward discrete stops in my view.

 

[inquis]

@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?