Light back gas deco

[Angelo Farina]

What I mean is by using nitrox you may eliminate a stop if you had used air for the same bottom time.

Ah, OK, Nitrox, not oxygen. But, as said, you cannot use Nitrox for a dive to 48-50 meters...
For oxygen I was meaning the common practice to have a tank of pure oxygen hanging down the boat, at 6m, attached to the deco rod.
This was in use already in the seventies of past century, when instead Nitrox was entirely unknown.
When you reach the deco stop at 6m you start breathing pure oxygen, and some advanced deco theories allow to shorten the deco time if during deco you breath pure oxygen.
As said, I did never thrust entirely these theories...

 

[UCFKnightDiver]

Ah, OK, Nitrox, not oxygen. But, as said, you cannot use Nitrox for a dive to 48-50 meters...
For oxygen I was meaning the common practice to have a tank of pure oxygen hanging down the boat, at 6m, attached to the deco rod.
This was in use already in the seventies of past century, when instead Nitrox was entirely unknown.
When you reach the deco stop at 6m you start breathing pure oxygen, and some advanced deco theories allow to shorten the deco time if during deco you breath pure oxygen.
As said, I did never thrust entirely these theories...

I'm curious, why don't you trust the practice/theory of accelerated deco using rich oxygen mixes? It's been used successfully for at least the last 50+ years and I'm sure longer than that.

 

[100days-a-year]

Within the context of the poll of dives with a deco obligation of + - 10min, accelerated deco is not that relevant. To save 2, 3 or even 5 minutes it's just not worth the drag and hassle of a separate deco tank.
I have stages, extra regs, the training and 20+ years experience using them but they aren't really useful until the deco obligations get longer.

 

[scubadada]

As per @100days-a-year poll asked only about light back gas deco

 

[Angelo Farina]

I'm curious, why don't you trust the practice/theory of accelerated deco using rich oxygen mixes? It's been used successfully for at least the last 50+ years and I'm sure longer than that.

We risk going off topic here... And my opinions are based on the knowledge on accelerated deco which as available 20 years ago, while some recent research could provide better info.
The first point, as clearly explained by @scubadada, is that accelerated deco is not worth the effort for these short deco times.
The second point is that we had reports of some bad reactions to gas switching: suddenly switching from from 21% oxygen to 100% oxygen at 6m depth has caused a number of passing-out or epileptic crisis, which is something better to be avoided underwater.
The third point is about the "model" employed for computing the length of deco stops. This model (Bulhmann) considers a number of fully-independent compartments (tissues), each trapping a certain amount of nitrogen, and releasing it based on the difference between the pressure of the trapped nitrogen and the partial pressure of nitrogen in the mixture in your lungs.
This is an over-simplistic model, not taking into account the transfer of nitrogen from one tissue to another.
Furthermore, the speed of release of nitrogen does not only depends on the pressure difference between the nitrogen in the tissue and the nitrogen in the lungs, it also depends on the ambient pressure (which does not change breathing pure oxygen).
In conclusion, I think that the Bulhmann model, albeit having proven reliable and reasonably safe for decompression in air, has not been tested enough for these accelerated deco profiles in pure oxygen.
As said, following research in the last 20 years has probably cleared these doubts.

 

[Gareth J]

Doing a dive as deco dive with air doesn’t make sense when you could probably do the same dive with nitrox without deco.

snipped

Doing deco dives with air as backgas and deco gas doesn’t make sense for me. You can probably do that dive without deco when using the right gas.

Remember it is also a question of logistics. By preference [1], we would select an optimum Nitrox mix, in the case this is not possible, a 'standard' gas, failing that air.

Nitrox is not always available.
We managed to squeeze a trip to Scotland in between lockdowns. There was limited O2, which wasn't guaranteed. (I took a spare twinset to swap out my CCR, just incase.)
Other than the gas we arrived with, we dived air. Decompressed on air (or the limited deco gas we arrived with).

We run trips to Scapa Flow every five years or so.
The first time I went, only 1 boat would provide Nitrox, by prior arrangement. My buddy and I where the only two on our boat using twinsets, we also took 3 litre cylinders. I used to get up early, to visit the boat to get the 3 litres filled with 50%. We dived air, and decompressed on 50% (on air tables). The longest stop was 45 minutes.
The second time, O2 clean twinsets, + 3 litre cylinder (50%). Optimum mix each dive. (Nitrox was now more available)
Now, O2 clean twinsets + deco cylinders. 36% mix on the second dive. Air top for the following morning first dive (but ensuring PO2 will be 1.4 or less). Even then, we still run stops on the first dives, often on the second.

If Nitrox is logistically difficult. It's easy to take a few stages with deco' mixes and use them over the week and use air as the diving gas.

I've been on trips where the O2 and Helium start to run low. You have to manage the available gas and modify the dive plans appropriately, or stop diving.
We've also taken our own J's of O2 to blend our own. But transporting O2 is complicated, especially when you put it on a ferry.


Gareth J


[1] Note a predominantly dive CCR these days

 

[Compressor]

For basic divers reading this thread, I'd like to point out that a dive does not have to have a mandatory decompression stop to be enjoyable. When you dive in such a way that there is a physiologic decompression requirement, you will increase the dive's risk. Good training is required to do decompress dives. If you develop an issue that requires surfacing and you have a mandatory decompression stop, there will be trouble,

 

[MichaelMc]

You can somewhat shorten deco stops using oxygen, if you thrust those theories which are behind accelerated deco.
I have always been quite dubious of these theories, so if I have oxygen or high Nitrox available, I am very happy breathing it during the deco stops, but I simply consider it as an added safety, and I DO NOT shorten my deco times.

And my opinions are based on the knowledge on accelerated deco which as available 20 years ago, while some recent research could provide better info.
....
As said, following research in the last 20 years has probably cleared these doubts.

Angelo,

Your historical perspective is often wonderful, but when your data is 20 years old and you respond in a non-historical context, you may be confusing other divers instead of helping them.

 

[Gareth J]

For basic divers reading this thread, I'd like to point out that a dive does not have to have a mandatory decompression stop to be enjoyable. When you dive in such a way that there is a physiologic decompression requirement, you will increase the dive's risk. Good training is required to do decompress dives. If you develop an issue that requires surfacing and you have a mandatory decompression stop, there will be trouble,

I fully agree..... I've solved the problem by diving a CCR .

Joking aside, you are correct, there is plenty of diving that doesn't require compulsory staged decompression. It depends on the diving you wish to do, and the sites available. No one should be pressured into doing diving that they don't enjoy or feel they are being pushed beyond their comfort zone.

If you are diving a single 12, you are unlikely to have sufficient gas to require stops in a single days diving. As the number of consecutive dives increase, then you have more potential.
Personally, if you don't want to do stops, all strength to you. Unless it is a dive I have been planning or wanting to do for some time, but then I would dive with a different buddy, but that is part of the preplanning. I generally dive on 'club dives', so I dive with everyone, from students too experienced divers, we all do. We dive within the limits of the most conservative or least experienced diver in the buddy pair.

What does worry me is people rushing the ascent because they see the NDL nearing 0. I've seen too many divers ascending far too quickly up the shot because they where told they where going to die if they dived beyond the NDL, or they would be banned from further diving that day, or any similar foolish statement about exceeding NDL.
OK, they made a mistake and overshot the NDL. They should know this is no problem, a controlled ascent to the stop depth is the action they should take and ride out the stops. As long as they have the gas, they stay and do the time, even if they drop below 50 bar (into the reserve).
The hard and fast rule is that you ensure you do surface, even if that means ultimately they skip the last of the stops. If you do that you tell the dive manager (DM, or skipper), and get on the O2 immediately and are monitored for DCI. - Then you certainly aren't diving for the next 48 hours.
We can fix a bend, we can't fix drowning.

 

[Julius SCHMIDT]

We get the speel this is a non deco dive boat, if you go into deco you won't be doing the second dive
If you treat us like children you get children

Interestingly I've seen divers on tech boats all clumped on the line together
so no communication on the boat to stagger them, and ascending far too slowly
all looking at each other to see who's going to move up to the next stop first, and stopping far too long