Why have slow ascents or a 3m stop on Oxygen?

[Janos]

Hi all,

One question is bothering me, which although it doesn't keep me awake at night, might be the start of an interesting discussion.

If you are decompressing on oxygen for your last stop, why not spend the extra time at 6m instead of the slow ascent and/or 3m stop. You've not got any inert gas in your breathing mix, so you're off-gassing just as quick.

Janos

[vid]

Quote: (Originally Posted by Janos)

If you are decompressing on oxygen for your last stop, why not spend the extra time at 6m instead of the slow ascent and/or 3m stop. You've not got any inert gas in your breathing mix, so you're off-gassing just as quick.

isn't it because GUE gas planning works on the assumption that the ascent from 6m should be 20% of the total period spent in O2 deco? so, if you add (unnecessary?) extra time at 6m - then you just lengthen the ascent too?

vid

[fraser]

AFAIUI we don't do 3m stops on O2.

My understanding is after the O2 stop is complete we do 6 mins to the surface, I'm assuming to keep things nice and slow.

This is really Tech2 though, or hopefully one of the more experienced guys can confirm?

Thanks,

Fraser.

[JohnKendall]

Quote: (Originally Posted by Janos)

Hi all,

One question is bothering me, which although it doesn't keep me awake at night, might be the start of an interesting discussion.

If you are decompressing on oxygen for your last stop, why not spend the extra time at 6m instead of the slow ascent and/or 3m stop. You've not got any inert gas in your breathing mix, so you're off-gassing just as quick.

Janos

Hi Janos,

Where did you get this from? I'm only asking so that I can see what situation it's suggested in.

In the ocean, on fairly short dives we will do the O2 stop at 6m, and then have a slow ascent (5-6 mins from 6-surface). However on longer dives then the 3m stop may be required, however it will still be quite short.

At least that is my understanding.

J

[Jack]

Quote: (Originally Posted by Janos)

Hi all,

One question is bothering me, which although it doesn't keep me awake at night, might be the start of an interesting discussion.

If you are decompressing on oxygen for your last stop, why not spend the extra time at 6m instead of the slow ascent and/or 3m stop. You've not got any inert gas in your breathing mix, so you're off-gassing just as quick.

Janos

This is not a GUE answer but................
6m = 1.6ppo2 and 1.6 bar absolute
5m = 1.5 ppo2 and 1.5 bar absolute
4m = 1.4 ppo2 and 1.4 bar absolute
3 m = 1.3 ppo2 and 1.3bar absolute
2m = 1.2 ppo2 and 1.2bar absolute
1m = 1.1ppo2 and 1.1 bar absolute
0m = 1.0 ppo2 and 1 bar/ata (5 min on o2 )
While you are not on gassing inert gas at any of these depths on o2 ,you are OFF GASSING and reducing ambient pressure on ascent and increasing the off gassing gradient, which helps with a cleaner deco on the slow tissues as you ascend slowly from 6m stop.

[Janos]

Quote: (Originally Posted by JohnKendall)

Where did you get this from? I'm only asking so that I can see what situation it's suggested in.

Sorry - it could well be my mistake. I'm sure that I've pciked up the idea that you do a 3m stop but I can't remember where from. I could well be wrong

Quote: (Originally Posted by JohnKendall)

In the ocean, on fairly short dives we will do the O2 stop at 6m, and then have a slow ascent (5-6 mins from 6-surface). However on longer dives then the 3m stop may be required, however it will still be quite short.

But from an off-gassing point of view, wouldn't it be just as productive [1] to spend the six minutes you spent ascending at 6m? Ie:

0 to 60min - some sort of dive and deco
61 min - 6m
62 min - 6m
63 min - 6m
64 min - 6m
65 min - 6m
66 min - Surface

Instead of:

61 min - 5m
62 min - 4m
63 min - 3m
64 min - 2m
65 min - 1m
66 min - Surface

?

I probably wouldn't do the first profile (if for nothing else it keeps be practiced for when I'm using say 50% as a deco gas) but I'm just querying the notion that slow ascents (compared to the same time at 6m) make any difference to your deco profile.

Janos

[1] - More so if you subscrive to the Oxygen Window concept

[Janos]

Quote: (Originally Posted by Jack)

increasing the off gassing gradient

Ah, but if you're on pure O2 then the PP of any inspired inert is going to be zero, irrespective of depth, so the off-gassing gradient is the same at 0m, 6m, and 60m.

The only way that I can see it would change the way you off-gassing is if you ascend enough to cause small bubbles to form which are filtered out by the lungs (ie bend and mend).

Janos

[JohnKendall]

Quote: (Originally Posted by Janos)

But from an off-gassing point of view, wouldn't it be just as productive [1] to spend the six minutes you spent ascending at 6m? Ie:

0 to 60min - some sort of dive and deco
61 min - 6m
62 min - 6m
63 min - 6m
64 min - 6m
65 min - 6m
66 min - Surface

Instead of:

61 min - 5m
62 min - 4m
63 min - 3m
64 min - 2m
65 min - 1m
66 min - Surface

?

I probably wouldn't do the first profile (if for nothing else it keeps be practiced for when I'm using say 50% as a deco gas) but I'm just querying the notion that slow ascents (compared to the same time at 6m) make any difference to your deco profile.

Janos

[1] - More so if you subscrive to the Oxygen Window concept

It's a matter of what you need to do. You could spend the extra time at 6m, and I have done dives where due to the conditions we have lengthened the 6m stop as we knew the ascent would be quicker. However from 6m-surface is the highest pressure change, so this is where you have to be most careful. By doing the slow ascent I feel much better after the dive, therefore in my mind it must be doing something right. This is the big thing with deco. You need to find out what works for you.

J

[Jack]

[quote=Janos]Ah, but if you're on pure O2 then the PP of any inspired inert is going to be zero, irrespective of depth, so the off-gassing gradient is the same at 0m, 6m, and 60m.

Correct, the inspired inert is zero. But what about the already inspired inert thats wanting to come back out



The only way that I can see it would change the way you off-gassing is if you ascend enough to cause small bubbles to form which are filtered out by the lungs (ie bend and mend).

Thats not what I said. We are talking about 6m to surface,yes?
This is a rough guide.
When you leave 6m, to all intents and purposes ,you have finished your "deco", i.e. the inert gases you inspired are at a rough state of equilibrium with the ambient pressure. Biggest pressure change is between surface and 5m. SOOOOOO, 6M -Surface = 100%,,, 6m-5m =16% ok? So you move from 6m to 5m you increase the OFFGASSING gradient by 16% CONTROLLED increase in offgassing. As JK said, he feels much better after this.

[Janos]

Quote: (Originally Posted by JohnKendall)

However from 6m-surface is the highest pressure change, so this is where you have to be most careful.

This is true if you are ascending close to your decompression ceiling [1]. But my point is that once you get to 6m, your ceiling will get higher and higher until it hits the surface. Once the ceiling is at the surface it's safe to come up. [3]

Now my understanding is that the DIR way is to ascend super-slowly until you hit the surface, but I can't see any reason why you can't spend that time at 6m and then come up instantly. [4]

Quote: (Originally Posted by JohnKendall)

By doing the slow ascent I feel much better after the dive, therefore in my mind it must be doing something right.

Ah, but what's the "control group" in this? You need to compare deco + a slow ascent to deco + 6mins extra at 6m.

Also, if you're feeling a bit dodgy without the super-slow ascent then doesn't that that mean that you're not "clean" when you finish your 6m stop?

Quote: (Originally Posted by JohnKendall)

This is the big thing with deco. You need to find out what works for you.

Very true.
Currently I get on to the 80% at 9m and I will move up slightly to about 5m after a while at 6m. But all that's changing and so I'm thinking about how I want to deco in the future.

Janos

[1] - Ie the shallowest depth which you're not going to get bent[2]
[2] - Yes I agree it's a probability thing - but everyone chooses their level of risk and that gives you your ceiling.
[3] - Is this the DIR way incidentally? Is the slow ascent a mandatory part of the deco plan.
[4] - Ignoring air expansion injuries of course.