Accelerated deco: low PO2 vs high PO2?

[Rage]

Not that I'm aware of any evidence that indicates decompressing on lower ppO2 mixes reduces the likelihood of DCS. If the instructor has some information, perhaps you can get him to share so that we can all learn.

Unfortunately he doesn't have anything solid, only personal observations. Like I said, it could be coincidence or due to a different factor. Thats why I am asking here, as some of you guys may through work or interest have a better and / or newer ideas of decompression than your every day tech diver / instructor.

Thanks,
S.C.

 

[100days-a-year]

From a DCS standpoint, both clinical and sub-clinical it would seem only sensible that more P02 is better.DCS is not however the only danger.Countercurrent?Blood has a total gas load ability,the speed with which they(gasses) come out is based on a lot of variables which from dive to dive and gas to gas change.There are very few absolutes in life.When every program you run shows you getting out faster with 80% on a specific dive,I would tend to think the programmers didn't intentionally all screw up at the same point.Standardization has it's benefits too tho.

 

[Genesis]

For single-gas deco dives (one decompression gas) I take a slung bottle of 50%. It can be breathed at 70', which is where I make my gas switch. This provides several benefits, the most important of which is that if I am forced out of the water early, I have gotten the benefit of SOME decompression.

I might see this differently in a cave, where being forced out is extremely unlikely. However, in the ocean its a real possibility. Predators are the most common reason, but not the only one - an approaching wall cloud is a damn good reason to get out of the water!

I do tend to keep runtimes to an hour or so though..... I might see it differently if it mean an extra hour of deco.

Some examples:

Dec to 140ft (2) on Nitrox 26.0, 50ft/min descent.
Level 140ft 27:12 (30) on Nitrox 26.0, 1.36 ppO2, 129ft ead
Asc to 100ft (31) on Nitrox 26.0, -30ft/min ascent.
Asc to 70ft (32) on Nitrox 26.0, -30ft/min ascent.
Stop at 70ft 1:40 (34) on Nitrox 50.0, 1.56 ppO2, 32ft ead
Stop at 50ft 1:40 (36) on Nitrox 50.0, 1.26 ppO2, 20ft ead
Stop at 40ft 3:00 (39) on Nitrox 50.0, 1.10 ppO2, 13ft ead
Stop at 30ft 4:00 (43) on Nitrox 50.0, 0.95 ppO2, 7ft ead
Stop at 20ft 19:00 (62) on Nitrox 50.0, 0.80 ppO2, 1ft ead
Asc to sfc. (62) on Nitrox 50.0, -30ft/min ascent.

Off gassing starts at 93.2ft

OTU's this dive: 74
CNS Total: 28.5%


Decompression model: VPM-B

DIVE PLAN
Surface interval = 2 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2

Dec to 140ft (2) on Nitrox 26.0, 50ft/min descent.
Level 140ft 27:12 (30) on Nitrox 26.0, 1.36 ppO2, 129ft ead
Asc to 100ft (31) on Nitrox 26.0, -30ft/min ascent.
Asc to 60ft (32) on Nitrox 26.0, -30ft/min ascent.
Stop at 60ft 0:20 (33) on Nitrox 26.0, 0.73 ppO2, 54ft ead
Stop at 50ft 3:00 (36) on Nitrox 26.0, 0.65 ppO2, 45ft ead
Stop at 40ft 4:00 (40) on Nitrox 26.0, 0.57 ppO2, 35ft ead
Stop at 30ft 5:00 (45) on Nitrox 26.0, 0.50 ppO2, 26ft ead
Stop at 20ft 12:00 (57) on Oxygen, 1.60 ppO2, 0ft ead
Asc to sfc. (57) on Oxygen, -30ft/min ascent.

Off gassing starts at 93.2ft

OTU's this dive: 74
CNS Total: 46.2%

Note that I get out of the water 5 minutes faster taking the 100%, but as the "penalty" for that I take a higher CNS hit. The OTU dose does not change though.

Now let's say that I get driven out of the water after 2 minutes @ 20' by a 12' Hammerhead who wants to make a lunch out of me. If this happens with the 100%, I've gotten almost no accelerated decompression benefit at all! On the other hand, if it happens on the 50%, I've gotten quite a bit of accelerated deco, and logic says that the overpressure of nitrogen is less in the case where I switch @ 70'.

So while both may well nail me, using the 50% provides a better shot at getting out of the water without a MAJOR DCS incident.

 

[padiscubapro]

For single-gas deco dives (one decompression gas) I take a slung bottle of 50%. It can be breathed at 70', which is where I make my gas switch. This provides several benefits, the most important of which is that if I am forced out of the water early, I have gotten the benefit of SOME decompression.

I might see this differently in a cave, where being forced out is extremely unlikely. However, in the ocean its a real possibility. Predators are the most common reason, but not the only one - an approaching wall cloud is a damn good reason to get out of the water!

I do tend to keep runtimes to an hour or so though..... I might see it differently if it mean an extra hour of deco.

Some examples:

Dec to 140ft (2) on Nitrox 26.0, 50ft/min descent.
Level 140ft 27:12 (30) on Nitrox 26.0, 1.36 ppO2, 129ft ead
Asc to 100ft (31) on Nitrox 26.0, -30ft/min ascent.
Asc to 70ft (32) on Nitrox 26.0, -30ft/min ascent.
Stop at 70ft 1:40 (34) on Nitrox 50.0, 1.56 ppO2, 32ft ead
Stop at 50ft 1:40 (36) on Nitrox 50.0, 1.26 ppO2, 20ft ead
Stop at 40ft 3:00 (39) on Nitrox 50.0, 1.10 ppO2, 13ft ead
Stop at 30ft 4:00 (43) on Nitrox 50.0, 0.95 ppO2, 7ft ead
Stop at 20ft 19:00 (62) on Nitrox 50.0, 0.80 ppO2, 1ft ead
Asc to sfc. (62) on Nitrox 50.0, -30ft/min ascent.

Off gassing starts at 93.2ft

OTU's this dive: 74
CNS Total: 28.5%


Decompression model: VPM-B

DIVE PLAN
Surface interval = 2 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2

Dec to 140ft (2) on Nitrox 26.0, 50ft/min descent.
Level 140ft 27:12 (30) on Nitrox 26.0, 1.36 ppO2, 129ft ead
Asc to 100ft (31) on Nitrox 26.0, -30ft/min ascent.
Asc to 60ft (32) on Nitrox 26.0, -30ft/min ascent.
Stop at 60ft 0:20 (33) on Nitrox 26.0, 0.73 ppO2, 54ft ead
Stop at 50ft 3:00 (36) on Nitrox 26.0, 0.65 ppO2, 45ft ead
Stop at 40ft 4:00 (40) on Nitrox 26.0, 0.57 ppO2, 35ft ead
Stop at 30ft 5:00 (45) on Nitrox 26.0, 0.50 ppO2, 26ft ead
Stop at 20ft 12:00 (57) on Oxygen, 1.60 ppO2, 0ft ead
Asc to sfc. (57) on Oxygen, -30ft/min ascent.

Off gassing starts at 93.2ft

OTU's this dive: 74
CNS Total: 46.2%

Note that I get out of the water 5 minutes faster taking the 100%, but as the "penalty" for that I take a higher CNS hit. The OTU dose does not change though.

Now let's say that I get driven out of the water after 2 minutes @ 20' by a 12' Hammerhead who wants to make a lunch out of me. If this happens with the 100%, I've gotten almost no accelerated decompression benefit at all! On the other hand, if it happens on the 50%, I've gotten quite a bit of accelerated deco, and logic says that the overpressure of nitrogen is less in the case where I switch @ 70'.

So while both may well nail me, using the 50% provides a better shot at getting out of the water without a MAJOR DCS incident.

Genesis I agree,

For the most part I train my students to plan their decos on 50%, and stage 100% near the exit point.. I don't teach them to carry a useless gas at depth.. The 50% is always with them so it can be switched to @100fsw in an emergency, do the entire deco on it if necessary, and if all goes well they can switch to it at 20fsw or 10fsw for some additional offgassing..
When the decompressions get very long 100% becomes a necessity, but for most people's diving 50% does a very good job.

 

[gilou28]

hum: Switch to 50% @100fsw.....

It has to be a real good emergency to do that.... At a PO2 above 2, better get to 70 ASAP...


I think that both to 50% and 100% will help shorten the deco time with the added safety of having an intermediate mix. Also, if you are coming from a high He trimix (ex 18/40), having an intermediate trimix (30/20) could be a good solution.

 

[Genesis]

I think that both to 50% and 100% will help shorten the deco time with the added safety of having an intermediate mix. Also, if you are coming from a high He trimix (ex 18/40), having an intermediate trimix (30/20) could be a good solution.

You need to look at this with an eye towards what carrying two bottles does to the REST of the dive planning and safety.

Sometimes its the right thing to do, but often its not, especially in the ocean. Its not a big deal in a cave where you can drop one (or both!) when you exceed their MODs, because you can't use them at that depth anyway, but this is definitely NOT true in open water.

Dropping deco bottles in the ocean has been implicated in several fatalities when the diver was unable to locate them on the way out/up and had insufficient gas to complete their decompression on their backs.

As someone who does only "light" deco dives right now (approximate runtimes of an hour) I will not drop a deco bottle. If I cannot take it with me, then wherever I want to go isn't somewhere I need to go.

While I probably would not kill myself if I lost the gas today with the dives that I'm doing, I might get hurt pretty badly, and setting up bad habits is never a good idea.

 

[Saturation]

Thanks for the replies so far. I've heard a few interesting things I'll look into. I am particularly interested in theories such as what Saturation mentioned regarding Heliox, though 100days comments led to some interesting reading too.

To rephrase the question: Is there any evidence to substantiate the theory that decoing on lower a PO2 (e.g 1.12) for a longer period, is safer (in terms of less likelyhood of suffering DCS), than decoing on a PO2 of 1.6 for a shorter time?

Thanks for reading,
S.C.

Here you go SC, in experiments using rats:

http://www.ncbi.nlm.nih.gov/entrez/...ve&db=PubMed&list_uids=11299250&dopt=Abstract

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8061554&dopt=Abstract

In human trials in chambers for treatment of DCI:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9074400&dopt=Abstract

As for Question #2, there is none. You didn't really mean CNS toxicity as another user showed using Vplanner? If so, the risk for CNS tox is greater with pp02 1.6 versus 1.12, but in deco, there is no known DCI protection using the lower 02.

 

[DepartureDiver]

As someone who does only "light" deco dives right now (approximate runtimes of an hour) I will not drop a deco bottle. If I cannot take it with me, then wherever I want to go isn't somewhere I need to go.

In open water, always carry all your gas with you. Also, while not a standard mix or "DIR", if carrying only one gas during a nitrox dive, consider switching to a 60-63% blend at 50'.

 

[Genesis]

In open water, always carry all your gas with you. Also, while not a standard mix or "DIR", if carrying only one gas during a nitrox dive, consider switching to a 60-63% blend at 50'.

Why 60%?

50% is a nice mix in that if I end up with a serious CF at 130-140 I can PROBABLY make a CESA to 70' where the 50% is breathable.

Now that's a VERY unlikely scenario, at least now, as I've taken to diving doubles for virtually everything, but it certainly wasn't all that unlikely when I was diving singles, without a H-valve, at those depths! (Yes, I was dumb not all that long ago in that regard....)

 

[gilou28]

Thanks Saturation for your links...

This brings a reflexion to my mind:

If I can only bring one deco bottle, should I opt for:

1- a 72/28 Heliox and start my deco at 40 fsw and keep that mix at 30, 20 and 10 fsw

2- a 100 O2 mix from 20 fsw.

Since each inert gaz off-gaz independantly, having the He at 33 fsw should not slower the N2 off-gas.

V-Planner confirms that the 2nd option will save about 6 minutes (7%) of deco on one test I did with a 220f 20 min with 18/40 trimix.

$$$ wise, it's about the same, since I save on my bottom mix, the real difference is less than dollars (CAD) total.

Have fun!