hypothetical deco regulator failure

[Trace Malinowski]

Hmm . . . in my classes which have discussed staged decompression, it was never suggested that 50% was a "better" gas for decompression from 150 feet. I was taught that the loss of decompression efficiency was offset by the improvement in rock bottom requirements. Rock bottom from 150 feet or more to a 20 foot gas switch becomes quite impressive, especially when you factor in lost deco gas requirements.

Which is really the only good reason and the only good reason I provided in our discussion, but at the end of the day, a poor reason not to go with a better gas switch. Therefore, minimal gas requirements is more easily served by a bottom stage with back gas bailout with a switch to 100% at 20 feet.

 

[Trace Malinowski]

Yes, it's not surprising that 100% works better than 50%, naive physics would tell you as much ... I hope that they didn't spend a whole lot of money or time figuring that one out.

Lynne's other point about calculating your turn point to get you and your buddy on back gas to either 20 feet if your carrying pure oxygen, or 80 feet for 50% (OK, I cheat a little and am OK with a ppO2 approaching 1.8) is a rather telling one.

That's why I favor a travel gas, say EAN-32 for deep dives (I can make my switch here at about 150) or EAN-36 for a shallower dive (switch at 130). But I am going to end up on 100% at 20 no matter what. Green gas is good gas.

I was going to also mention that strategy in my original reply. It's less expensive when using helium as your bottom mix, but for the sake of a bottom failure, the bottom gas would have to go in the back gas and the bailout mix in the stage.

 

[Thalassamania]

That is the usual way of things.

 

[DA Aquamaster]

IIRCC I don't think isobaric counter diffusion is a major concern with normoxic mixes and depths and resulting helium percentages in the 30-35% range. I also have encountered some conditions with large waves and swells where a 10 ft stop can be hard to hold with precision. Consequently I tend to plan my last stop depth to be at 20 ft. Both those factors tend to influence my deco gas planning directly or indirectly.

The downside with 100% O2 if you do your last stop at 20' is that you spend a lot of time at a PPO2 of 1.61 and that really ratchets up the CNS clock, especially when it comes on top of a long bottom portion of the dive at a PPO2 around 1.3-1.4 and on the heals of deeper stops from 70' on up on 50%.

In short, I think the fairly common 50% and 100% deco gasses are not always great choices for a two gas deco dive, especially offshore.

My preference is to use a travel/first deco gas with about 30%-32% O2 (and depending on the max depth and the helium percentage used you can add 25-30% helium to the mix as well if you are concerned about ICD). And then use 80% for the second deco gas (which also leaves some room on top for some helium if ICD is viewed as a concern).

In terms of deco efficiency, run times with 80% versus 100% are virtually identical but the hit on the CNS clock is greatly reduced with 80%. You can get on it sooner (30 ft) but the PPO2 at 30 ft is still lower than O2 at 20' and the PPO2 is reasonable to tolerate for the entire final stop at 20' where it can be problematic on 100% O2.

In terms of "rock bottom" calculations, the travel gas in the 30% range helps a lot to increase the available back gas and if you lose access to the 80% it is still a reasonable gas to do most of the deco on.

I do not like to rely on deco on back gas as a contingency plan for any dives below 150 ft as the run time gets to be prohibitive, even if you could carry that much gas. You very quickly reach a point if you are only using one deco gas where lost gas planning severely limits your bottom time due to the back gas required, and in any event doubling or tripling your deco can become a major problem for the boat and everyone else on board and the odds ar ethey are going to bring you a tank to get you out of the water faster and I think putting them in that position is exceptionally bad form.

Now, in cave diving, the circumstances are different but depending on the cave profle and where the deep bits are, a travel gas in addition to a deco gas still makes a great deal of sense to increase back gas reserves and improve deco efficiency and lost gas planning.

 

[Cave Diver]

If its deep enough for helium its deep enough for 2 deco gases

You need two deco gasses for a 100' dive?

 

[PfcAJ]

Which is really the only good reason and the only good reason I provided in our discussion, but at the end of the day, a poor reason not to go with a better gas switch. Therefore, minimal gas requirements is more easily served by a bottom stage with back gas bailout with a switch to 100% at 20 feet.

So its somehow better to take 2 bottles (a bottom stage and o2) than to take 50% and have a deco gas option at 70ft? Throwing gear at the problem isn't the best solution, imo.

50% (only 50%) is a great choice for dives in the 130-170ft range with a total deco time of 30ish mins.

 

[TSandM]

Well, you can talk optimal strategy all day, but the 150 foot/60 minute run time dive for the beginning technical diver is made adequately safe in gas reserves AND decompression time with a 70 foot switch, as well as made simpler by carrying only one deco bottle. There are a lot of considerations that go into selecting a strategy.

 

[Trace Malinowski]

So its somehow better to take 2 bottles (a bottom stage and o2) than to take 50% and have a deco gas option at 70ft? Throwing gear at the problem isn't the best solution, imo.

50% (only 50%) is a great choice for dives in the 130-170ft range with a total deco time of 30ish mins.

The scenario presented by the OP was one in which you have a single decompression bottle.

Diving the bottom stage and leaving the back gas untouched isn't throwing gear at the problem. The benefit of having extra gas is a by-product of logistics allowing you to conserve gas supplies for repetitive or subsequent dives such as when trimix fills aren't immediately available. However, once employing that strategy, the benefit of having extra gas in the team is a nice cushion.

I know two GUE instructors who carry additional gas during Tech 1 classes and dives. If the Tech 1 gas strategy was bullet-proof why would they do that?

Well, you can talk optimal strategy all day, but the 150 foot/60 minute run time dive for the beginning technical diver is made adequately safe in gas reserves AND decompression time with a 70 foot switch, as well as made simpler by carrying only one deco bottle. There are a lot of considerations that go into selecting a strategy.

I always thought Nitrox 50 was a great choice myself. By the end of the evening in which I learned about the findings of the study, my mind was opened to the possibility of selecting oxygen over Nitrox 50 as the preferred "go to gas" for those profiles.

Our discussion began with questions such as:

"When you cave dive, which deco gas do you use and why?"

"Why would you use oxygen when cave diving and not use it for open water deep diving?"

"How do you feel after cave diving and switching to oxygen compared to open water diving and switching to Nitrox 50?"

In comparing my real-world experience to the scientific information provided to me regarding DCS recorded in the study, several things made sense. I realized that I rarely, if ever, felt anything unusual after oxygen decompression, but I've had decompression stress and some unusual sensations that heightened my internal awareness to the possibility of DCS being present after switching to Nitrox 50 alone. Nothing major. Just things that make you go "Hmmm."

Nitrox 50 was always my favorite gas both logistically and psychologically. No air breaks. Simple, single gas choice. Easy. Love it!

Yet, in my real-world experience, it was a conservative Tech 1 dive which resulted in the need for a teammate to be chambered. No issues have occurred while cave diving with oxygen switches in my personal circle. While such things are only anecdotal, personal experience corresponds to the science presented to me.

Yes, there are a lot of considerations that go into selecting a strategy. Often, the forums are one-sided when it comes to decompression strategies. I thought I'd take the opportunity to present alternative considerations for single deco gas diving.

 

[TSandM]

Thought-provoking post, Trace; thank you. I think there are some differences in gas planning between caves and open water that weigh in here, but I'll have to think about it a bit before I can articulate it intelligently.

 

[LiteHedded]

The scenario presented by the OP was one in which you have a single decompression bottle.

Diving the bottom stage and leaving the back gas untouched isn't throwing gear at the problem. The benefit of having extra gas is a by-product of logistics allowing you to conserve gas supplies for repetitive or subsequent dives such as when trimix fills aren't immediately available. However, once employing that strategy, the benefit of having extra gas in the team is a nice cushion.

I know two GUE instructors who carry additional gas during Tech 1 classes and dives. If the Tech 1 gas strategy was bullet-proof why would they do that?



I always thought Nitrox 50 was a great choice myself. By the end of the evening in which I learned about the findings of the study, my mind was opened to the possibility of selecting oxygen over Nitrox 50 as the preferred "go to gas" for those profiles.

Our discussion began with questions such as:

"When you cave dive, which deco gas do you use and why?"

"Why would you use oxygen when cave diving and not use it for open water deep diving?"

"How do you feel after cave diving and switching to oxygen compared to open water diving and switching to Nitrox 50?"

In comparing my real-world experience to the scientific information provided to me regarding DCS recorded in the study, several things made sense. I realized that I rarely, if ever, felt anything unusual after oxygen decompression, but I've had decompression stress and some unusual sensations that heightened my internal awareness to the possibility of DCS being present after switching to Nitrox 50 alone. Nothing major. Just things that make you go "Hmmm."

Nitrox 50 was always my favorite gas both logistically and psychologically. No air breaks. Simple, single gas choice. Easy. Love it!

Yet, in my real-world experience, it was a conservative Tech 1 dive which resulted in the need for a teammate to be chambered. No issues have occurred while cave diving with oxygen switches in my personal circle. While such things are only anecdotal, personal experience corresponds to the science presented to me.

Yes, there are a lot of considerations that go into selecting a strategy. Often, the forums are one-sided when it comes to decompression strategies. I thought I'd take the opportunity to present alternative considerations for single deco gas diving.

well that depends on the cave...