Shearwater's take on the helium penalty

[Superlyte27]

Did any of you guys read how small their sample size was? I won't be changing anything anytime soon.

 

[rjack321]

This begs the question: For those of us planning OC trimix dives, could we plan the ascent based purely on the FO2 being used and ignore the Helium. I.e. set your Petrel or Perdix to the correct FO2 of the gas(es) you're breathing, but leave the He content set to 0, even if you're actually using trimix? Or, for example, when planning in Multi Deco, set the gases as if they were Nitrox, with the same FO2?

I just talking about doing this to determine the ascent stops and times. If you're really using trimix, obviously there are some other aspects of the planning you'd have to do based on the actual gas you plan to use.

It seems like this would allow you to plan ascents with less time spent doing deco, with, apparently (according to the NEDU study), no increase in risk of DCS.

Or what am I missing?

You missed that as you go deeper the probability of DCS is rising. Buhlmann is not a constant risk model. One way this is ameliorated is because buhlmann is penalizing you for the helium. So its adding deco time, which lowers the risk of DCS. If you ignore the helium and treat all inerts as equal you are technically correct, they are behaving equally. But because ZHL-16 is not an equal risk model you are increasing your risk on the deeper deco dives compared to the shallower ones. (equal risk models exist just not for recreational deco divers)

 

[stuartv]

You missed that as you go deeper the probability of DCS is rising. Buhlmann is not a constant risk model. One way this is ameliorated is because buhlmann is penalizing you for the helium. So its adding deco time, which lowers the risk of DCS. If you ignore the helium and treat all inerts as equal you are technically correct, they are behaving equally. But because ZHL-16 is not an equal risk model you are increasing your risk on the deeper deco dives compared to the shallower ones. (equal risk models exist just not for recreational deco divers)

I understand what you're saying.

Just to clarify a minor point in my own mind, you mentioned deeper dives as being the issue. My understanding from the article was that risk increases as deco time increases. So, a deep dive with a short bottom time and a shallower dive with a longer bottom time could both have the same risk of DCS, right?

And, for that matter, since people typically use less or no Helium on shallower dives (say, in the 130 - 180 foot range), I might actually suppose that dives in that range using only Nitrox are actually more risky than deeper dives on trimix, because the computers we're using today would not add the Helium Penalty to the ascent. Correct?

 

[tursiops]

since people typically use less or no Helium on shallower dives (say, in the 130 - 180 foot range)

Why do you think this?

 

[stuartv]

Why do you think this?

I know there are people who use trimix below 100', but I think there are LOTS of people who dive to 130' and do not use trimix.

I also think there are plenty of people who are certified as TDI AN/DP, which means they are certified to 150', but not certified for any helium. I think that all of those people typically do dives with no Helium at any depth, including depths in the 130 to 150' range.

 

[victorzamora]

It seems like this would allow you to plan ascents with less time spent doing deco, with, apparently (according to the NEDU study), no increase in risk of DCS.

Or what am I missing?

It's conceivable that....we are doing the right amount of deco but probably for the wrong reason.

I think this is the most relevant quote to your question. Why can't we just blow off (potentially) hours of deco by telling our computers to ignore the He in our tanks? Because it'd likely increase the pDCS beyond our comfort level.

 

[Beau640]

It seems like our models fall apart for these longer, deeper profiles where this fake "helium" penalty may actually be the thing saving us on those profiles, but for the wrong reason.

But what about using helium on shallow dives? Is there truly no difference between nitrogen and helium? The military study suggests that may be true. I don't think we have enough info to make the decision to lie to our computers about our He %, but what this study does say is that our current He models are wrong and we need more research to revise them correctly. It seems like we shouldn't be doing things to lower deco in the meantime. If anything we should be more conservative until our models are corrected.

 

[PfcAJ]

It seems like our models fall apart for these longer, deeper profiles where this fake "helium" penalty may actually be the thing saving us on those profiles, but for the wrong reason.

But what about using helium on shallow dives? Is there truly no difference between nitrogen and helium? The military study suggests that may be true. I don't think we have enough info to make the decision to lie to our computers about our He %, but what this study does say is that our current He models are wrong and we need more research to revise them correctly. It seems like we shouldn't be doing things to lower deco in the meantime. If anything we should be more conservative until our models are corrected.

I've been diving 3030 and 32% the same and so far so good.

n=1

 

[kensuf]

I've been diving 3030 and 32% the same and so far so good.

n=1

Must be nice to be a rich guy.

 

[Dr Simon Mitchell]

Did any of you guys read how small their sample size was? I won't be changing anything anytime soon.

Hello,

I get why you might say this, but I think you have misunderstood the purpose and methodology of the study.

The context for the USN was that several other Navies are developing trimix tables for use with CCRs, with one of the putative advantages being shorter decompression (than for a heliox diluent). Based on work by their own scientists (David Doolette), the USN suspected that this advantage was more assumed than real so they set out to investigate this in the study.

They reported an experiment designed to answer the question of whether decompression from a trimix dive is more efficient than a heliox dive. Identical dives (under tightly controlled conditions) to 200' (60m) for 40 minutes bottom time with 119 minutes of decompression were performed by rebreather divers using a PO2 set point of 1.3 and either heliox (oxygen 12% helium 88%) or trimix (oxygen 12%, helium 44%, nitrogen 44%) diluent. To be clear, the dives were identical in every way, other than the diluent used. The primary outcome measure was decompression sickness.

It is crucial to understand that this was what is referred to as a non-inferiority trial with pre-defined stopping criteria to be invoked if it became apparent, to the chosen level of statistical certainty, that heliox was not inferior as a decompression gas to trimix. They were not interested in proving that one of the gases was better than the other (which as Superlyte27 implies would require a much larger trial).... indeed, if you think about it, given Doolette's animal work which showed no difference in helium and nitrogen kinetics in two relevant tissues, why would they hypothesize that one or the other would be superior? What they were interested in determining was whether the gas they currently use (heliox) was no worse in terms of decompression efficiency than the one they could potentially change to (trimix).

After 50 heliox dives were completed with no DCS and 46 trimix dives were completed with 2 cases of DCS the stopping criteria were met. The essence of this result is that there is a very low statistical probability that if they continued with 100s more dives, that trimix would eventually prove to be more efficient. Thus, they have not demonstrated that heliox is better (please be clear about that) but they have demonstrated that its very unlikely that heliox is worse. That is what is meant by a "non-inferiority" trial. The result is confluent with the animal work.

So if I'm reading this right, there's thought that there is no true helium penalty in that just because we add helium to a mix, it doesn't necessarily mean we need to perform more deco by adding deep stops for helium which add in more nitrogen tissue on gassing during those stops. But there is also a "deep" penalty because our models fall apart at greater depths and we know we need more deco. If i'm reading the article right, it's implying that we are doing the right amount of deco for our trimix, but for the wrong reason. We are doing the right amount of deco because of he depth, but not because of the helium.

That is exactly right. The conundrum is in knowing how to practically apply these findings. Can we all tell our computers that we are diving air diluent when we are really diving trimix? The study does not answer that question unfortunately. Although it suggests that there is no evidence for treating helium and nitrogen differently (as reported by Doolette in animals) it is conceivable that the long held belief that helium needs more decompression has compensated for underestimation of required decompression (to achieve acceptable levels of risk) by the decompression algorithms we all use, and that we are doing the right amount of deco from very deep dives but probably for the wrong reason. I probably would not change much at this point.

Simon M