Requiring shallow helium mixes?

[RyanT]

Yeah,the chance of osteonecrosis and permanent lung damage doesn't sound high but it's there.
PubMed is the place get the scoop on this stuff if you're interested either way.

@100days-a-year, a big thanks! I've been reading the literature quite a bit, but I have not seen this paper! I have often wondered about longer term effects of the production of ROS from high O2 exposure, but until this, I had not seen any evidence.

 

[northernone]

Oxygen Toxicity and Special Operations Forces Diving: Hidden and Dangerous. - PubMed - NCBI

Yeah,the chance of osteonecrosis and permanent lung damage doesn't sound high but it's there.
PubMed is the place get the scoop on this stuff if you're interested either way.

Thank you. I used an o2 rebreather quite extensively for photography, I'm going to pay more caution to my exposure levels.

Cheers,
Cameron

 

[Kay Dee]

Those who might be comfortable with 1.6 ppo2 rather than 1.8 or 1.4. It was (is) a fairly common maximum so there might be a lot of divers. Would you prefer the term pod, or school, perhaps gaggle? Joking aside maybe just (unable to add an option for ppo2 1.6) and leave it at that

Pardon my misinterpretation of your wording then, but it (erroneously) appeared to me that you may have been - I'm struggling for the right term here - but lets say 'putting down' the '1.6 crowd', a 'limit' (i.e. the old NOAA max 45min @1.6) I was taught back in the Dark Ages long ago and followed over the years without incident, save for HIM. Anyway, thanks for clarifying my misunderstanding of your wording.

He is referencing PO2 - Partial Pressure of 1.6 rather than 1.4 thought/proven to be generally safer..

I totally understood what the 1.6 refers to, and am comfortable with that exposure for a time, but was a little puzzled shall we say at how he phrased his wording is all. Clarified now though.

 

[Kay Dee]

The poll is about what you "require". What is the depth where you would not plan to go deeper, period, unless you had helium?

Yes, and that is something I misconstrued as it were at first (just paint me slow, or a poor reader, duh!) and inverted it in my head to mean what was the deepest one would go on air before they felt they required using helium. Now while I'd use it shallower, and do / have, my first post answer / suggestion (that I erased when I realised my mistake), was I'd have no qualms going to 220ft / 66m on air (or a little deeper for a very short time) in the 'right conditions' and have done so on many an occasion on OC back in the day (that old 1.6 bar/ata maximum, remember?) before I'd definitely switch to He if, repeat 'if' it was available. And some places we dived it simply wasn't. That is, much to some peoples surprise who now have He on tap as it were, it ain't always available everywhere in ze world.

And just for the record, all the many years of CCR diving I have done, I have done so with a setpoint of 1.4, and at times well exceeded the so-called CNS clock, as we did - on OC - back in the day when decoing from what were once referred to as very deep gas dives (to 430ft/130m in my case), but now seem relatively common place to some folks, and that depth well and truly exceeded today (but not by me anymore).

 

[Kevrumbo]

With the research in gas densities and narcosis being significant at shallower depths then historically considered what is your personal minimum depth where helium becomes a reasonable necessity for your dive plan? How deep a dive would you cancel and not dive if helium was unavailable? Side note: recognizing conditions, availability and cost also dictate this decision. Making a meaningful poll factors are limited. Assume 'normal' conditions for you.

Rough poll attached. Also curious what has helped form your current assessment.

Regards,
Cameron

40m max deep air limit for gas density considerations, or even shallower if anticipating physical exertion at depth such as working against a current. Refer to summary of recent research below and links to the excellent articles below:

Hello,

As clearly illustrated by the discussion above there are multiple risks in breathing air at progressively greater depths, but with one exception (see below) there are few data defining clear thresholds at which the "big three" risks (narcosis, oxygen toxicity, gas density and CO2 retention) become intolerable. Indeed, as has been pointed out, there is strong evidence that there is considerable variability (both between and within individuals) in the expression of these problems.

Arguably, of these risks, the one with a threshold that is now best defined by data is the risk of CO2 retention due to rising gas density. As pointed out by Kev, the relevant data were compiled by Gavin Anthony of QinetiQ from a database of test dives, and we published those data together last year. To summarise, when gas density exceeded 6g/L during modest work (100 - 125 watts) underwater there was a sharp inflection in the risk (to over 40% of dives) of developing dangerously high levels of body CO2. 6g/L corresponds to use of air at about 40m.

Right there we now have a strong evidence-based argument for avoiding use of air deeper than recommended by the recreational training agencies. I would remind you that development of a high CO2 level is also a significant risk factor for oxygen toxicity. So, using deep air has been clearly demonstrated to increase the risk of CO2 retention, which in turn also indisputably increases the risk of another catastrophic complication.

Yes, I know that many vocal divers have used air at great depths. That, of itself, is not a valid justification of the practice in the face of hard evidence that it is dangerous. These people are a self selected population of survivors, and are around to talk about it, but the ones that died doing it are not.

Simon

Advanced Knowledge Series: The Gas Density Conundrum

Advanced Knowledge Series: Basic Carbon Dioxide Physiology

Advanced Knowledge Series: Carbon Dioxide Retention

 

[100days-a-year]

"Self selected population of survivors" I thought for a minute he was talking about rebreather divers.

 

[KenGordon]

Do you have any links to documentation of this? I have never heard that before.

AFAIK, most trimix computers use published algorithms (e.g. Buhlmann ZHL-16C or VPM-B) and, at least with Buhlmann, Helium has its own set of M-values and there is no "cheat" that has to be programmed. The helium is simply modeled using its parameters, which are different than the nitrogen parameters.

Also, what tables are there for diving with helium? My training for diving on air or Nitrox included use of tables, but my training for using helium did not offer any tables at all. All dive planning to use helium was done using computers.

Can you point us at the helium m values for the the B and C variants of ZHL16?

 

[stuartv]

Can you point us at the helium m values for the the B and C variants of ZHL16?

I think Subsurface only does C. I also *think* the B values are very close, but I don't have them at hand.

Here is what Subsurface uses:

const double buehlmann_N2_a[] = { 1.1696, 1.0, 0.8618, 0.7562,
                  0.62, 0.5043, 0.441, 0.4,
                  0.375, 0.35, 0.3295, 0.3065,
                  0.2835, 0.261, 0.248, 0.2327 };

const double buehlmann_N2_b[] = { 0.5578, 0.6514, 0.7222, 0.7825,
                  0.8126, 0.8434, 0.8693, 0.8910,
                  0.9092, 0.9222, 0.9319, 0.9403,
                  0.9477, 0.9544, 0.9602, 0.9653 };

const double buehlmann_He_a[] = { 1.6189, 1.383, 1.1919, 1.0458,
                  0.922, 0.8205, 0.7305, 0.6502,
                  0.595, 0.5545, 0.5333, 0.5189,
                  0.5181, 0.5176, 0.5172, 0.5119 };

const double buehlmann_He_b[] = { 0.4770, 0.5747, 0.6527, 0.7223,
                  0.7582, 0.7957, 0.8279, 0.8553,
                  0.8757, 0.8903, 0.8997, 0.9073,
                  0.9122, 0.9171, 0.9217, 0.9267 };

 

[_Ralph]

Can you point us at the helium m values for the the B and C variants of ZHL16?

There are no M values for "B" and "C" variants in regards to Helium (only nitrogen. The "A" variant Helium component was derived from the Nitrogen component ratios)... sadly Buhlman passed away before getting down to modifying those like he did with the "B" and "C" (making them more conservative in the "fast/mid" tissues). So all devices using a ZH-L16 algorithm (in whichever flavour) reference the original Helium values (unless they've gone and made their own values).

The C variant was intended for dive computer use, as it would track the actual dive better.


_R

 

[Kevrumbo]

I think Subsurface only does C. I also *think* the B values are very close, but I don't have them at hand.

Here is what Subsurface uses:

const double buehlmann_N2_a[] = { 1.1696, 1.0, 0.8618, 0.7562,
                  0.62, 0.5043, 0.441, 0.4,
                  0.375, 0.35, 0.3295, 0.3065,
                  0.2835, 0.261, 0.248, 0.2327 };

const double buehlmann_N2_b[] = { 0.5578, 0.6514, 0.7222, 0.7825,
                  0.8126, 0.8434, 0.8693, 0.8910,
                  0.9092, 0.9222, 0.9319, 0.9403,
                  0.9477, 0.9544, 0.9602, 0.9653 };

. . .The C variant was intended for dive computer use, as it would track the actual dive better.

_R

The above table matrix in Subsurface looks like the "a" and "b" coefficients of the ZHL-16C Nitrogen Tissue Model.

const double buehlmann_He_a[] = { 1.6189, 1.383, 1.1919, 1.0458,
                  0.922, 0.8205, 0.7305, 0.6502,
                  0.595, 0.5545, 0.5333, 0.5189,
                  0.5181, 0.5176, 0.5172, 0.5119 };

const double buehlmann_He_b[] = { 0.4770, 0.5747, 0.6527, 0.7223,
                  0.7582, 0.7957, 0.8279, 0.8553,
                  0.8757, 0.8903, 0.8997, 0.9073,
                  0.9122, 0.9171, 0.9217, 0.9267 };

There are no M values for "B" and "C" variants in regards to Helium (only nitrogen. The "A" variant Helium component was derived from the Nitrogen component ratios)... sadly Buhlman passed away before getting down to modifying those like he did with the "B" and "C" (making them more conservative in the "fast/mid" tissues). So all devices using a ZH-L16 algorithm (in whichever flavour) reference the original Helium values (unless they've gone and made their own values). . .

ZHL-16A Helium Tissue Model.