Equivalent Air Depth question

[Angelo Farina]

Okay so I have a question. I’m currently working towards my ADCI card. (commercial diver card). On the commercial side we use Bimix so HeO2. Why on the TECH or REC side do we use TRIMIX? Wouldn’t it be easier and actually safer to take nitrogen out of the picture? I understand by using Archimedes principle we can find out the mixture we need. So a 80/20 or 79/21 mix would work great up to around 4ata off of the top of my head then we just reduce the O2 in the mix for greater depth. Of course with deco past probably 180 we would do a 50/50 nitrogen/O2 deco stop for a minute. Anyway all of my top of the head math aside…why does tech/rec use Trimix instead of bimix?

The main reason is that helium is very expensive, nitrogen is free.
So a fill with 40% N2, 40% He, 20% O2 costs half than 80% He, 20% O2.
And such a trimix is effective avoiding narcosis at, say, 60m max depth.
Regarding the resulting deco obligation I do not possess enough technical knowledge for evaluating if the trimix is better or worst than Heliox.

 

[Akimbo]

Why on the TECH or REC side do we use TRIMIX?

Very simple, cost. The cost of gas comes out of recreational divers' pockets. The client pays for gas offshore. Gas is largely recycled on sat systems so the economic incentive to use Trimix does not offset the added complexity.

 

[Kay Dee]

I have known quite a few other scuba divers who appear to function reasonably well in 180 feet of water on air.

I have known quite a few other divers that performed very well at (and beyond) that depth. But that is not a recommendation by me to do it, just an observation.

And @Wibble, my apolgies, I somehow missed your answer to my questions above, so now......... a belated thank you.

As for the "why not Bimix", others have answered that. But cost and deco obligation being the prime I would assume .
.

 

[Dwmathews88]

The main reason is that helium is very expensive, nitrogen is free.
So a fill with 40% N2, 40% He, 20% O2 costs half than 80% He, 20% O2.
And such a trimix is effective avoiding narcosis at, say, 60m max depth.
Regarding the resulting deco obligation I do not possess enough technical knowledge for evaluating if the trimix is better or worst than Heliox.

Thank you very much for this answer.

 

[John C. Ratliff]

While I have not read this whole thread, it appears that there may be a lot of misinformation presented as fact about oxygen and its narcotic ability. It appears that this is a classic example of internet information (or misinformation) being broadcast. I would talk to DAN about this, and seek the scientific studies on this situation, rather than relying upon any information here.

SeaRat

 

[Dr Simon Mitchell]

While I have not read this whole thread, it appears that there may be a lot of misinformation presented as fact about oxygen and its narcotic ability. It appears that this is a classic example of internet information (or misinformation) being broadcast. I would talk to DAN about this, and seek the scientific studies on this situation, rather than relying upon any information here.

Hello,

Prescient words John. There seems to be a general sense on this thread that oxygen has been demonstrated to be as narcotic as nitrogen. This is not true. It is a complicated matter and still debated in the scientific literature. Some older papers did suggest narcotic effects by oxygen, but to my knowledge only one paper using modern experimental techniques has raised that possibility, though the study was not designed to isolate the effects of oxygen alone, and the measured effect may have been hyperexcitability rather than narcosis. [1]

In contrast, more recent studies have found that oxygen enhances cognitive function at surface pressure [2] and that nitrox produces less cognitive impairment, and may actually enhance cognitive function at depth. In one fascinating study this even seemed to apply to the elevated PO2 in air on arrival at 30msw equivalent in a dry recompression chamber, with nitrox being even better. [3] That positive effect was attributed to excitability / arousal.

More recently our group (specifically, one of my PhD students - now a post doc, Dr Xavier Vrijdag) has tried to hone in on the effects of individual gases using quantitative electroencephalogram (EEG) analyses. Xavier developed an analysis technique that demonstrated dose dependent EEG change during hyperbaric nitrogen exposures, but not helium exposures. [4] This represents the first EEG algorithm that appears capable of quantifying nitrogen narcosis.

The latest study in this series applied that same algorithm to divers exposed to hyperbaric oxygen at 142 and 284 kPa (1.4 and 2.8 ATA - representing typical maximum exposures to oxygen in diving and hyperbaric chambers respectively). Oxygen at these pressures did not cause the same EEG changes (or changes in psychometric test performance) as hyperbaric nitrogen, suggesting that oxygen is not narcotic, or at least not in the same way as nitrogen. It did cause EEG changes suggestive of hyper-excitability. Those results have just been accepted for publication [5] and as soon as the paper is available I will post a link.

So, overall, I would characterise the modern literature as trending away from attributing a nitrogen-equivalent narcotic effect to oxygen. Oxygen may even enhance cognitive performance. However, work in this space continues. We are also in the middle of a series of experiments aiming to evaluate how CO2 fits into the mix.

Simon M

References:
1. Freiberger JJ, Derrick BJ, Natoli MJ, Akushevich I, Schinazi EA, Parker C, Stolp BW, Bennett PB, Vann RD, Dunworth SAS, Moon RE. Assessment of the interaction of hyperbaric N2, CO2, and O2 on psychomotor performance in divers. J Appl Physiol 121: 953–964, 2016.
2. Scholey AB, Moss MC, Neave N, Wesnes K. Cognitive performance, hyperoxia, and heart rate following oxygen administration in healthy young adults. Physiol Behav 67: 783–789, 1999.
3. Germonpré P, Balestra C, Hemelryck W, Buzzacott P, Lafère P. Objective vs. Subjective Evaluation of Cognitive Performance During 0.4-MPa Dives Breathing Air or Nitrox. Aerosp Med Hum Perform 88: 469–475, 2017.
4. Vrijdag XCE, van Waart H, Pullon RM, Sames C, Mitchell SJ, Sleigh JW. EEG functional connectivity is sensitive for nitrogen narcosis at 608 kPa. Sci Rep 12: 4880, 2022.
5. Vrijdag XCE, van Waart H, Sames C, Mitchell SJ, Sleigh JW. Does hyperbaric oxygen cause narcosis or hyperexcitability? A quantitative EEG analysis. Physiological Rep. In press, 2022

 

[johndiver999]

delete please

 

[LFMarm]

Very simple, cost. The cost of gas comes out of recreational divers' pockets. The client pays for gas offshore. Gas is largely recycled on sat systems so the economic incentive to use Trimix does not offset the added complexity.

If cost is the only driver for trimix vs heliotrox, wouldn’t it make sense to use the latter for CCR where the dil cost per dive is 10-15 times lower than OC?

I am not trying to make a point but rather genuinely trying to learn

 

[Akimbo]

If cost is the only driver for trimix vs heliotrox, wouldn’t it make sense to use the latter for CCR where the dil cost per dive is 10-15 times lower than OC?

There is more to the decision matrix.

Gas Cost
There is still a significant cost differential between Trimix and HeO2, even on a rebreather. The gas used to inflate the loop at the maximum depth is vented into the water on ascent.

Modern commercial and military saturation diving operations recycle gas from the diver's hats, chambers and bells on ascent, when utility locks and transfer trunks are vented, and even gas analysis. Of course all this requires enormous amounts of space on the DSV (Diving Support Vessel), electrical power, multiple tube-trailer levels of gas storage, machinery, umbilicals, and skilled technicians.

Heat Loss
Respiratory heat loss is much higher on HeO2, which sat divers manage with open circuit hot water heated wetsuits and gas heaters.

HPNS
High Pressure Nervous Syndrome is largely mitigated in shallower water by Trimix, like less than about 500'/150m. Saturation divers manage HPNS and Compression Neuralgia by compressing divers very slowly, on the order of 1'/minute.

Gas Density:
The lower gas density of HeO2 is more important to divers who are physically working hard and may be locked out for 4-8 hours. Deep rebreather dives are relatively shallow, short, and low exertion.

Simplicity:
A DSV may move from a worksite at 200'/60m where Trimix might be fine to over 1000'/300m where the value of Trimix would be marginal. The complexity in machinery, analysis, purification, and training to support Trimix would have vary marginal returns.

Does all this make sense?

 

[Dr Simon Mitchell]

The word "nitrogen" in the bolded section is a typo right? You mean oxygen...?

Yes, correct, sorry! Now fixed. Thanks.