Min Deco for 30/30

[sea_otter]

I've been looking at dive planners for a couple 30/30 dives, comparing GUE's minimum deco tables and multideco, and I was quite surprised to see a significant discrepancy between the two.

For example, if I'm planning a dive to 100 ft on 30/30 for 25 minutes (per GUE, within min deco limits with some margin), a planner set to 20/85 gives me a mandatory 7 minute stop at 20'. If I push this to the GUE min deco limit of 30 minutes, I see a mandatory 11 minute stop at 20' (this assumes staying on back gas to the surface - not plan A in our case - but an option).

I questioned my instructor on what I thought was a slight variance (though after punching the numbers, I'd call that more than a slight difference), and his response was that the constants for Bühlmann's constants related to helium were never really tested when the algorithm was developed, and that as a gas, it behaves more like nitrogen. I'm curious to learn more about this - is it really valid to have tables that show the same min-deco limits for 32% as 30/30?

 

[mikeny9]

Your instructor is right. Most Buhlmann-based planners penalize you for using Helium because Buhlmann's studies did not actually use any Helium, he died before it was considered in diving. His model uses Graham's law to describe solubility of a particular gas in a tissue. If you follow this model, then the Helium in tissues would saturate 2.65 times faster than Nitrogen. The trouble with that model is that the speed at which a gas moves across the membrane has nothing to do with it's solubility (see the Meyer-Overton rule). Because Helium moves across the membrane fast, it can also move back out just as fast because it's not really dissolved at all.

Most of those tables use equivalent air depth to figure out what a gas mix would be equivalent to in air if you consider the nitrogen portion in the mix. We metabolize oxygen, and helium is not significantly soluble for the calculation. To calculate air depth in feet: (Depth + 33) × Fraction of N2 / 0.79 − 33. So for 30/30, it'd be 34 feet. Erring on the side of caution, you'd use a 40' air table for NDL. The NOAA one gives 163 minutes. I'm guessing GUE is just making things simple by using a 32/00 table. UTD does the same thing with 25/25 (25/25 is UTD's standard mix for 100'-130' range), and they use the same table for 25/25 as the 32/00 table. 25/25 at 100' would give an EAD of 51'; the NOAA air table at 50' gives 92 min for NDL. Now the NOAA table was made from analyzing healthy US Navy divers, so I believe both agencies are making things simpler, and safer, by using more conservative NDLs in their tables.

 

[LiteHedded]

on a min-deco dive i treat 3030 like 32% and i dive them exactly the same
always have

 

[sea_otter]

and helium is not significantly soluble for the calculation. To calculate air depth in feet: (Depth + 33) × Fraction of N2 / 0.79 − 33. So for 30/30, it'd be 34 feet.

I can understand treating helium the same as (or at least no worse than) nitrogen, but ignoring it altogether? That seems... not right. Maybe I am misunderstanding your comment.

 

[PfcAJ]

I can understand treating helium the same as (or at least no worse than) nitrogen, but ignoring it altogether? That seems... not right. Maybe I am misunderstanding your comment.

Agreed. You can't just pretend to have less inert gas. Maybe ignore 2% but not 30%.

 

[LiteHedded]

i mean, if i get 30.5% at the fill station i'm not cutting new tables. i would still call it 32%. i treat 3030 the same as 32%

I will and do pretend to have less inert gas

 

[mikeny9]

Sorry if I made it seem like we should ignore it all together, that's certainly not what I meant. We should account for it but not to the extent we do with Nitrogen.

Ignoring it all together is what the EAD would give you. Helium of course isn't completely insoluble but it's very much less soluble compared to Nitrogen based on the solubility indexes in the Meyer-Overton rule. So we do have to account for Helium in some way. The problem is there is no research done with it to easily say how much we need to account for it by, other than solubility models.

In the example above, the EAD we used was 40' for which allows for 163 minutes of NDL on the NOAA air table. Keep in mind thats as if no Helium component was in the mix. If we treated Helium the same as we do Nitrogen, the FN2 would be .7 for 30/30, which gives an EAD of 85'. Rounding up using 90' and the NOAA air table gives you 30 min of NDL, which sounds similar to the Nitrox 32% table. Nitrox 32 EAD is 82', similar to the previous calculation using 85' for the EAD.

So using a 32% table is definitely on the safer side because of the solubility factor of Helium compared to Nitrogen. In reality we could probably stay down much longer with 30/30 at 100' than 30 min, but definitely not 163 minutes that the EAD at 40' would give.

I think where Helium shines is not so much on NDL but on calculating stop times. Since Buhlmann models are all about getting rid dissolved gas, the real benefit of Helium shows up when the gas is coming back out during decompression due to the solubility. In my opinion it's incorrect to penalize the ascent profile for a Helium mix, it should be the opposite, since it doesn't dissolve nearly as much and crosses the membrane (gets in and out of your system) much faster.

 

[PfcAJ]

It's "faster" in that a tissue reaches saturation faster due to helium being less soluble.

I wouldn't object to your ideas if your suggestion was to treat helium the same as nitrogen for decompression purposes, but that's not what you're saying.

Pretending that 25/25 can be dives the same way as 32% is just reckless. But I guess I have come to expect that from anything UTD puts out regarding deco.

 

[mikeny9]

It's faster to move in and out of the tissue but doesn't dissolve nearly as much based on the solubility indexes mentioned in the Meyer-Overton rule.

How exactly is it wreckless to treat 25/25 the same as 32/00 for NDL?

Attacking UTD and not giving a good explanation doesn't really help anybody. For the record I've train with both GUE and UTD, if agencies matter.

 

[PfcAJ]

It's faster to move in and out of the tissue but doesn't dissolve nearly as much based on theory of the Meyer-Overton rule.

How exactly is it wreckless to treat 25/25 the same as 32/00 for NDL?

Attacking UTD and not giving a good explanation doesn't really help anybody. For the record I've train with both GUE and UTD, if agencies matter.

Myer-Overton is a lipid solubility/ narcosis thing. I don't even know why you're bringing that up. I think you're just trying to talk about solubility. Helium is less soluble. The issue comes out of a tissue reaching saturation sooner and therefore being supersaturated sooner on ascent.


There's some good discussion and references over on this thread:

Helium vs nitrogen decompression