deco myths

[L13]

The thread is in the "New divers and those considering diving" forum. Additionally the OP was a video mostly targeted at recreational divers.
@LI-er is more than welcome to chip into the conversation.

I may have misread your tone but maybe you should apologize?

This thread appears to have been moderated, and some (most?) of the posts that motivated that comment have been removed.

A person making authoritative comments on deco should at least understand GFs even if they don't use them or accept them, or at least have significant experience doing deco without them.

However, you are correct about my tone. I apologize for that, and for making a post that did not add usefully to the dialog.

 

[L13]

Robert Helling, who's responsible for most the deco code in subsurface, has written a long blog post on it.

N2 vs. He, what’s the difference? – The Theoretical Diver

thetheoreticaldiver.org

Rober Helling did not address one additional difference between He and N2. Our tissues start with 0.79 ATA partial preasure of N2, but with 0.0 ATA of He partial pressure. This effects the rate of change of the total inert gas loading in tissues. The effect is approximately equivalent to He on-gassing as if you were ~8 m (~25 feet) deeper than for N2.

 

[Celt]

I think the most important thing for divers is to understand and recognise how they are personally affected by decompression stress and to extend decompression stop times or shorten bottom time to account for how they feel on the day. If you hit the first stop lethargic, with a mouth full of froth and limbs like lead you’ve overdone it no matter what your computer says.

 

[JMarc]

We are getting way beyond what should be in the "New Divers & Those Considering Diving" forum

Agree, and I fear I'll add to the problem. I'm too much a newcomer on this forum to know what's the appropriate action.

Thévenin's theorem would be correct for modeling the circuit as seen from outside at it's interface.

I wasn't so much thinking of using Thévenin's theorem after having modelized haldanian models as electrical circuits but of using the ideas behind's its proof.

As said, I perceive there is a problem of using many independent compartments when there are strong pressure gradients between them, neglecting any gas transfer directly from one compartment to another.

I think I understood that; but I wasn't -- and still am not -- convinced that's this is an issue. In the blog I discovered due to this message:

Robert Helling, who's responsible for most the deco code in subsurface, has written a long blog post on it.

N2 vs. He, what’s the difference? – The Theoretical Diver

thetheoreticaldiver.org

there is an interesting entry on this subject, with a more mathematical reasonning than my intuition and analogies, with the same conclusion.

https://thetheoreticaldiver.org/wordpress/index.php/2019/11/12/why-are-tissues-independent/

For a single inert gaz, where I think -- perhaps due to my lack of knowledge of past studies -- haldanian models are lacking is that they don't take enough into account how severe was previous recent desaturation. Bubble models were an approach, but their popularity seems to have vanished, but I don't know if that impression is due to a ... bubble effect. The blog has also a relevant entry.

 

[bipolarbear]

Tech diving does not always mean long stops. Depending on the dive site, you can plan the profile so that the deco obligation is mostly gone before you start your final ascent. Then with 100% oxygen your stops can be very short indeed.

Agreed. Depending on your site, it can also simply mean planning a profile that lets you deco up onto a reef or pass another feature of interest.

I just finished a dive two hours ago. 30 mins cruising along a wall at 40-50m. Racked up around 25-30mins deco (we run very conservative GFs). We had scooters so after we hit our turn mark (based on TTS) we scooted up towards a reef at around 21m. Switched to our 50% and then had a lovely 10 or 15 mins as we worked our way up to the reef. Hit the 6m stop, switched to O2 and had a ~12 minute hang where I got to play with a banded sea snake and frogfish. Plenty of deco diving doesn't necessarily involve hanging on a line or doing drift deco in blue water.

 

[JMarc]

Plenty of deco diving doesn't necessarily involve hanging on a line

I often dive with insta-buddies. I've met some who insisted to do a "safety stop" hanging on the anchor line at 3 meters when we had spent the last 20 minutes at 6 meters deep and never had a mandatory stop. I was very tempted to leave them alone on the line, ... It seems some do not consider that decompression can occur anywhere but only on the line.