Riding GF99 instead of mandatory/safety stops

[old frogman]

You do understand that I was not attempting to validate the efficacy of the drop in venous gas pressure as a decompression benefit, I was only explaining what Powell said in his book in 2008.

Understood. Efficacy is debateable.

 

[rjack321]

I am not sure what point you are trying to make here.

Isobaric counter diffusion (ICD) may occur in dives shallower than 100 meters. Depth is not the issue. Switching from helium-based gas to nitrogen-based gas. If the percentage of nitrogen is too high ICD may occur resulting in a vestibular DCS hit.

ICD is a phenomenon where two different gases diffuse in opposite directions under, wait for it, constant ambient pressure.

I was interested in your actual 100m trimix diving training and experience. These statements appear to based on paraphrasing what you can google - they are also misleading and inaccurate.

For the record I have probably 250 dives in the hypoxic range >200ft. Forty-five or fifty of those below 250ft and a couple in the 300-330ft range. Also close to 400 dives with a "IBCD warning" in multideco because the default helium percentage difference flag in that widely used software is totally bogus.

For all practical purposes, for normal non-Shek Exley type sports diving purposes, IBCD doesn't actually exist.

 

[old frogman]

I was interested in your actual 100m trimix diving training and experience. These statements appear to based on paraphrasing what you can google - they are also misleading and inaccurate.

For the record I have probably 250 dives in the hypoxic range >200ft. Forty-five or fifty of those below 250ft and a couple in the 300-330ft range. Also close to 400 dives with a "IBCD warning" in multideco because the default helium percentage difference flag in that widely used software is totally bogus.

For all practical purposes, for normal non-Shek Exley type sports diving purposes, IBCD doesn't actually exist.

We appear to be going off on a tangent in this discussion. Let me summarise the discussion so far.

First, some people on this site appear to be under the impression that ICD is caused by a pressure gradient. How can that be so, when ICD occurs at a stop (there is no pressure gradient). Physics of thermodynamics (the relationship between pressure, temperature and solubility as applied to gases) provide an explanation of ICD, based on the solubility of different gases. Solubility appears to control ICD.

Second, you state that ICD cannot occur below 100m so we should not be concerned. History has shown that it has occurred below 100m. You mentioned Sheck Exley who used to push the envelope. Remember, at the time Sheck did not have the knowledge that you now fortunately have.

Third, It's important that we all understand ICD so we do not repeat history and wind up seriously injured or dead. Lest we forget.

Fourth, I agree with you ICD is not the problem it once was because we now know how to manage gas composition and switch depths better on ascent. Recently, some agencies suggest, for dives shallower than 100m to go from bottom gas directly to deco gas e.g. Nitrox 50 at 21m or Oxygen at 6m. I believe they call it the "zero order rule".

 

[lermontov]

I am not sure what point you are trying to make here.

I think hes meaning those that are diving these depths on a regular basis have based their decision on real world experience rather than a software model - No doubt your have hear of DR simon Mitchell who is one of the worlds leading hyperbaric specialists haas this to say about it and the bro science rule of fifths to try and avert ICBD

Same goes for CNS its nigh impossible to keep your CNS % under 100% on a deep dive

Personally, I doubt isobaric counter-diffusion is a very relevant concern in modern technical diving where most divers doing deep trimix are using rebreathers. These days I just stay on bottom diluent to the surface, and don't bother with diluent switches. The rebreather takes care of increasing my inspired oxygen fraction as I get shallower. Open circuit is obviously different. Even though the rule of fifths is based on a flawed premise it would likely prevent any inner ear problems caused by isobaric counter-diffusion if followed. But it is very conservative and may create more risk (eg the need to carry more intermediate gases and more switches than necessary) than it resolves. In some ways it can be seen in a similar light to the original 'oxygen clock' recommendations - a rule with little physiological pedigree, and often ignored, but nevertheless a 'calibration point' from which people can work.

Simon M