ICD and gas swaps

[Dr Simon Mitchell]

I want to thank you for the contribution in this discussion and for the elements of reflection that you raised.

And I want to thank you for the polite and receptive way you have addressed what is essentially a challenge to your established beliefs.

Simon M

 

[boulderjohn]

Yes, potentially, I acknowledged that. But I have never known a technical diver to put helium in their drysuit.

I can only speak for my personal experiences.

When I first started technical diving, I was told that I should never use a helium mix in my drysuit. I didn't.

A few years ago, i was on a dive boat filled with technical divers, and I overheard a conversation in which people were laughing about that. They were using whatever they had in their back gas for suit inflation. I looked around the boat, and I saw that a lot of people were using back gas for suit inflation. These were not high helium mixes--maybe 18/45 at the most.

On a two month trip to Florida the year after that, I realized that I had left my argon regulator at home in Colorado. I did all my dives with my suit inflated with back gas. Again, I don't think I did anything over 18/45 then.

When I did my trimix instructor internship, I had to fly to the site, and I conferred with the instructor about what I should bring and what I could get there. When I asked about bringing my argon bottle, he was incredulous. Just use back gas, he said. That's what he always did. We used it for mixes down near 300 feet.

For a while back home, I used back gas for mixes around 60% helium for a while, but I was feeling cold on long dives, so I eventually resurrected an argon bottle (filled with air).

So no technical divers I know are using helium alone to inflate their dry suits, but some are indeed using helium mixes to do it.

 

[Dr Simon Mitchell]

Hi John,

Interesting. To be an issue you need high ambient pressures and high surrounding helium, so the risk of what you are describing is probably small. Still, best if you don't do it though. In any event, the consequence of that particular brand of isobaric counterdiffusion is itch and skin DCS, so typically not serious.

Simon

 

[Germie]

But even a 21/35 makes you colder than air in your drysuit. When I started technical diving I did not have an drysuitinflationbottle.I was doing the Advanced recreational trimixcourse. I opted to the instructor to use the EAN50 in my suit, but an inflatorhose on my stagereg was notdone, it was not 'DIR', so not allowed. In 6C water I used 21/35 and was freezing. Since then there is an inflatorhose on my stageregs

 

[rjack321]

Bringing a whole bunch of bottles/gases to try and mitigate IBCD seems to substitute the very minor risk of IBCD for the significant risk of swapping to the wrong gas.

 

[TheSnake]

Bringing a whole bunch of bottles/gases to try and mitigate IBCD seems to substitute the very minor risk of IBCD for the significant risk of swapping to the wrong gas.

Which is you plan for a 100 meters / 328 feet dive?

 

[ofg-1]

You know, you can avoid all of these issues. Just do what we did back in the early 70's, dive air.

 

[TheSnake]

and what about of oxygen toxicity and narcosis ? it would be interesting to try to make a 300m / 984 feet air dive ....
Stats are not your side.

 

[TrimixToo]

Bringing a whole bunch of bottles/gases to try and mitigate IBCD seems to substitute the very minor risk of IBCD for the significant risk of swapping to the wrong gas.

I am extremely conservative about this (more conservative than need be, I know), and I have yet to need an additional mix to manage the risk. I add some He to my deep and intermediate deco gases to keep both the planned PPinerts decreasing throughout the ascent. This way, I am always offgassing them both. This adds complexity to mixing, but adds no additional risks at gas switch time. It might also add more headroom when things go off-plan and my inert gas loading is higher than planned.

This does, of course, cost more, particularly for dives past the 300' mark where a deep-ish (e.g., MOD 150') deco gas gets you out of the water a lot faster, and He is expensive. But I don't do many of those, and for shallower dives (to 250' or so), it's far less pricey. Pay yer money and take yer pick.

 

[boulderjohn]

This does, of course, cost more, particularly for dives past the 300' mark where a deep-ish (e.g., MOD 150') deco gas gets you out of the water a lot faster, and He is expensive.

It depends on the diving for the entire trip and how you get your gas. If you are blending your own gas, have a booster, and doing multiple dives over several days, it is cheaper to go with the extra gas bottle. That way you are keeping more high helium back gas for the next dive and using less helium in the middle range of the dive.