Is ppO2 change too large?

[Tomeck]

how deep had they been?

157 feet during 20 minutes with 21/15.

 

[rjack321]

157 feet during 20 minutes with 21/15.

If you are going to experience a vestibular (inner ear) hit its mostly likely when switching onto O2. Although that is really only known for divers with a PFO.

For a 157ft dive I would probably not use 21/15 (If I am going to bother with He, more is better) and I would not use O2 as my sole deco gas. I suspect there is more to your buddy's problem than the mere gas switch. For instance, switching at 15ft is a bit odd and rather "late" in the profile.

 

[DA Aquamaster]

Helium is a much faster gas and ascent rates are much more critical. They get even more so shallow where the rate of change per ft is so much greater. In that regard, switching to a mix like 32% or 50% much deeper makes a lot more sense. Add 6' swells and it is a no brainer that you do not want to be on a high He/low O2 mix at 30'.

Then there is also the practical gas planning side of it - if you have to do all the stops to the 20' stop on back gas, you are going to need a lot of back gas and will need a much healthier reserve and will have more difficulty with lost gas planning. Consequently from a gas planning standpoint, it makes no sense to do a deep trimix dive with just O2. If you are doing deco on just O2, you are most likely only doing so as a lost deco gas contingency.

 

[Mike Edmonston]

Helium is a much faster gas and ascent rates are much more critical. They get even more so shallow where the rate of change per ft is so much greater. In that regard, switching to a mix like 32% or 50% much deeper makes a lot more sense. Add 6' swells and it is a no brainer that you do not want to be on a high He/low O2 mix at 30'.

Unless holding a stop is really a major problem for you, there is no need to change your mix for swells.

Then there is also the practical gas planning side of it - if you have to do all the stops to the 20' stop on back gas, you are going to need a lot of back gas and will need a much healthier reserve and will have more difficulty with lost gas planning. .

Sorry, but it's much more practical to have the same contingency gas as your bottom gas if the $h!t hits the fan. No need to worry which gas has what MOD etc... As long as you don't grab your O2 reg.

Consequently from a gas planning standpoint, it makes no sense to do a deep trimix dive with just O2. If you are doing deco on just O2, you are most likely only doing so as a lost deco gas contingency.

So are you saying that going from a NON Hypoxic mix to o2 is not a good thing? Please explain to me why you would want to go to a higher nitrogen loading gas in a decompression OFFGASSING phase of the dive by introducing another gas? Maybe you'll shave a couple minutes off your Deco... So what. Is it worth the extra task loading, ICD potential, and extra Nitrogen? Nope...

Take a look at RGBM tables, they go straight from Helium rich mixes to O2. Travel gas is used in Hypoxic environments. Thousands of dives have been conducted and researched using RGBM tables, and although no Deco algorythm is 100% foolproof, the RGBM record speaks for itself.

Now, I am not advocating only RGBM tables. I also use VPM and Buhlman depending on the dive, but I wanted to give you an example of a Decompression model that takes advantage of the Helium to O2 dive plan.

Safe Diving

 

[dab]

So are you saying that going from a NON Hypoxic mix to o2 is not a good thing? Please explain to me why you would want to go to a higher nitrogen loading gas in a decompression OFFGASSING phase of the dive by introducing another gas? Maybe you'll shave a couple minutes off your Deco... So what. Is it worth the extra task loading, ICD potential, and extra Nitrogen? Nope...
Safe Diving

I think you may be reading more into his statement that what was intended. I read it to imply simply that a deep trimix dive will involve the use of intermediate deco gases prior to 100% 02 and if one has to go from a high He content gas directly to 02 it could very likely be due to the fact those intermediate deco gases have been lost.

 

[LiteHedded]

I agree that an intermediate gas (such as EAN 50 at 70 fsw) should 'likely be involved', but I didn't get the feeling that the OP was asking that question.

Tomeck clearly stated he was going from his bottom gas (18/35) to O2.

he wasn't
but a deeper deco gas would alleviate the rock bottom concern and also make the ppo2 difference a little less pronounced (if that's an issue)

 

[Meng_Tze]

Why not dive to 1.6, or sligthly less then the 'jump' would not be as great?


http://www.scubaboard.com/forums/3669180-post103.html

?????

 

[Mike Edmonston]

Why not dive to 1.6, or sligthly less then the 'jump' would not be as great?


http://www.scubaboard.com/forums/3669180-post103.html

?????

Freddy, your uncanny recollection of past posts is magical!

Take care

 

[rjack321]

Why not dive to 1.6, or sligthly less then the 'jump' would not be as great?

http://www.scubaboard.com/forums/3669180-post103.html

?????

 

[Tomeck]

Why not dive to 1.6, or sligthly less then the 'jump' would not be as great?


http://www.scubaboard.com/forums/3669180-post103.html

?????

I don't understand your post. If I dive at 66 meters with air (ppO2 of 1.6), it's OK, but if I dive at 6 meters with air (ppO2 of 0.34)?