Drawbacks to Helium??

[DevonDiver]

Helium bend underwater? I am no doctor nor am I a phd. Either you are bent or your not, You can either follow a schedule or you can not. Care to elaborate and advance my understanding...
Eric

New phrase for the day..... Isobaric Counter-Diffusion.

Steve Burton's ICD Article

 

[DevonDiver]

But it a maximum FHe we're talking about, not a minimum FO2. Could be hyperoxic.

My fault... I didn't make it clear in my reply that I was addressing sunkmail's post (air top-up).

Just a guess -

Using a blending program, If you have a completely empty tank and fill it to 17% He, and top up with air, you get ~17% O2.

Perhaps the 17% He is an attempt to keep minimum PPO2?

(I could be way off here, I know, but it seems to fit with being overly cautious)

 

[waterpirate]

I was afraid this is where this was going to go, ICD. My solution he, Ross Hemingway's explanation of the benefits of he in your deco gasses should put any worry about ICD to rest, and it certainly is NOT an issue at the tech 1 level.
Eric

 

[lamont]

New phrase for the day..... Isobaric Counter-Diffusion.

Steve Burton's ICD Article

That strategy in there is obviously wrong since T1 divers do 21/35 to 50% switches all the time without worrying about IBCD inner ear hits.

Here's a substantially better article:

Biophysical basis for inner ear decompression sickness ? Journal of Applied Physiology

 

[Kern]

New phrase for the day..... Isobaric Counter-Diffusion.

Steve Burton's ICD Article

The likelihood of IBCD for a diver using normoxic mix's is probably lower than that of a rec. diver getting an undeserved hit. I'd actually say it was infinitesimally small.

Even for the hypoxic diver doing dives below 90/100mt, IBCD risk can be almost completely removed by using the correct deco mix's/strategies. Having used, what might be considered non optimal deco mix's (for inner ear decompression) on several dives in the 70-75mt range, I don't even consider IBCD until I pass this point.

For whom, is IBCD a "new phrase for the day".

 

[DevonDiver]

For whom, is IBCD a "new phrase for the day".

I was addressing waterpirate's earlier post. Got internet connection problems... I thought my post would just slot underneath his... but turns out that it took a while to make the post and several others had posted between

Helium bend underwater? I am no doctor nor am I a phd. Either you are bent or your not, You can either follow a schedule or you can not. Care to elaborate and advance my understanding...
Eric

 

[Kevrumbo]

That strategy in there is obviously wrong since T1 divers do 21/35 to 50% switches all the time without worrying about IBCD inner ear hits.

The likelihood of IBCD for a diver using normoxic mix's is probably lower than that of a rec. diver getting an undeserved hit. I'd actually say it was infinitesimally small.

Even for the hypoxic diver doing dives below 90/100mt, IBCD risk can be almost completely removed by using the correct deco mix's/strategies. Having used, what might be considered non optimal deco mix's (for inner ear decompression) on several dives in the 70-75mt range, I don't even consider IBCD until I pass this point.

For whom, is IBCD a "new phrase for the day".

Notwithstanding the low incidence of IBCD at the T1 level-- this doesn't imply it's more correct to change to a higher N2 inert ingassing deco mix of Eanx50 (50% O2 & 50% Nitrogen), instead of 50/25/25 (50% O2, 25% He, and 25% N2), and a final inert clean-up using pure Oxygen, all coming off a bottom mix of 21/35/44 (21% O2, 35% He, and 44% N2). . .

If you have the means (partial pressure blending for best deco mix) and additional training, why not be consistent and always try to reduce or hold steady the fraction of inspired inert gases in your intermediate deco stops, and always utilize a 100% Oxygen stop for final full inert clean-up? What is fundamentally or qualitatively wrong about such a strategy? By the following below, doesn't this make more logical sense??

. . .It should be intrinsically obvious that removal of a gas from tissue can be speeded by elimination of the gas from the inspired mixture. If the arterial partial pressure of a gas is zero, then no gas will diffuse into tissue while the gas is diffusing out of the tissue. . .The decompression obligation of a tissue compartment is based on the sum of gas partial pressures in the compartment. This means that if a tissue is loaded with N2 as He is being removed, its tissue has a greater decompression obligation than when no N2 is added to tissue during He off-gassing. . .
p.11-12, Gas Exchange, Partial Pressure Gradients, and the Oxygen Window, Johnny E. Brian, Jr., M.D.

 

[Kern]

Because on these dives, it adds so little to reducing the risk of IBCD, that it''s just not necessary.

Of course every divers physiological response is different. If for any reason I felt not allowing the ppN₂ to be raised during the ascent, & total washout using O₂ was required, I'd do things differently.

 

[Kevrumbo]

Because on these dives, it adds so little to reducing the risk of IBCD, that it''s just not necessary.

Of course every divers physiological response is different. If for any reason I felt not allowing the ppN₂ to be raised during the ascent, & total washout using O₂ was required, I'd do things differently.

IMO it's NAUI-tec's contention that ICD (or IBCD) phenomena should be made aware of, and all means of reducing its potential effects --for both shallow and deeper/longer dives-- should be implemented or at least considered if practically possible.

Their arguments based on Wienke's empirical work on RGBM may seem over-cautious for dives in the 30m to 45m range --but that doesn't mean they're "wrong". . .or should be summarily dismissed as "just not necessary":

. . .What this all adds up to is the following isobaric switch
prescription, which is, of course, outside of most dive
logistics. But useful on first principles -- and some places
DO have the support in place for just such logistics.

So, it goes like this.

Best deco strategy is to increase O2 on way up in same proportion
as He is reduced, while keeping N2 relatively constant. Switch to
a nitrox mix with less N2, and thus outgas both He and N2. . .
the lower N2 nitrox switches avoid gradient slams and further ingassing of
N2.

Practically, this means all the above plus EAN50 at the 70 fsw
level for long exposures, and/or He to the surface for shallow
exposures, plus O2 in the 20 fsw zone either way.

Cheers, and safe diving always,

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Ldr

--------------------------------------------------------------------------------

Let me see if I can reduce this....

down to something a bit more practical.

Is the bottom line that from a bubble mechanics point of view, one should never increase the percentage of an inert gas as one "rides up" the decompression chain?

So, for instance, if you are using Trimix 18/45, the FN2 in the bottom mix is 37%. Thus, switching to a gas with more than 37% FN2 as a deco mix is inadvisable?! That precludes a 70' 50/50 bottle, but might allow a 60/40 bottle, which is pretty close to 37%.

Or, for example, if you're using 21/35, the FN2 is 44% in that mix; as a consequence the "maximum FN2 permitted" is 44%? That might permit 50/50, as that's pretty close, but even better might be that 60/40 mix again....

In both of the above cases you COULD ride the Trimix up to the 30' depth, in that neither would become hypoxic and, in fact, you could breathe the 18/45 on the surface (its a bit hypoxic but not significantly so.)

I think I understand what the prescription is here, in that increasing the Fx of an inert gas can cause counterdiffusion (actually increasing your inert gas load rather than decreasing it in that gas, even as the other gas vacates your body) but this is majorly counter to what I've understood about general deco strategy.

(The odd thing is that for a deco dive on Nitrox, for example, taking a single bottle of 50/50 might remain an optimum strategy, in that there is no counterdiffusion problem since the FN2 is always decreasing, and if something goes wrong on the shallow stops you've done some of the deco with an advantageous oxygen window .vs. no benefit from that, but that same deco gas option on a 21/35 dive would be inadvisable, with either a single bottle of 80/20 or 100% being preferred.)

Did my distillation end up correct?

--------------------------------------------------------------------------------

Optimal 70 fsw mix

BRW;

Sounds like you would advocate 50/25/25 at the 70 over 50/50 nitrox coming off, say, a 15/50 at 250 kind of dive. Is that correct?

http://www.scubaboard.com/forums/as...c-countertransport-mix-switch-strategies.html

 

[Kern]

I haven't said anything was wrong. I haven't "summarily dismissed" anything. I'v just decided to stick with a lifetime policy I learnt in the army, "KISS". If, NAUI, Wienke, or yourself feel the need to take tools that I don't think are required for a given job, that's OK with me. As long as the tools I use, get the job done just as well, I'll continue to use them.

PS. I have actually tried the sort of approach suggested by Wienke, before deciding that the added logistics, & expense, v's the benefits, weren't worth it to me.