why helium?

[Mike Edmonston]

Rebreathers and T bottles????

Uhhh... I'm confused....

Weren't we talking about Helium or something???

 

[Bismark]

Rebreathers and T bottles????

Uhhh... I'm confused....

Weren't we talking about Helium or something???

You might just have missed the last page of posts. BTW, your photo in your avatar looks suspiciously like a set of manifolded doubles..........

 

[wedivebc]

Rebreathers and T bottles????

Uhhh... I'm confused....

Weren't we talking about Helium or something???

Well as is typical when debating belief sysems with the assimilated, as soon as they run out of valid arguments to support their position they obfuscate the argument with nonsense.

 

[grazie42]

Well, manifolded doubles dont fit the criteria. I dont know enough about rebreathers but since those divers have to carry bailout gas, I am guessing they aren't "fully redundant" surface-supplied -- someone still has to change the T-bottle.

Maybe you could let us know what you were meaning ?

Bailout is fully redundant from the RB since it´s (at least usually) comprised of completely separate OC-tanks and since you carry enough to end your dive on the bailout (while breathed OC)...some units have/are modified to connect off-board bailout but I don´t think anyone plans their bailout on the assumption that they´ll be able to...at least, from my newbie-perspective, that´s a bad idea...

 

[Bismark]

I think if we aren't diving completely redundant systems which allow problem solving independant of a either a buddy or the surface we have no business diving trimix. quote]


You still haven't answered Lymex's question. What do you mean? You are not trying to obfuscate here are you Dave. Are you or are you not referring to independent doubles?

As a further question, if you are an advocate of independent doubles, In the interests of furthering my knowledge, I would like if someone would take the time to explain to me the benifits of using them versus manifolded doubles. I am not trying to be a pain here but I truly would like to know. As you might guess, I don't dive with divers who use them so I would truly like the rationale behind them. You can PM me if you like so I don't hijack the thread. Now does this sound like someone who has been assimilated Dave? ( I am serious btw, not trying to make some kind of joke)

 

[Bismark]

Bailout is fully redundant from the RB since it´s (at least usually) comprised of completely separate OC-tanks and since you carry enough to end your dive on the bailout (while breathed OC)...some units have/are modified to connect off-board bailout but I don´t think anyone plans their bailout on the assumption that they´ll be able to...at least, from my newbie-perspective, that´s a bad idea...

Okay, so now I have another question: I don't dive a CCR so please help me with this one. What do RB divers do for bailout gas for a dive to say, 200' with, say a deco of 35 minutes? Do you actually carry enough bailout gas for the ascent and the deco (different bail out gases?) assuming the **** hits the fan at the last minute of the dive?

 

[wedivebc]

[
You still haven't answered Lymex's question. What do you mean? You are not trying to obfuscate here are you Dave. Are you or are you not referring to independent doubles?

I am sorry, I thought redundant breathing system was commonly understood to mean either doubles other than a single outlet crossover bar or a single with adequate bailout tank. By my definition an RBS could be doubles, RB with bailout or a single tank with pony bottle. I guess most of you guys stay away from the solo diving forum so wouldn't know this

Bismark, I would gladly debate independant vs manifold issues but not on this thread. Care to start a new one I'll be there

 

[Charlie99]

I just read those posts. Can someone explain the difference to me between ongassing and solubility? I thought ongassing involved diffussion/solubility of the biomass. Just trying to learn something here!

To put things into perspective, remember that the early responses in this thread were about the overriding reason for helium --- lower narcosis. Divers may quibble over whether the proper maximum Equivalent Narcotic Depth is 30m/100', or 40m/132', or 50m/165', but at some combinations of dive conditions and depth it is wise to use helium to have less narcosis.

Then the discussion got into the relative performance of N2 and He in decompression. The post and links by Kevrumbo are a pretty good summary of the relative advantages and disadvantages.

==========

As to your question on the difference between ongassing and solubility ........

all gases will be absorbed into the bodies tissues. For any particular combination of gas and tissue there is 1) a speed at which it ongasses/offgasses, and a 2) maximum volume (per ata) of gas that will be absorbed by the tissue.

Helium, being a lighter molecule, is absorbed about 3 times faster than N2 for any given tissue. OTOH, the solubility of helium is lower in most body tissues, so the total amount of gas absorbed when saturated is lower.

On long, deep dives the lower solubility of helium is the important factor. Dual phase decompression models (aka bubble models) such as VPM and RGBM estimate the free phase volume (fancy term for bubble volume) and set the decompression profile and times to keep the free phase volume on ascent below a certain limit. Since for the same deep/long profile the total starting volume of dissolved He on a trimix or heliox dive will be lower than the total starting volume of N2 for the same profile nitrox dive, a larger fraction of the dissolved helium is allowed to come out of solution into free phase.

OTOH, for a shorter, shallower dive the faster absorption of helium as compared to N2 means that helium requires a bit more decompression for those dives. Or to put it another way, NDLs will be shorter for helium than for nitrox. The crossover point between deep/long dives where helium has the advantage vs. short/shallow where nitrox has shorter deco and/or longer NDL is roughly at the depth-time combinations that are the air NDLs.

Of course, this means that in the depth-time areas where nitrox has an advantage, that you aren't doing much deco or total ascent time, and the slight disadvantage of helium on these dives doesn't really matter much ---- other than to generate arguments on Scubaboard.

 

[Bismark]

Charlie99 You are the Man! Most excellent precis. I might just have to print that out for future reference.

 

[JeffG]

Well as is typical when debating belief sysems with the assimilated, as soon as they run out of valid arguments to support their position they obfuscate the argument with nonsense.

We could just call you names then.