Trimix - He percentage?

[MechDiver]

You see alot of information regarding percentage of O2 used in nitrox and trimix, along with reasons as to what is good/bad, but I can't find zip for helium.

Speaking strickly normoxic, there appears to be some fairly "standard" mixes floating about; 20/40, 30/30, 25/35 and maybe a couple others, but I cannot find anything on WHY its 30/30 and not 30/50, or 20/50 instead of 20/40. Realizing cost of He being what it is most places, and discounting that, are there any defined guidelines out there?

In the "lessor" trimix depth ranges (<200ft) the percent of He has an effect on deco time, but not significant once you have to be there, as it where. IOW, once you have to deal with 40% He, going to 50% doesn't appear to make much difference.

I have also seen reference to GUE standard mixes, but the little I have gotten from that is they are more for uniformity, and possilbly ease of mixing/storage than anything.

So, is the He percent more how much ya gotta pay for it, or is there some good reasoning behind what % to use?

Phil

 

[chickdiver]

Mechdiver-

Personally I base the percentage of helium in a given mix on the target depth and target END, keeping the END at less than 100'. I do use "standard" mixes for most dives, simply because they are required for some of the things I do, and I have mixed them so many times its easy, consequently sometimes I end up with REAl shallow END's. I would far rather have a shallow END, though, than not enough He in the mix.

 

[MechDiver]

chickdiver once bubbled...
Mechdiver-

Personally I base the percentage of helium in a given mix on the target depth and target END, keeping the END at less than 100'. I do use "standard" mixes for most dives, simply because they are required for some of the things I do, and I have mixed them so many times its easy, consequently sometimes I end up with REAl shallow END's. I would far rather have a shallow END, though, than not enough He in the mix.

Okay, so END is a good starting point. Why are some of the standard mixes you use "required"? Is this because of instructing? And, if you could, what mixes do you consider standard?

Oh, great reply to the troll. I especially enjoyed the "darlin'". ROFL

Phil

 

[chickdiver]

I work with a couple of projects that require standardized mixes. Here is what we use:

120' and shallower 21/35
190' -120' 18/45
300'- 190' 12/60 or greater, depending (some of us like to put the He REALLy high here, I typically run 12/70)

Deco mixes are 50/50 at 70' and 100% at 20

re: Darlin'- Im from the south, it was 5:30 Am, whatta ya want???

 

[MechDiver]

chickdiver once bubbled...
I work with a couple of projects that require standardized mixes. Here is what we use:

120' and shallower 21/35
190' -120' 18/45
300'- 190' 12/60 or greater, depending (some of us like to put the He REALLy high here, I typically run 12/70)

Very informative. Thank you.
What is the reason for using 21% for <120 instead of higher O2, 30 or 32 perhaps?

As we already talked about, I'm still in the 'normoxic' realm, so maybe a dumb question, but do you have any trouble with the 18% mix as far as surface to depth goes? I "think" I read somewhere that the Navy considered 16% and above okay all the way to the surface, but, IANTD at least, seems to frown on anything less than 19% without travel gas.

Phil

 

[chickdiver]

We run 30/35 for 120 deco bottles, but for actual diving I like to keep the working Po2 low. If I know the bottom is at 100', obviously we adjust for amore hyperoxic mix, I was just trying to generalize for you.

We run 18/45 from the surface all the time, no problems at all. Your descents are fast enough that the mildly hypoxic mix at the surface really isnt and issue (especially on scooters).

 

[MechDiver]

chickdiver once bubbled...
We run 30/35 for 120 deco bottles, but for actual diving I like to keep the working Po2 low. If I know the bottom is at 100', obviously we adjust for amore hyperoxic mix, I was just trying to generalize for you.

We run 18/45 from the surface all the time, no problems at all. Your descents are fast enough that the mildly hypoxic mix at the surface really isnt and issue (especially on scooters).

Sigh. So much to learn...so little time

Thank you again,

Phil

 

[chickdiver]

Glad to help, Anytime!!

 

[nradov]

MechDiver once bubbled...
Speaking strickly normoxic, there appears to be some fairly "standard" mixes floating about; 20/40, 30/30, 25/35 and maybe a couple others, but I cannot find anything on WHY its 30/30 and not 30/50, or 20/50 instead of 20/40. Realizing cost of He being what it is most places, and discounting that, are there any defined guidelines out there?

About 30% helium is really the minimum to get any noticable benefit on narcosis and ease of breathing. Much less than 30% and you might as well not even bother. And you want to keep the equivalent narcotic depth as low as possible, certainly no more than 100ft.

In the "lessor" trimix depth ranges (<200ft) the percent of He has an effect on deco time, but not significant once you have to be there, as it where. IOW, once you have to deal with 40% He, going to 50% doesn't appear to make much difference.

This seems to be a common misconception; it probably arises from the obsolete and inaccurate Buhlmann decompression model which calls for longer and deeper deco stops with helium. In the real world helium does not impose any sort of "penalty".

I have also seen reference to GUE standard mixes, but the little I have gotten from that is they are more for uniformity, and possilbly ease of mixing/storage than anything.

Standard mixes also make it easier for new divers to learn proper decompression profiles. Note that the standard mixes quote a minimum helium content suitable for most dives in that depth range; if you want to add more you're free to do so. We often dive off a charter boat that can fill nitrox on board, but no helium. So we bring tanks with an extra high helium content and then still have a reasonable mix after topping off.

So, is the He percent more how much ya gotta pay for it, or is there some good reasoning behind what % to use?

It's mainly a cost issue. If I could afford it I would use heliox on every dive.

-Nick

 

[Aquamaniac]

chickdiver once bubbled...
Glad to help, Anytime!!

Thanks Obi Won, great information..........looks like i got to do MORE reading.