Trimix pros and cons...

[TSandM]

Well, from physics, maybe? Helium is fairly insoluble in water and in lipids. On the other hand, it is small and diffuses rapidly. So it reaches saturation quickly, but there is very little of it in solution at saturation. The implications of the combination of poor solubility and rapid diffusion are controversial.

 

[battles2a5]

Another issue w/ trimix that hasn't been mentioned is logistics, especially on the carolinas coast (or anywhere else w/ unpredictable, hard to reach locations. Let's say you plan a weekend of deep offshore wrecks, stuff 2 sets of doubles full of trimix, and then the dives get blown out. Now you are stuck either a) not diving those doubles until another trip is set (not easy in these parts given the distance from shore) b) diving your $150 trimix fill on a ridiculously shallow dive, and/or c) buying more doubles. I have 4 sets of doubles now and it is still a problem. This is when rebreathers start to make a lot of sense.

Not sure I buy into the weight advantages of stages w/ uneeded helium, especially considering the cost. If weight is that much of an issue either spend some more time swimming them or do a set-up dive so you're not carrying so many full ones.

 

[Rhone Man]

I think the other point that is rarely said (I haven't seen it in this thread yet) is that there is a lot less history in relation to decompression obligations and helium. We have lots of theoretical models, but because trimix divers are so much rarer and more recent than air divers, there is a much shorter and narrower track record to look at in terms of practice rather than theory.

I'd be interested to hear from the experienced trimix divers on the board whether there is much experience of trimix divers having an "unearned hit" whilst following computers or tables. My exposure is very limited, but I have never heard of this on trimix (as opposed to air, where it is a common enough tale), which makes me feel that calculations on helium are probably pitched quite far inside the lines. I love to hear thoughts on that topic from the trimix gurus on the board.

 

[battles2a5]

While trimix diving is farily new to the sport diving world, commercial and military diving ops have been using is for a looonngg time. So I don't think there is anything inherently more dangerous with using helium vs. air tables.

That being said, with helium comes the possibliity for much deeper dives, and the deeper we go the wider the standard distribution of incidents will become over a given population. All of the algorithms are based on assumptions and theory, so it will never apply to any one person exactly. The further you push them, the less accurate they become. So you always have to be aware of the fact that you are conducting a science experiment every time you do a big dive, and you are the specimen.

When I started diving trimix I started with conservative plans and slowly worked my way up through bigger dives. I took careful note of how I felt when I got out of the water, of any variances in the dive plan, and now I have a feeling for what I can/ cannot get away with on bigger dives. I have never taken a hit, but I have had some minor niggles when running more aggresive profiles. Really the term "unearned hit" is a misnomer since nobody is going to guarantee that any of these algo's will get you of the water clean.

 

[Blackwood]

Now you are stuck either a) not diving those doubles until another trip is set (not easy in these parts given the distance from shore) b) diving your $150 trimix fill on a ridiculously shallow dive, and/or c) buying more doubles.

I started off in the "a" and "b" category. Didn't want to buy more (I borrowed).

Now I'm a "d" diver.

d) trans filling to stage bottles

After breaking apart my double 80s, I have two sets of doubles. My general plan for dives in the <160avg range is to always dedicate one set for non-helium dives, and to use the other set plus a stage on two dive trips. I have a three dive trip coming up for which I need helium in two sets and a stage, but two-a-days are much more regular.

Well, from physics, maybe? Helium is fairly insoluble in water and in lipids. On the other hand, it is small and diffuses rapidly. So it reaches saturation quickly, but there is very little of it in solution at saturation. The implications of the combination of poor solubility and rapid diffusion are controversial.

Yah, from physics.

And yah, I'll freely admit that I don't know how that plays into decompression algorithms, just thought I would point it out.

Know of any heliox tables that take solubility into account?

 

[ivobj]

One issue that I didn't see mentioned here is that when you dive trimix, specially if the helium percentage is high, you should not use it in your drysuit, you must necessarily carry a pony bottle with either argon (cold water) or at least regular air, because helium conducts heat very well, so it would transfer the water temperature to your body very easily exposing you to a hipotermia risk in long and/or cold water dives.

 

[Meng_Tze]

I have done mix dives to 300ft and runtimes of 3+ hours (incl deco obviously). Both OC and CC mix dives. No hits, niggles or anything in relation to DCS when diving mix. But then I was in no rush and had ample time to think things through...

 

[Deefstes]

Wow, this is a fascinating thread. I'm no advanced diver and nowhere near the point where I'll be diving fancy mixes but it does interest me so I enjoy reading this.

I'm just curious about this idea of adding some He to your deco mix simply so the stage cylinder rides better. Seems awfully expensive to me for that luxury - but then I have no idea how hasslesome a stage bottle really is. Would it not be an option to simply attach a piece of polystyrene or something buoyant to the cylinder?

 

[battles2a5]

I'm just curious about this idea of adding some He to your deco mix simply so the stage cylinder rides better. Seems awfully expensive to me for that luxury - but then I have no idea how hasslesome a stage bottle really is. Would it not be an option to simply attach a piece of polystyrene or something buoyant to the cylinder?

Like I said in my earlier post, this is the last reason I would add He to a mix. There are a million things you can do to a stage to adjust the way it rides that cost $0. If pushing that extra 1lb of mass through the water is that significant to you then you shouldn't be swimming stages. At most, a better riding stage is a pleasant side effect of using mix, but not a reason for using mix. There are plenty of other reasons to use He in stages, but ride quality is not one of them, IMO.

 

[Rick Inman]

I'm just curious about this idea of adding some He to your deco mix simply so the stage cylinder rides better. Seems awfully expensive to me for that luxury - but then I have no idea how hasslesome a stage bottle really is. Would it not be an option to simply attach a piece of polystyrene or something buoyant to the cylinder?

Actually, a fill to 2200psi will do the same thing... A lot of divers who don't have access to O2 to boosters will just decant the O2 into an 80 with a light fill. You get more than enough gas, and it rides on the leash on your butt like a dream. Of course, this has nothing to do with stages with mix.