Calculating Ideal Gas Mix?

[Kingpatzer]

You guys are providing some great info here -- THANKS!
I followed up with my instructor with an e-mail asking the same question, and he included a lot of similar thoughts in his response: that it's more of an exercise for leading into future course work; that the dive team as a whole needs to be considered; that site selection issues can be at work; that published depths for sites aren't always correct; and so on.

So I'm getting the idea that there's a lot more to consider than doing the math -- which I definitely suspected in asking the question.

One of the things that is concerning me about going down this road is that I'm still at the point where I don't know what I don't know (if that makes sense). I'm reading as much as I can outside of the course material, but so much of what I'm finding seems to be mixing personal preferences, opinions and organizational differences with best practices, fact, and real wisdom that it is often a bit difficult for a n00b to discern what is worth knowing and what is merely worth being aware of.

I know that's why mentorship is important -- something I've strove to incorporate into my journey as a diver from the very first dives -- but right now it's a bit much. It's not really information over-load, what I need to learn to pass the class is pretty straightforward. Rather, I'm struggling with figuring out how to really understand the "why" behind things.

I much prefer being in the position of being able to do more than rote recall than not. I guess I'll get there soon enough, but in the meantime it's a bit frustrating.

 

[Slamfire]

If you compare a rebreather diver using 1.2 throughout the dive and an OC diver using 1.4 for bottom gas and well selected 1.6 PO2 deco gases then the OC diver will probably incur less deco on dives in the 200ft+ range. If that same RB diver used 1.4 thought the dive he would probably regain that advantage but high constant PO2s on CCR are not a good idea.

Is programming a variable pO2 too complicated with common current CCR technology? How about doing an "air break" strategy? Obviously you wouldn't necessarily use air because at those depths the sudden narcotic hit could be less than helpful. But you could use a low pO2 mix for the break. Aren't there ways around this? Or is it that the difference between OC and CCR deco schedules is not that great to be worrying about (akin to using 25% instead of 28% for a 130' dive)?

King, sorry about hijacking your thread with all these rebreather questions. I would just add that patience is your friend against frustration. On the other hand frustration is a great motivator for getting rid of ignorance. I would also add that, IMO, it is to your own benefit to get answers from different diving philosophies. You can then objectively analyze the pros and cons and make a choice that is best for you. Don't get too militant within one camp and discard others based on subjective cliqueish motivations. You could end up doing yourself a disservice.

 

[wedivebc]

Is programming a variable pO2 too complicated with common current CCR technology? How about doing an "air break" strategy? Obviously you wouldn't necessarily use air because at those depths the sudden narcotic hit could be less than helpful. But you could use a low pO2 mix for the break. Aren't there ways around this? Or is it that the difference between OC and CCR deco schedules is not that great to be worrying about (akin to using 25% instead of 28% for a 130' dive)?

No variable PO2 is not too complicated with the technology but there is just no advantage to complicate your dive unnecessarily for the slight decompression benefit it might provide. Same with air breaks, keep the PO2 lower and there is no need to use them.

 

[rjack321]

If you compare a rebreather diver using 1.2 throughout the dive and an OC diver using 1.4 for bottom gas and well selected 1.6 PO2 deco gases then the OC diver will probably incur less deco on dives in the 200ft+ range. If that same RB diver used 1.4 thought the dive he would probably regain that advantage but high constant PO2s on CCR are not a good idea.

What Dave said.

Running a high (e.g. 1.4) ppO2 for the entire bottom phase and ascent gets tough to do within CNS limits and it can be hard from a pulmonary perspective as well. Its especially tough to do below somewhere in the 200-240ft range depending on BT.

The OC diver can run a low ppO2 on the bottom (where it makes surprisingly little difference in deco) with spikes and declines in ppO2 as they switch gases. The RB will generally end up with less deco <200ft while once >200ft the OC diver ends up with less deco time within their respective gas constraints. Net: Best not to chose a RB with the objective of attempting to minimizing deco via "best mix" for especially deep dives. Of course deciding to do a 400ft dive on OC just to have a shorter runtime isn't the best idea either.