Calculating Ideal Gas Mix?

[Kingpatzer]

So, I'm in the midst of my TDI class, and I am doing some problems on calculating best gas, using the formula: (max ppO₂ / ATA).

And it all more or less makes sense.

But it raises a few questions for me.

First, I assume that in the real world, most of the time one's tanks aren't going to be custom mixed to each dive, are they? If I'm dropping down to 150' for dive one, and 130' for dive 2, am I going to mix up one set of doubles at 25% O₂ and the other at 28%, or am I just going to say "well, 25% is good enough for the shallower dive as well." Or more likely "ok, those are close enough to air to just do an air fill -- since I'm not (yet) up to doing trimix.

Second, is it the view that the 1.4 ppO₂ value offers a sufficiently conservative approach to not worry about over-shooting the target depth (in terms of the gas being used, not in terms of the rest of the dive plan!!)

 

[ucfdiver]

So, I'm in the midst of my TDI class, and I am doing some problems on calculating best gas, using the formula: (max ppO₂ / ATA).

And it all more or less makes sense.

But it raises a few questions for me.

First, I assume that in the real world, most of the time one's tanks aren't going to be custom mixed to each dive, are they? If I'm dropping down to 150' for dive one, and 130' for dive 2, am I going to mix up one set of doubles at 25% O₂ and the other at 28%, or am I just going to say "well, 25% is good enough for the shallower dive as well." Or more likely "ok, those are close enough to air to just do an air fill -- since I'm not (yet) up to doing trimix.

Second, is it the view that the 1.4 ppO₂ value offers a sufficiently conservative approach to not worry about over-shooting the target depth (in terms of the gas being used, not in terms of the rest of the dive plan!!)

Most of us tend to use standard gas mixes because they're easier to mix, and also cover a wide range of diving. This helps if your diving plans change because a cave is blown out, or a wreck has too strong of a current to dive that day. There's also issues of using standard gases so that you're familiar with deco profiles if you happen to overstay out original plan, or covering lost/stolen deco bottles.

As for the second, I plan for a 1.4 max ppo2, and 1.2 or less average ppo2. Too many people have toxed randomly to try and save a few minutes of deco by pushing it.

 

[rjack321]

Second, is it the view that the 1.4 ppO₂ value offers a sufficiently conservative approach to not worry about over-shooting the target depth (in terms of the gas being used, not in terms of the rest of the dive plan!!)

1.4 is a working max for most agencies today. So if you subscribe to the "best mix" logic you very well may end up 20ft above the wreck at your MOD unable to descend further. Its the reason most of us use "standard" mixes. I don't know how these apply to your course because they assume you are able to get trimix fills. There's nothing magic about trimix but in your case I'm guessing you're stuck with air - or backing off on the "best" part of best mix to give you some buffer for an unexpectably deep site.

 

[w ripley]

I just want to mention that a diver needs to be careful about straying from how he was taught. You've recieved good advice so far on this thread but such will not always the case.

You're going thru training for a reason, and that's to learn the safe way to dive. Appreciate that. Do what your instructor tells you. You're spending good money to help you stay alive.

With technical diving comes the understanding that the tolerances for error begin to get pretty small, and error can mean death. Stick with what you've been taught, as it comes to you as the result of many people learning the hard way. Little short cuts at the beginning of your technical career can lead to bigger ones down the road, which will come back to bite you when you're at 400 feet.

If you haven't done so, pick up a program like V-Planner or Deco Planner. You're going to need it to plan your technical dives and they come with great tools for determing your best mix, your best depth, what you get with a half tank of this when you top it off with so much air, etc, and you will need it when you get into trimix. The one thing I can tell you is that once out of class it's all electronic. We all use programs and carry our laptops, etc., with us when we're doing more than one dive.

To answer your questions:

If you don't have two sets of doubles or access to a compressor on your boat or where your diving to top off your tanks for the next dive, then using the mix from your first/deepest dive is usually fine to use for your shallower/second dive, assuming you have enough of it. The only way you will know is to run the numbers. BTW, if you don't have an oxygen analyzer get one so that you can always know what it is you are breathing.

Most of us won't dive OC or CCR with a diver who has a bottom mix above 1.4 (1.6 for OC decos), and even lower on more demanding dives. And, most of us want to dive with divers who are more concerned about being safe than getting out of the water fast. Nothing wrong with smart dives that have shorter decos (especially in cold water), as long as it's not just spiking the O2 to get out faster.

Sorry to go on, but I think you asked important questions. Best wishes with the class.

 

[rjack321]

The one thing I can tell you is that once out of class it's all electronic. We all use programs and carry our laptops, etc., with us when we're doing more than one dive.

I use an excel spreadsheet to make sure I don't eff up the math when changing mixes, but otherwise haven't used a computer of any type to plan or execute a dive or repetitive dive in years.

Its good to know "best mix" but at least around here, I don't know anyone actually planning EAN25 vs EAN28 dives anymore. From a practical perspective, the OP's instructor may or may not expect this. They are your best resource for this question anyway.

 

[AndyNZ]

As a TDI instructor, I can safely say that "best mix" is poorly defined in TDI course materials.

Some of the questions in the books/exams are ridiculous - particularly in Advanced Nitrox where you end up calculating a "best mix" of 54% or something daft like that.

I would take the manuals and questions as simply being a case of showing you can do the calculations - it doesn't mean that the answers are "right".

From a practical point of view, you have to look at the trade off between the hassles of getting 23% over the advantages of using 21% (for example). When you are talking a couple of percent in the mix.... well, if you are using EADs and tables any "benefit" is likely to be lost in rounding of depths and times etc.

Standardised gasses, as touched on by others, have a number of advantages. The one that is often skipped over is that the team have the same gas. If you turn up for your dive with EAN23 back gas and O2 for deco, and your buddy turns up with EAN21 and EAN50.... your deco schedules will be so different that you are virtually useless at helping out if something goes wrong.

Other than that, standard gasses are a convenient subset of all the available gasses to simplify the selection of "best mix" (whatever that really means) for a given dive.

 

[Rhone Man]

As for the second, I plan for a 1.4 max ppo2, and 1.2 or less average ppo2. Too many people have toxed randomly to try and save a few minutes of deco by pushing it.

James makes a really good point. I remember in my class they taught very much on a 'square profile' basis, ie. calculating best mix for max depth. In reality, I have come to suspect that average ppO2 is much more important in risk planning (which I don't remember being covered at all in my TDI class).

When I was diving in Cyprus, the dive plans had us going down to a depth where our initial ppO2 was 1.5 ATA, but we were only there a few minutes before the majority of the dive proceeded at a depth with a ppO2 of around 1.0 ATA. At first I thought it was pretty hairy breaking the 1.4 glass ceiling, but on the profiles we were doing it actually made a lot of sense. Plus European divers are all crazy anyhow. :shocked2:

 

[Rhone Man]

As a TDI instructor, I can safely say that "best mix" is poorly defined in TDI course materials.

As a non-instructor, I'd agree, although I can see the benefit of giving students really off the wall examples to work through, just to make sure they fully understand the concepts and the formulae.

 

[AndyNZ]

As a non-instructor, I'd agree, although I can see the benefit of giving students really off the wall examples to work through, just to make sure they fully understand the concepts and the formulae.

Yup, provided they examples get used appropriately then it works well. For example, the particular question that I am thinking of that results in a >40% backgas mix (for a single tank, non-deco dive) allows you to really start discussing what "best mix" is.

Typically when talking about that question the discussion goes:

Me: I noticed that for question 5, you calculated best mix as being 54%?
Student: Yes, I did 1.4/Data
Me: So is your single tank set up oxygen clean to allow you to use this mix?
Student: Ah..... no.
Me: Will you oxygen clean it for this one dive, then maintain it's oxygen clean nature until next time you use >40%?
Student: Probably not.
Me: What is your CNS exposure for that dive, anyway?
Student: I didn't work that out.
Me: OK, let's do that..... ah look, it's up there at around 70% for a single dive. Does that seem high to you?
Student: Yeah, I guess so... but if I do a 90 minute surface interval instead of an hour that would drop to 35% so it's not too bad.
Me: Do you think that the hassles and disadvantages of doing this dive on 54% really warrant the gas choice? Or can you do the same dive on 40%, or even 32%?

etc etc

 

[Slamfire]

Best mix does not mean mandatory mix for any particular depth. Not using the best mix for any particular depth in a dive does not mean that you're departing from the best diving practices. There could be many specific situations -- some exemplified in the posts above -- were using the best mix may not be best decision. In your example, I think it is perfectly acceptable to do a 130' dive using 25%. Overall run time differences should not be very dramatic for dives within the TDI deco procedures range.

As for the 2nd question, I do not think that having 1.4 as max operating ppO₂ was done so that it becomes ok for people to blow through their planned max depth. There's no hard line drawn in the sand below which you will certainly not tox. In that way, it is similar to DCS. Tolerance will vary from individual to individual and from day to day. So overall the largely accepted standards are 1.6 as max deco and 1.4 as max working/operating. You may decide to change this for your diving. Some people will like keeping it below 1.3 or even as low 1.2 as ucfdiver does. Nothing wrong with keeping it more conservative.

Others will point out to the time-dose relationship to oxygen exposure. They'll say that a long exposure at 1.4 could be more dangerous than a short exposure to 1.6. Therefore they'll feel its acceptable to go beyond 1.4 for short periods of time.

The course is just the beginning point. You should strive to gain more knowledge (as I'm sure you are) that will allow you to make you're own well informed decisions about the kind of risks you are willing to take.