Calculating deco and mix

[Reg Braithwaite]

Ok, for starters I am NOT a tech diver, but I am trying to understand the basics behind planning all types of dives. I took my PADI Nitrox and found it much more informative about Nitrogen and Oxygen and the calculations behind dives and intervals than the Open Water (obviously). I then bought the TDI manuals for Advanced Nitrox, Decompression Procedures, and Extended Range Diving, but there is a simpel question that I could not figure out: maybe I missed the obvious...

When working with enriched air mixtures, it seems straightforward to calculate EAD and use that to turn a 21/79 table into an enriched air table. Likewise, the CNS limits and accumulation seem straightforward.

However, I do not understand how breathing enriched air changes off-gassing nitrogen. For example, let's say I plan my first dive as a no-deco dive to 100' on air, with a 1 minute safety stop at 50' on the way back up and 3 minutes at 15'. Depending on the table I use, there is going to be a certain carry-over of residual nitrogen for my next dive, let us say it is forty minutes later.

What happens if I take a 50/50 "deco mix" with me and breath that starting at my 1 minute stop at 50', and continue breathing 50/50 for the remainder of the ascent, including my 3 minute stop? There's a picture in the TDI Advance Nitrox manual of someone breathing enriched air on the boat after diving. What effect would this have on a surface interval? On time before it is safe to fly?

The books talk about doing this kind of thing to add a safety margin, and they talk about using software to plan dives (where the software obviously models this type of dive). But before I even think about training for this kind of thing, I would like to understand the principles.

Again, maybe I missed it and the answer is simply "Turn to page xxx and Read The Fine Manual." But I would appreciate a pointer, thanks! So... where can I find some information on calculating the effect of using enriched mixtures for "deco?"

 

[jimdiverman]

There are tables that address accelerated deco. I use IANTD tables that give you reduced deco for 50% or greater O2 deco gas. There are other tables for higher percentages as well, obviously at shallower depths. Always pay attention to the MOD. Standard tables don't give you deco credit for breathing higher oxygen mixes. You need the accelerated tables. Check the IANTD website, and I am sure there are others where you can find that information.

As far as breathing EANx on the boat, well there are lots of potential reasons for that, but none of them has anything to do directly with stop times in the tables.

When you take an advanced nitrox course, your instructor should take you through this stuff. I took IANTD and PSAI Advanced Nitrox last year, and found them very informative and a great help in understanding gas planning and management.

Dive safe!

 

[do it easy]

If you want to go beyond tables, you would have to dive into the actual models to see how they deal with ongassing and offgassing with various mixtures. John Haldane, Robert Workman, and Albert Buhlmann all have relatively simple models that would be a good place to start.

 

[TSandM]

I can't tell if you are asking for specific numbers (as in, 50/50 cuts the no-fly time in half) or just the underlying principle.

The principle's pretty simple: The gradient driving the movement of nitrogen is the difference between the nitrogen in the tissues and the nitrogen in the lungs, which is what you are breathing. To maximize the gradient while breathing the same gas you were using at depth, you need to get quite shallow, which encourages bubble formation. By dropping the nitrogen percentage in the gas you are breathing, you can achieve the same gradient at a deeper depth, controlling bubble formation; alternatively, at the same depth, you have increased the gradient to drive nitrogen out of the tissues more quickly. This is the idea behind rich mixes for "accelerated deco".

How much credit you get for a given decrease in nitrogen percentage in the deco mix will depend in part on the model you are using to determine your deco in the first place.

As has been discussed here before (and not too long ago, IIRC), the risks involved in doing gas switches and maintaining depth accurately with rich mixes are generally considered to be greater than the benefit to SIT or no-fly time from dives within recreational limits. Most people survive Type 1 hits; very few survive oxygen toxicity seizures.

 

[Bismark]

One other thing you might find interesting is the "oxygen window". Google it for information but if you can't find anything or need a "quick and dirty" summary, just ask. Really a very cool thing that we use to help with decompression. I once looked at the TDI books but I don't remember if they mentioned anything. I am sure TS&M can provide you a pretty detailed description from a medical point of view also......sorry to set that up for you Lynne..........try to keep it inside 5 pages..........

 

[Reg Braithwaite]

"As has been discussed here before (and not too long ago, IIRC), the risks involved in doing gas switches and maintaining depth accurately with rich mixes are generally considered to be greater than the benefit to SIT or no-fly time from dives within recreational limits. Most people survive Type 1 hits; very few survive oxygen toxicity seizures."

Very good point, and most interesting to consider. Vaguely reminds me of the math behind certain forms of cancer tests: if the patient doesn't display enough indicators, the risk of a false result outweighs the benefit of testing.

 

[do it easy]

One other thing you might find interesting is the "oxygen window". Google it for information but if you can't find anything or need a "quick and dirty" summary, just ask. Really a very cool thing that we use to help with decompression. I once looked at the TDI books but I don't remember if they mentioned anything. I am sure TS&M can provide you a pretty detailed description from a medical point of view also......sorry to set that up for you Lynne..........try to keep it inside 5 pages..........

At the risk of getting flamed, I'm skeptical about the oxygen window- I don't think there is anything magical about breathing O2 at 1.6 partial pressure, other than the fact that you are breathing less inert gas that you are trying to get rid of (driving the gas gradient). If I recall correctly, even the researcher himself said the effect would be small or inconclusive.

 

[texdiveguy]

..... very few survive oxygen toxicity seizures.

Does anyone have statistics on the subject of CNS OxTox hits in terms of incidents and conclusions of the hit? CNS OxTox is very rare just as is ICD in our sport....this is a 'good thing'. I never have really seen a good report on the actual numbers, would find it interesting.

 

[Bismark]

Yikes, you don't believe the world is flat too do you? Just kidding.

Try this:

"Gas Exchange, Partial Pressure Gradients, and the Oxygen Window" by Johnny E Brian, Jr., M.D., Associate Professor, Department of Anesthesia, University of Iowa College of Medicine.

You won't just be able to skim that baby' in the bathroom, trust me.

I will expect a single page book report, double spaced.....(just kidding)

 

[JS1scuba]

A no-stop dive by definition has no required decompression. So shifting to a 50/50 ean mix for an ascent from a no-stop dive really does nothing at all. The only time using an additional decompression mix is warranted is when there is required decompression that you want to reduce the time for.

Cheers

JDS