Tracking Nitrogen in Tec Diving

[steady1570]

So, a VERY simplistic view of diving, and particularly ascent profiles, is managing the different pressure gradients of inert gas in your tissues. As others have said, there are varieties of models for doing this, but they all seek to control the rate of offgassing. Different tissues offgas at different speeds, and so you need to control (slow) the ascent to make sure that the difference in the pressure surrounding the body is less than some critical amount to control the rate of gas leaving the tissue. The non-stop (recreational NDL) tables basically are designed such that all the different compartments (or however the tissues are designated) are “allowed” to go to the surface before this critical pressure gradient is breached. Typically (Always?) the inert gas here is Nitrogen.

Tracking the Nitrogen (Inerts) groups allows repetitive diving because it considers the fact that tissues take time to offgas and so they may not all be clean – that is back to “normal atmospheric” levels – before you dive again. Depending on how much gas got into your tissues on dive one determines how much more gas you can put in your tissues and still keep the critical pressure gradient at or above the surface (and so need no in water decompression). With the relative safety figures built into these models that usually boils down to one “compartment” being the “controlling” one, and so the maths is relatively simple and can go on a pre-calculated table. (And why you are recommended to have extra “safety” stops if you are near the edges of the table).

When tec diving, we typically do not want these restrictions, and so control the offgassing such that the route to the surface involves staged decompression; that is, stopping when the pressure gradient for the inert gas (Nitrogen or Helium or both here) gets to a certain level and waiting for the body to offgas before moving upwards. The modelling of this is quite complex and so best done on a computer, although there are ways to approximate the models “on the fly” (eg Ratio Deco), but that is something that must be properly studied and the limitations appreciated before using in anger. But overall, tec diving DOES monitor the Nitrogen loading, its just that the outputs are typically not as “simple” as a single group after surfacing. Repetitive diving does have to take into account previous dives (and all of the popular algorithms and computer programs used do incorporate this), but again, there are several factors to take into account, rather than the “simple” pressure group of typical NDL tables.

As ever, this is a VERY simplistic explanation and its worth about as much as you paid for it from this amateur diver!

-Mark

 

[IDS_Bill]

Can you give us a scenario? Are you asking about an emergency situation? In that case you would have to have blown your profile, your contingency profile(s), had dual computer failure (primary and backup). Not outside the realm of possibilities but that would be a rare situation.

It's still a good question though! Remember, in rec diving, you can use the tables but they assume a constant depth throughout the dive (or simple multi-level). They aren't that accurate. I can do a lot of math in my head but nitrogen off-gassing for accelerated decompression would be pretty brutal.

 

[abnfrog]

james you will find out on the tech courses why we use tables to track n2 there is no reason to do it manually we use a p c computer for tri mix . you will have enough calculations for the nitrox to deal with

 

[rossh]

How do you manually track nitrogen when deco is involved and especially when deco using different gases is involved?

In no-stop recreational diving you can track nitrogen accumulation by letter groups on the tables. So now if we are going into decompression how do we manually calculate how much time we need to decompress if we aren't using a dive computer and we have accidentally over-stayed our bottom time as a recreational diver?

Getting into tec diving, how do you calculate how much time you need to remain at a deco stop with 21% O2 versus 100% O2?

I'm studying for my tec courses now but so far the only numerical calculations involve short and long term O2 exposure. Are we to rely on a dive computer to track nitrogen when decompression is involved? Maybe I am brain farting somewhere here but it feels like I'm missing a piece of information.

Simple tables for the NDL are possible, because the variables are 2 or 3 dimensions, and a flexible table can be created from that

But as you suggest, in tech diving, the number of variables can grow beyond the point where they can be managed easily in your head (or a table). Hence the need for computers to do the job.

However It can be done, as in the RD method, but that comes with strict controls on mix types and range limits for various formula.

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