CCR Mode

The biggest change in CCR mode form dive physiology, science (math) and API point of view is, that the content of the gas in the loop is dynamic.

There are couple of factors how it's dynamic:

• It's possible to change set point (ppO2) on the fly with most units
• The mix of diluent changes as function of depth, while the unit is maintaining the set point

Set points (ppO2)

The ppO2 varies between different units and also by diving training / practice. Typically it's something between low (0.7) to high(1.2 -1.3). The low set point is used on the surface and the high set point during the dive. Then there are Oxygen breaks and also some people drive the unit on higher set point during deco.

So it has to be possible to set the current set point for that point-in-time deco calculation. In other words: it can't be constant dive parameter.

Dynamic loop content

Let's assume that we are doing 90m mine dive, and for that dive we have 12/65 (or 10/70) diluent. The dive starts with traverse first on the level 30m and then with second traverse on level 60m, until finally we dip into 90m level.

The ppO2 of diluent is 0.48 (4 * 0.12) at 30m, but this is already part of the dive where high set point is used. So the unit will inject O2 until the ppO2 is at the high set point (1.3). But this will dilute the diluent (pun intended), so the mix doesn't have 65% of He. Same happens with 60m level.

At the bottom phase of the dive, at the 90m , the ppO2 of diluent is 1.2 (10* 0.12). This is still under the set point (1.3), so the unit is again adding some Oxygen, but this is desired situation, so that's easier to do diluent flush if needed (the diluent is not so "hot").

Same happens in the reverse while we are doing deco: the He is reduced gradually until the loop is 100% oxygen (set point 1.3 @ 3-4m depth).

So the content of mix has to be function of the original (or "nominal") mix and the current diving depth.

Other factors

Then there are Oxygen breaks (to reduce risk CNS Oxygen toxicity) and some people are also diving with different diluents (at least in theory).

Hopefully this gives some starting points, this is probably the biggest change (from math point of view) compared to OC mode.

P.S. It's mostly about He in above, but same applies of course to Nitrogen.