I was recently asked by a good friend why it is possible to do all of your oxygen stops at 6m instead of having to move up to 3m eventually. And this seemingly without any penalty to your decompression time. To explain this counterintuitive phenomenon we need to know a bit about on and off-gassing.

To explain on and off-gassing in diving we will use a single compartment “Haldane” model with a 10min half time.

On-gassing

We will start our dive and instantaneously descent to 20m or 3ata. We are diving air and to simplify we say we have 21% oxygen and 79% nitrogen. As we know from Daltons Law the total pressure is the sum of all partial pressures. (Ptotal = P1 + P2 + P3 + … + Pn) So the partial pressure of O2 and N2 in the breathing gas is 3x0.21=0.63ppO2 and 3x0.79=2.37ppN2. Since we just were on the surface the partial pressure in the tissues is still 0.21ppO2 and 0.79ppN2.

Henry’s law tells us that the pressure will equalize to the surrounding pressure, and in our very simple single-compartment model with 10min half-time the pressure in the tissues will half the difference in the tissue and the breathing gas every 10min.

Mathematically we can write it like this:

10min (0.79 + 3*0.79)/2 = 1.58 ppN2
20min (1.58 + 3*0.79)/2 = 1.97 ppN2
30min (1.97 + 3*0.79)/2 = 2.17 ppN2
40min (2.17 + 3*0.79)/2 = 2.27 ppN2
50min (2.27 + 3*0.79)/2 = 2.32 ppN2
60min (2.32 + 3*0.79)/2 = 2.34 ppN2

After 60min and 6 half times the tissue is approximately in saturation. half times is just an approximation and will never actually become 100% saturated mathematically so we just call it saturation after 60min (2.37ppN2)

Off-gassing

We ascent to 6m (1.6ata) and will wait at this depth until the ppN2 is no longer oversaturated.

10min (2.37 + 1.6*0.79)/2 = 1.87 ppN2
20min (1.87 + 1.6*0.79)/2 = 1.57 ppN2
30min (1.57 + 1.6*0.79)/2 = 1.42 ppN2
40min (1.42 + 1.6*0.79)/2 = 1.34 ppN2
50min (1.34 + 1.6*0.79)/2 = 1.30 ppN2
60min (1.30 + 1.6*0.79)/2 = 1.28 ppN2

And we see again, after 6 half times the tissue is in equilibrium and no longer oversaturated as its about 1.264ppN2

The same happens when we go up to the 3m stop. But since we only ascended 3 meters, it took a lot shorter time before we were close to equilibrium. We can see that off-gassing increased when we got closer to the surface, and the partial pressure fell.

10min (1.26 + 1.3*0.79)/2 = 1.14 ppN2
20min (1.14 + 1.3*0.79)/2 = 1.08 ppN2
30min (1.08 + 1.3*0.79)/2 = 1.05 ppN2
40min (1.05 + 1.3*0.79)/2 = 1.04 ppN2
50min (1.04 + 1.3*0.79)/2 = 1.03 ppN2
60min (1.03 + 1.3*0.79)/2 = 1.03 ppN2

So what have all this to do with Oxygen?

Oxygen Deco

If we instead of breathing air when we arrived at 6m we used a bottle of O2 we would suddenly have zero, 0.0, ppN2 in our breathing gas. And this is the key to why we can stay at 6m from an off-gassing perspective if we use 100% oxygen.

Let us look at our off-gassing with pure oxygen.

10min (2.37 + 1.6*0.0)/2 = 1.19 ppN2
20min (1.19 + 1.6*0.0)/2 = 0.60 ppN2
30min (0.60 + 1.6*0.0)/2 = 0.30 ppN2
40min (0.30 + 1.6*0.0)/2 = 0.15 ppN2
50min (0.15 + 1.6*0.0)/2 = 0.08 ppN2
60min (0.08 + 1.6*0.0)/2 = 0.04 ppN2

Multiplying anything with 0 is of course non-sense so we could have been writing this as 2.37/2^6 (0.037). So again after 6 half-times, we are extremely close to 0 ppN2. And more importantly, we see that it’s not more efficient to ascent to 3m (1.3 ATA) as you still have to multiply the depth with the inert gas to find the partial pressure of the inert gas as you can see from the following calculations.

10min (2.37 + 1.3*0.0)/2 = 1.19 ppN2
20min (1.19 + 1.3*0.0)/2 = 0.60 ppN2
30min (0.60 + 1.3*0.0)/2 = 0.30 ppN2
40min (0.30 + 1.3*0.0)/2 = 0.15 ppN2
50min (0.15 + 1.3*0.0)/2 = 0.08 ppN2
60min (0.08 + 1.3*0.0)/2 = 0.04 ppN2

Real decompression

Our bodies do not act like our single tissue with a 10min half time used in the examples above. But is instead put together of multiple tissues with very different half times. But the concept is identical, ether you use my single tissue 10min half time from the examples, a classic Haldane 5 tissue model with 5, 10, 20, 40 and 75min half times or Bühlmann ZHL-16B (where 16 stands for 16 tissue compartments) model. The only difference is that tissues with different half-times will become the controlling tissue in a different part of the ascent. But as soon as you switch to pure oxygen you have reached the maximum off-gassing gradient and can’t improve upon it by going shallower.

But in addition to off-gassing, we need to control bubble growth when we ascent, so if you choose to stay both your 6 and 3m oxygen stop on 6m you still have to make a slow ascent to the surface. This is one of the very good reasons to make a 1m/1min ascent when you end your dive. During your o2 decompression, your deco ceiling will gradually become shallower until the ceiling reaches the surface. That’s why I often do most of my deco at 6m I start going shallower to 3m at the end of my decompression and then finish with a 1m/1min ascent from 3m to the surface.

If you dive in the ocean, swells and waves might be an issue, and staying at 6m for all of your 02 deco might be very well advised followed by a 1m/1min ascent to the surface. It is however important that you only ascent. If you overshoot on this very slow ascent rate just stop and wait, don’t go down again. As the very reason for the slow ascent is to control bubble growth and recompressing bubbles is exactly the opposite of what you want to do.

Here is a real-life example of a deco profile from a long exploration dive this month. First with no last stop defined, and then with the last stop at 6m.

 andreas@blackops ~ deco -d 18 -t 345 -g ean31,o2 -f 0.7,0.85
  Dpt   O2 18m:
   18   31  345
    6  100    6
    3  100   33
   Deco      40

 andreas@blackops ~ deco -d 18 -t 345 -g ean31,o2 -f 0.7,0.85 -s 6
  Dpt   O2 18m:
   18   31  345
    6  100   39
   Deco      40

What I actually did was something closer to this. As there is a nice place to deco under the cave ceiling at 5m. It makes absolutely no difference as the deco ceiling when I started the deco was between 4 and 3m and the decompression time is 39min.

 andreas@blackops ~ deco -d 18 -t 345 -g ean31,o2 -f 0.7,0.85 -s 5
  Dpt   O2 18m:
   18   31  345
    5  100   39
   Deco      40