Pushing limits – diving longer with Nitrox
No decompression limits and tissue saturation with air and nitrox
If you look at the no-decompression tables for air and various nitrox mixtures, it is clearly visible that you can simply dive longer with nitrox – provided your own breathing allows it. Nevertheless, you still saturate with nitrogen, it just takes a little longer. Exploiting the new limits of nitrox has made the dive longer, but not safer.
That’s exactly what we’re going to look at step by step here.
Here we would like to look at a dive to 30m and compare how the safety advantage of using an EAN32 instead of air on the same dive is – and also how far you can extend the dive by doing so.
On the first picture you can see a typical dive profile:
You reach the 30m maximum depth within three minutes and stay there until the no-decompression limit. According to the table, that’s 20 minutes at 30m, so we’ll just plan for that. We go up slowly, taking 5 minutes until we are back at 10m. We stay there for another 10 minutes and then return to the surface with a safety stop.
Below the profile you can see the heatmap for this dive. This tells you which tissues are currently saturating, which are desaturating, and how oversaturated they are. The top line corresponds to the fastest tissue, the bottom to the slowest. What is important here is the moment when the surface is reached again. In this example, the middle tissues are at their saturation limit here (the redder, the more borderline).
The schematic below shows exactly the same dive, but now with an EAN32. We look at the decompression phase, surfacing, and most importantly, how supersaturated you arrive at the surface. Even the middle tissues are far from the orange or red range, not reaching 50% of the M values,
As you can clearly see here, you actually come out of the water much less saturated, so that would have made your dive safer.
Things get different when you use nitrox to extend the no-stop time. Although in this plan you can stay at 30m for 32 minutes instead of 20 – but then you come out of the water just as saturated as in the shorter dive with air. This dive is just as “risky” as the one with air.
Nitrox is a great thing, sure. Taking in less nitrogen is so good that you’re happy to put up with a little oxidative stress for it. But it doesn’t make the dive safer per se: if you use it to push the limits, you’ll come out just as saturated, and you’ll also have exposed yourself to even higher oxygen pressures. A little distance from what the models and computers say you’re allowed to do might actually be a good idea.
You can also create such profiles yourself with Subsurface – open source, free, for planning only usable on PC, great: Subsurface Dive Planner
Optimize gases and times: What you can calculate
Of course we can find tables and technical aids for everything. The Analyzer simply displays the MOD, we can read EADs from a table, apps calculate the best mix for us… So why still calculate yourself?
Maybe just because we can. But maybe also because it’s good to be able to at least roughly estimate what can be true and what isn’t, and where it becomes dangerous. The app is not there when you decide underwater whether you really want to risk 2m deeper or not.
In the next videos we will show you step by step how to calculate your MOD, Best Mix and EAD. Under each video you will find a few values to calculate it yourself.
Dalton’s Diamond – or: How am I supposed to remember all the formulas?
Who feels like me: Too many characters at once just don’t stick in the brain. You don’t even need it, if you remember the one important context…
After all, it’s always about three things: The ambient pressure – that is, the depth at which you are. Then the percentage of oxygen in the gas, what percentage does the mixture have. And, of course, it’s all about the partial pressure of oxygen.
These three units are connected to each other. What’s really easy to remember is that pO2 increases as pressure increases, and it increases in proportion to pressure. P, the pressure, times fO2, the oxygen content in a gas, gives pO2, the partial pressure of oxygen.
P x fO2 = pO2
So far, so simple, right? Depending on what you are looking for, you can fill two of the placeholders with values and calculate the third.
To remember, you can use “Dalton’s Diamond” – and because it’s not only about oxygen, but about any gas, we just take g for gas instead of O2.
Calculate MOD
How deep can I go with my mix? The Maximum Operating Depth (MOD) is the maximum operating depth of a gas, in our case the depth at which we reach an oxygen partial pressure (pO2) of 1.4 bar.
In this case, we already have the gas, so we know the oxygen content. Let’s assume that we measured 32% – this corresponds to a proportion of 0.32. And we know the pO2, precisely the 1.4 bar that we do not want to exceed. While we’re at it, this is what our Diamond looks like:
We only have to “pull out” the value we are looking for, P, and arrive – as can be seen in the calculation – at 4.37 bar.
So far, so simple. 4.37 bar pressure prevails at 33.7m, something like that (every 10m one bar, and one that is already there at the surface…). That would be, rounded to the safe side, a MOD of 33m.
All clear? Once you understand how to do it, try it yourself:
What is the MOD of EAN31, EAN 29 and EAN37?
Solution
| pO2 | 1.4 bar/ pO2 | (x-1)*10 | MOD |
| 0,31 | 4,52 | 35,2 | 35m |
| 0,29 | 4,83 | 38,3 | 38m |
| 0,37 | 3,78 | 27,8 | 28m |
Select Best Mix
You know how deep you want to dive, and you have the freedom to choose which mix you want to use. Most of the time, we then want an oxygen level that gives us the longest possible no-decompression time, but is still safe – that is, the one at which we reach 1.4 bar pO2 at the deepest point of the planned dive. How can you calculate which mixture is ideal for this?
For this we know the depth, let’s say we want to go to a wreck whose deepest point is at 30m. At 30m, the pressure is 4 bar – so far, so simple. Even now, we first use what we know:
And again, we just need to isolate the missing value.
Which brings us to an EAN35, a blend with 35% oxygen, as the Best Mix.
When you get it, it’s your turn again:
What is the best mix for 28m, 32m and 40m?
Solution
| m depth | P = ÷10 + 1 | 1.4 bar ÷ P | Best Mix |
| 28 | 3,8 | 0,368 | EAN36 |
| 32 | 4,2 | 0,333 | EAN33 |
| 40 | 5 | 0,28 | EAN28 |
EAD – Equivalent Air Depth
Equivalent Air Depth refers to the depth at which air reaches the same partial pressure of nitrogen as a given nitrox mix at a given depth. We will never really calculate this, because we can also plan directly with Nitrox – but back when the tables were delivered for air only, this calculation was used to plan the dive. One then simply pretends to be correspondingly flatter…. Today, the main point is to see vividly what you actually gain by mixing nitrox. For this, let’s take a very oxygen-rich mixture, an EAN40, as an example and dive with it to 25m. The question now is: At what depth would I absorb as much nitrogen with air? So how much longer can I dive?
Here it gets a tiny bit more complicated, because we are comparing two gases, so we have to fill in the diamond twice. And then there’s a little trap: we’re comparing nitrogen here, not oxygen.
Therefore, let’s look at the example step by step.
I have an EAN40 on 25m. The nitrogen content is 60%, so my fN2 is 0.6. To find out the pN2 on 25m, we take the Diamond again.
This gives me the nitrogen partial pressure of my EAN40 at 25m depth:
When would nitrogen have a partial pressure of 2.1 bar with air? Air has only 21% oxygen, but 79% nitrogen, i.e. the fN2 is 0.79.
We have a pressure of 2.68 bar at 17m. This means that if we dive to 25m with EAN40, it is as if we were at 17m with air. Accordingly, you can dive longer – how much that makes up, everyone can now look for themselves.
You can also calculate this yourself using the example:
What is the EAD for an EAN32 at 30m, for an EAN36 at 27m, and for an EAN29 at 35m?
Solution
| depth | P | fO2 | fN2 | PxfN2 | EAD |
| 30 | 4 | 0,32 | 0,68 | 3.44 bars | 24m |
| 27 | 3,7 | 0,36 | 0,64 | 3 bars | 20m |
| 35 | 4,5 | 0,29 | 0,71 | 4.04 bars | 30m |
