Nitrox-diving – Part 2: How deep is deep? EAD and Best Mix made easy

Nitrox makes many aspects of diving more enjoyable: less nitrogen means longer bottom times and a lower risk of nitrogen narcosis or decompression sickness. But how exactly does the lower nitrogen content affect the diving depth? And how do I find the optimal gas mixture for a planned dive?

Foto: Patrick Oswald

1. Equivalent Air Depth (EAD) – when 40 m suddenly feels like 35 m

EAD is a calculated depth that shows how deep you would have to dive with air to achieve the same nitrogen partial pressure as with your nitrox mixture.

Why is this important?

If you plan with Nitrox using air tables or use the air mode on your computer, you may want to know how much nitrogen your body is actually exposed to.

Example: EAN28 at 40 m

Let's assume you are diving with Nitrox28 (i.e., 28% O₂ and 72% N₂) at a depth of 40 m. At 40 m, the ambient pressure is 5 bar.

Since you are breathing Nitrox28, the nitrogen content (FIN2) in the gas is 72%. To calculate the inhaled nitrogen partial pressure (PpN₂), simply multiply the ambient pressure by the nitrogen content:

PpN₂ = 5 bar × 0,72 = 3,6 bar

For comparison: A diver using compressed air (79% N₂) would have to dive to 35.5 m to achieve the same nitrogen partial pressure. This is because at 35.5 m (≈ 4.55 bar ambient pressure), the following applies:

4,55 bar × 0,79 ≈ 3,6 bar

As you can see, EAN28 at 40 m corresponds to an air dive at approximately 35.5 m in terms of nitrogen exposure – and that is precisely your equivalent air depth.

The formula for this is:

Equivalent Air Depth (EAD) m = ((Tiefe m / 10 + 1) x FIN2 / 0.79  - 1) x 10

If you work with air tables or your computer is running in air mode, you can use the EAD to determine your no-decompression time, decompression stops, and surface intervals.

2. Best Mix – the perfect gas for your depth

Just as every depth generates a specific partial pressure of oxygen, every depth also has an ideal Nitrox mixture that allows you to utilize the maximum permitted PpO₂ of 1.4 bar without exceeding it. This mixture is called the Best Mix.

The formula for this is simple:

Best Mix = Target PpO₂ / max. ambient pressure

Example:

You are planning a dive to a depth of 40 m

• Ambient pressure = 5 bar

• Target PpO₂ = 1.4 bar

• Best Mix = 1.4 bar / 5 bar = 0.28 → Nitrox28

With EAN28, you can therefore exploit the maximum permissible oxygen partial pressure at 40 m without exceeding the critical 1.4 bar limit. This gives you the greatest possible nitrogen reduction – and thus longer no-decompression times with a high level of safety.

Conclusion: Planning is everything – and it's easy

With EAD and Best Mix, you have two simple tools at your disposal to dive not only more safely but also more efficiently with Nitrox.

• EAD shows you at which depth your body – measured in terms of nitrogen – actually experiences the dive.

• Best Mix gives you the ideal mixture for your planned depth.

Both concepts are easy to calculate – or you can simply use an app or dive computer. In any case, the following applies:

Plan your dive – dive your plan!