Once you progress from diving with air to alternate gasses it is important to know what the Equivalent Air Depth is. Basically if we dive with higher levels of oxygen or if we add helium to the mix something has to be removed from the gas. That something is nitrogen. By reducing the levels of nitrogen in the mix we can increase the times that we can spend at depth by reducing the effects of nitrogen narcosis, increasing the no decompression limit and reducing our decompression obligation.

### The calculation

How does one calculate the equivalent air depth for a particular gas mix? First you have to know the gas mix that you are planning to use and the depth that you are going to (the calculation for MOD can be found here). Obviously the mix has to be usable at that depth so you. Once you have the depth and the gas mix you can use the following formula to work out the equivalent air depth.

D is the depth that you are diving to. FN_{2} is the percentage of nitrogen in the gas that is in the mix.

So let’s do an example with the formula so that we can see how it works.

### Example

Diving to 30.5m/100ft using 32% Nitrox (NOAA1).

Metric | |

FN_{2} |
1.00-0.32 = 0.68 |

Depth | 30.5m |

Calculation | |

Imperial | |

FN_{2} |
1.00-0.32 = 0.68 |

Depth | 100ft |

Calculation |

How does the calculation change if you are using Helium in the gas mix? To work out the EAD for Trimix 32/15 for the above depths we would add the percentages of Oxygen and Helium together giving us 47% non-Nitrogen gas in the mix. We then subtract this from 100% and this gives us the percentage of Nitrogen in the mix, 53%. We would then just use the formula above with an FN_{2} of 0.53. If you follow the calculation through you would find that you have an EAD of 16.8m / 56ft. This is a considerable difference and would greatly reduce the nitrogen narcosis exposure a diver would encounter.

*First and foremost let me state that you must receive formal training from one of the many dive organisations before attempting a dive. Without formal training you risk injuring yourself. Please get training before attempting anything you have read or seen in this post. I will not be held responsible for you attempting to use the methods contained in this post. The following post is for illustration purposes only.*

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