The equivalent air depth (EAD) is a way of approximating the decompression requirements of breathing gas mixtures that contain nitrogen and oxygen in different proportions to those in air, known as nitrox. The equivalent air depth, for a given nitrox mix and depth, is the depth of a dive when breathing air that would have the same partial pressure of nitrogen. So, for example, a gas mix containing 36% oxygen (EAN36) being used at has an EAD of . Calculations in metres The equivalent air depth can be calculated for depths in metres as follows: EAD = (Depth + 10) × Fraction of N2 / 0.79 − 10 Working the earlier example, for a nitrox mix containing 64% nitrogen (EAN36) being used at 27 metres, the EAD is: EAD = (27 + 10) × 0.64 / 0.79 − 10 EAD = 37 × 0.81 − 10 EAD = 30 − 10 EAD = 20 metres So at 27 metres on this mix, the diver would calculate their decompression requirements as if on air at 20 metres. Calculations in feet The equivalent air depth can be calculated for depths in feet as follows: EAD = (Depth + 33) × Fraction of N2 / 0.79 − 33 Working the earlier example, for a nitrox mix containing 64% nitrogen (EAN36) being used at 90 feet, the EAD is: EAD = (90 + 33) × 0.64 / 0.79 − 33 EAD = 123 × 0.81 − 33 EAD = 100 − 33 EAD = 67 feet So at 90 feet on this mix, the diver would calculate their decompression requirements as if on air at 67 feet. Derivation of the formulas For a given nitrox mixture and a given depth, the equivalent air depth expresses the theoretical depth that would produce the same partial pressure of nitrogen if regular air (79% nitrogen) was used instead: Hence, following the definition of partial pressure: with expressing the fraction of nitrogen and expressing the pressure at the given depth. Solving for then yields a general formula: In this formula, and are absolute pressures. In practice, it is much more convenient to work with the equivalent columns of seawater depth, because the depth can be read off directly from the depth gauge or dive computer. The relationship between pressure and depth is governed by Pascal's law: Using the SI system with pressures expressed in pascal, we have: Expressing