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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 27 metres (89 ft) has an EAD of 20 metres (66 ft).
Equivalent narcotic depth (END) (historically also equivalent nitrogen depth) is used in technical diving as a way of estimating the narcotic effect of a breathing gas mixture, such as nitrox, heliox or trimix. The method is used, for a given breathing gas mix and dive depth, to calculate the equivalent depth which would produce about the same ...
For example, based on the PADI nitrox recommendations, the maximum operating depth for EAN45 would be 21 metres (69 ft) and the maximum dive time available at this depth even with EAN36 is nearly 1 hour 15 minutes: a diver with a breathing rate of 20 litres per minute using twin 10-litre, 230-bar (about double 85 cu. ft.) cylinders would have ...
The formula simply divides the absolute partial pressure of oxygen which can be tolerated (expressed in atm or bar) by the fraction of oxygen in the breathing gas, to calculate the absolute pressure at which the mix can be breathed. (for example, 50% nitrox can be breathed at twice the pressure of 100% oxygen, so divide by 0.5, etc.).
Training dives use 46% Nitrox and can exceed six hours at a maximum depth of 40 ffw (12 mfw) for a maximum equivalent air depth (EAD) of 24 fsw (7 msw). NASA guidelines for EADs of 20–50 fsw (6–15 msw) with maximum dive durations of 100–400 minutes allow either air or oxygen to be breathed in the preflight surface intervals.
Nitrox is a mixture of nitrogen and oxygen. Technically this can include air and hypoxic nitrox mixtures, where the gas fraction of oxygen is less than in air (21%), [4]: Ch. 3 but these are not generally used. Nitrox is generally understood as air enriched by additional oxygen, as that is the usual method for producing it.
The difference in PO 2 can be used to calculate PHe, or conversely, a desired trimix product partial pressures can be used to calculate the PO 2 for the nitrox and trimix stages of the blend. Example: Desired product 50% helium, 16% oxygen, remainder nitrogen (34%). The PO 2 after the helium addition must be 0.16 bar if there is negligible ...
The ratio of gases in a particular mix is chosen to give a safe maximum operating depth and comfortable equivalent narcotic depth for the planned dive. Safe limits for mix of gases in trimix are generally accepted to be a maximum partial pressure of oxygen (PO 2 —see Dalton's law ) of 1.0 to 1.6 bar and maximum equivalent narcotic depth of 30 ...