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the 10 fsw (3 msw) depth. This new format makes it eas- ier for divers and their supervisors to plan both single and repetitive dives using the standard NOAA nitrox gas mixes. The new NOAA Nitrox Decompression Tables fall within the NOAA oxygen exposure limits. Normal exposures to 130 fsw (40 msw) can be done with NN32, and to 110 fsw (34 msw) with NN36. The allowed times are sufficiently generous that the oxygen exposure time limits are not likely to be exceeded in normal no-stop scuba diving. That is to say, it is difficult for the average diver using single scuba cylinders to carry enough breathing gas to stay on the bottom long enough to reach the oxygen exposure time limit. If, however, the diver uses larger capacity cylinders, or uses surface-sup- plied equipment, or conducts decompression dives using the nitrox decompression tables, then the oxygen expo- sure time limits can be reached and in some cases exceeded in normal operations. 15.6.2 Equivalent Air Depth Principle The equivalent air depth (EAD) is the depth based on the partial pressure of nitrogen in the gas mixture to be breathed, rather than the actual depth of the dive. For a breathing mixture with less nitrogen than air, the equivalent depth is shallower than if air were being breathed (see Table 15.5). Although a diver is physically at a specific depth, physiologically the body is absorbing nitrogen equivalent to a shallower depth, since it is the partial pressure of the breathing gas that determines gas loading. EAD decompression is based not on the actual depth of the dive, but on the equivalent inert gas expo- sure depth experienced by the diver. Once the EAD has been determined, the diver can use the “equivalent air depth” with any air diving table to find the resulting no- stop and decompression stop dive times, and the repeti- tive criteria. The EAD may be calculated using a formula, or tabulated using a look-up table. Table 15.6 provides equivalent air depths for oxygen percentages between 28–40%. To use the table, locate the oxygen percent for the nitrox mixture to be used. Moving down the column locate the exact or next greater depth of the dive. Move horizontally on the same row to the column at the far left labeled “EAD.” This is the EAD for the dive. An exceptional exposure depth (dark gray) is included for each mix illustrated for emergency purposes only. 15.6.3 Custom Tables There may be situations where a table for a specific mix and depth are needed for a special diving project. In these cases, the diver has two choices. The first option is to select a table using the EAD technique. Here the diver determines the equivalent air depth for the specific depths and mixes, either selected from a table or calculat- ed using the formulas, then selects the desired air diving tables based on the equivalent depths. The second option is to have special tables developed specifically for the project.

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