To avoid the mathematical calculations, divers often rely upon published EAD tables; a very useful version of EAD tables is incorporated into the TDI-SDI combined nitrox tables, in both Metric and Imperial versions (depicted below).
To determine an EAD with these tables, simply look across the top row to find the nitrox mix (based upon the fraction of oxygen, expressed as a percentage), then proceed down that column under the mix to find the actual or next greater depth (appearing in bold type, in that column), and finally read across that row to the far left to identify the equivalent air depth.
Example (Metric): When a diver is using EAN36 at an actual depth of 15 m, the EAD is 12 m.
Example (Imperial): When a diver is using EAN36 at an actual depth of 50 ft, the EAD is 40 ft.
Using the EAD with Air Dive Tables
In nitrox dive planning, the EAD information (once determined) is simply applied to a standard set of air dive tables; the tables plot the dose of nitrogen experienced by the diver, based upon depth and time. In practice, a diver certainly may use any air dive tables with which he is comfortable. The TDI version of the US Navy dive tables again are used here for demonstration, with the same tables depicting both Metric and Imperial values (depicted below). When using nitrox, the no-decompression limit is calculated as usual on the air dive tables, simply with the EAD used in place of the actual depth.
When using EAN36 at 15 m / 50 ft, the EAD is 12 m / 40 ft, and thus the applicable no decompression limit is now 200 minutes. The ending pressure group, for any dive, is also calculated in the usual manner, again with the EAD used in place of the actual depth.
After completing a dive with EAN36 for 60 minutes at an EAD of 12 m / 40 ft, a diver will surface in pressure group G. Surface interval credit is calculated in the normal fashion (after all, regardless of the mix that a diver may use at depth, he will still be breathing air back on the surface during the surface interval).
Surfacing in pressure group G, followed by a two hour surface interval, the diver will then be in pressure group D. In planning for a repetitive dive, the diver may use the same or a different nitrox mix; in either case, the EAD is again used to determine the residual nitrogen time.
IMPORTANT: In calculating residual nitrogen time, the diver should base these calculations on the nitrox mix that will be used on the repetitive dive.
After completing his surface interval, a diver in pressure group D, planning another dive with EAN40 to an EAD of 9 m / 30 ft, will have a residual nitrogen time of 54 minutes. The residual nitrogen time is then added to the actual time at the EAD, to determine the ending pressure group.
A diver in pressure group D, completing a dive to an EAD of 9 m / 30 ft for 60 minutes, will have a total bottom time of 114 minutes (60 actual plus 54 residual), and will surface in pressure group G.