Nitrogen exposures are omnipresent in diving, and must be carefully monitored for every dive. These exposures are related to the risk of DCS (Decompression Sickness). In order to properly manage this risk, appropriate exposure limits have been identified for air when used as the diver’s breathing mix. These limits have been detailed in the US Navy dive tables and other similar references, in terms of no-decompression limits over a selected range of depths.
When using nitrox, the diver is exposed to a reduced PN2 (partial pressure of nitrogen), compared to using air for the same dive; though reduced, the diver’s level of nitrogen exposure nevertheless remains a concern. At the same time the diver will now be exposed to an increased PO2 (partial pressure of oxygen); though oxygen exposure is not an issue with air within recreational depths, the elevated exposure levels with nitrox now become an additional concern.
It may be helpful to think of oxygen and nitrogen in terms of drugs. The effect of each drug, as experienced by the diver, will be dependent upon the dose (determined by its partial pressure), combined with the duration of exposure (determined by dive time).
Equivalent Air Depth
When using nitrox, the diver is exposed to a reduced dose (lower partial pressure) of nitrogen, compared to air. In effect, when using nitrox at a specific depth over a specific period of time, it is the equivalent of this diver breathing air at a shallower depth for the same period of time. By accurately calculating this equivalent shallower depth, it is then possible to simply use this shallower depth in place of the actual depth, with any air dive tables, for all standard dive calculations. This concept is known as equivalent air depth (abbreviated as EAD).
To demonstrate the concept of equivalent air depth, consider Table 7 (below), which depicts the partial pressure of nitrogen in air, as well as in the mixes of EAN32 and EAN40, at various depths.
In reviewing Table 7, you will note, as highlighted, that the PN2 of EAN40 at 30 m / 99 ft is approximately equal to the PN2 of air at 20 m / 66 ft. Because the rate of nitrogen absorption is directly dependent upon the partial pressure of nitrogen, a diver breathing EAN40 at 30 m / 99 ft would therefore be expected to on-gas nitrogen at the same rate, during this deeper dive, as he would while breathing air at 20 m / 66 ft. And because he is on-gassing at the rate of the shallower depth, the same no-decompression time limit from that shallower depth would also now apply to the deeper dive; effectively, the diver has significantly extended his no decompression limit at the deeper depth, simply by using EAN40 instead of air at that depth.
As in prior discussions concerning tables based upon increments of 1 bar / 1 atm and 10 m / 33 ft, such measurements sometimes are too broad and unwieldy for practical use in diving; at times, more precise measurements will prove to be beneficial.
The EAD for any nitrox mix, at any depth, can be manually calculated, by first creating a ratio where the fraction of nitrogen in nitrox is divided by the faction of nitrogen in air, then multiplying that ratio by the ambient pressure expressed in m / ft, then converting that figure back to depth. This mathematical procedure is depicted in Metric Formula 7 and Imperial Formula 8 (below).
Metric: Equivalent Air Depth
EAD = [ (FN2 / .79) x (D + 10) ] – 10
Imperial: Equivalent Air Depth
EAD = [ (FN2 / .79) x (D + 33) ] – 33