I understand the basic premise of Nitrox, and I also understand the rationale behind the maths of calculating an Equilvanet Air Depth (EAD) for nitrogen exposure, and using this figure with the tables to calculate NDLs and reptitive dive groups.
My question is this: what I don't understand is why the computation is done on the basis of calculating an EAD rather than Equivalent Air Time (EAT)? The tables work in 10 foot increments, so by using Nitrox on the tables, you invariably "lose" a good proportion of your benefit because the tables always get rounded up to the next deepest 10 feet (in some cases you actually would get no extension of NDL at all by being on Nitrox - in other cases, even though your EAD is about 19 feet shallower, you only get the benefit of 10 feet because of the need to round up). On the other hand, if you reflecting the lower nitrogen exposure by round DOWN your time by the relevant factor, you would get the full mathematical benefit of your Nitrox.
Using EAT instead of EAD also fits with prevailing decompression theory. The basis of that theory (at least according to the PADI Multilevel Diver textbook) is that different tissues absorb nitrogen at different speeds and at different depths. By "pretending" that the diver is actually at a shallower depth they are distorting the model - the tissue will still absorb nitrogen at the rate appropriate to the true depth; it is just the exposure is less, which would more accurately be reflected by reducing the time spent at depth by the relevant factor.
Am I completed mad? Or did the world just take a wrong turn on day 1 with Nitrox and "just keep going" as the Bruce Springsteen song goes?
My question is this: what I don't understand is why the computation is done on the basis of calculating an EAD rather than Equivalent Air Time (EAT)? The tables work in 10 foot increments, so by using Nitrox on the tables, you invariably "lose" a good proportion of your benefit because the tables always get rounded up to the next deepest 10 feet (in some cases you actually would get no extension of NDL at all by being on Nitrox - in other cases, even though your EAD is about 19 feet shallower, you only get the benefit of 10 feet because of the need to round up). On the other hand, if you reflecting the lower nitrogen exposure by round DOWN your time by the relevant factor, you would get the full mathematical benefit of your Nitrox.
Using EAT instead of EAD also fits with prevailing decompression theory. The basis of that theory (at least according to the PADI Multilevel Diver textbook) is that different tissues absorb nitrogen at different speeds and at different depths. By "pretending" that the diver is actually at a shallower depth they are distorting the model - the tissue will still absorb nitrogen at the rate appropriate to the true depth; it is just the exposure is less, which would more accurately be reflected by reducing the time spent at depth by the relevant factor.
Am I completed mad? Or did the world just take a wrong turn on day 1 with Nitrox and "just keep going" as the Bruce Springsteen song goes?