Basic question about NITROX

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?

Round is never a good idea! Use a dive computer, read the manual regarding the Mathematical Method of calculation when Nitrox and Maximize your bottom time, taking all your respect to the maximum depth to be diving with Nitrox.

DSAO

Rhone Man:

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;

Click to expand...

No thats incorrect

Ongassing rate of nitrogen is determined by the partial pressure of the nitrogen,so the nitrogen in nitrox will be absorbed at the same rate that it would be from air at the EAD.

The depth does not matter,only the partial pressure of nitrogen is important. (although for the same gas as depth increases so does the partial pressure)

Thats why it is usually just either to do the math calculations for everything or just buy a dive computer good for air and nitrox. Like the sunto computers.

For ongassing, the relevant factor is the gradient between inspired gas and blood nitrogen tension. If I move rapidly from the surface to, say, 80 fsw, taking perhaps 2 to 3 minutes to get there, the nitrogen tension in my body tissues is going to be very close to what it was on the surface (except for the blood, perhaps, which has had half of a half-life by the time I get down there). If I'm on air, the ppN2 in the inspired gas is approximately 2.7, so I have a gradient of 2.7 to 1.

If, on the other hand, I'm at 100 fsw on 32%, the ppN2 is also 2.7. Ambient pressure at this point doesn't matter; the gradient is what is important. Ambient pressure becomes relevant on ASCENT, because ambient pressure is what is keeping bubbles from forming, or keeping them small. Therefore, on ascent, one wants to maximize the gradient between the blood and the inspired gas, while keeping the ambient pressure as high as possible -- Thus the theory behind rich Nitrox mixes for decompression.

So, since you have the same gradient, given equal amounts of time, you have equal nitrogen absorption.

Ianr33 has the correct explanation, and you can easily verify that by looking at the ppN2 for a few combinations of nitrox depth and equivalent air depth from your table.

If you don't like the rounding up to the next 10' on the air table, solve the EAD equation so that you plug in an even 10' depth on air and get the equivalent nitrox depth. See the attached spreadsheet.

For example, a valid dive plan on EAN32 would be 40 minutes to 86', since the equivalent air depth of EAN32 at 86' is 70' on air.

Charlie Allen