I think there may be some confusion of terms. I didn't see
Teljkon mention going below MOD.
When you mention
bottom time, you're really short handing what should be called your
max bottom time or
No-Deco Limit (NDL), which depends on how quickly your body is absorbing nitrogen. Your depth effects how quickly you on-gas. So to short hand that, people often say NDL decreases with depth; ie you lose [
max] bottom time with depth.
Nitrox has a different partial pressure of Oxygen and Nitrogen compared with Air.
There are different processes for getting a tank of Nitrox, I'll just simplify it. You pump a tank with some pure O2, and top it off with air. You'll get a higher PPO2 in that tank because you
supplemented it with
pure O2. That decreases the PPN2 in that tank. It's a matter of ratios. More Oxygen molecules VS Nitrogen molecules in your tank than in just plain air.
So with a decreased PPN2, you'll on-gas nitrogen at a slower rate because there's less saturation of nitrogen in your mix. That's the dumb-ed down version.
So you're thinking of it wrong.
The PPO2 in your tank doesn't effect your No-Deco Limit (NDL) directly, what it does is it decreases the PPN2 in your tank. The PPN2 is what effects your NDL.
So am I mistaken in saying that ppO2 or oxygen does not build up in your system over time? Much like over dosing on a drug you either are with in your bodies tolerance for oxygen or your not?
You body does absorb and buildup O2 as well. The difference is O2 is used in your metabolism. N2 is not.
When you become too saturated with O2, ie your PPO2 goes above 1.6 bar, then you get into trouble.