Rick Inman:Minor thing, but at 33' you are still breathing the same percentage of O2 as you are on the surface. But you are breathing twice as much O2 - twice the volume - at 2 ATAs. The amount of O2 per breath increases as you descend, but the percentage of each gas per breath stays the same.
Thank you Rick. As with Tobin's response, I see my mistake, and thank you for pointing it out.
~ Jason