Hello,
It is pretty much what the others have said in various ways.
Distilling it down, what your instructor has displayed is a profound lack of understanding of the difference between inspired fraction of oxygen and inspired pressure of oxygen. From a physiological perspective, the body only cares about the inspired pressure of oxygen. It does not care how you achieve it.
For example, we function well with an inspired pressure of oxygen of 0.21 atm. You can get that breathing air (21% oxygen) at the surface (1 atm ambient pressure), or you could achieve it breathing 1% oxygen at 200m (21 atm ambient pressure). I would not recommend the latter in diving for other reasons, but from an oxygenation perspective you would be fine. Put another way, 1% oxygen breathed at 21 atm pressure is not a hypoxic gas. It would be, however, if you tried to breathe it at the surface!
So yes, as you descend on a functioning constant PO2 rebreather the inspired fraction of oxygen falls, but the inspired pressure of oxygen remains constant.
Simon M
It is pretty much what the others have said in various ways.
Distilling it down, what your instructor has displayed is a profound lack of understanding of the difference between inspired fraction of oxygen and inspired pressure of oxygen. From a physiological perspective, the body only cares about the inspired pressure of oxygen. It does not care how you achieve it.
For example, we function well with an inspired pressure of oxygen of 0.21 atm. You can get that breathing air (21% oxygen) at the surface (1 atm ambient pressure), or you could achieve it breathing 1% oxygen at 200m (21 atm ambient pressure). I would not recommend the latter in diving for other reasons, but from an oxygenation perspective you would be fine. Put another way, 1% oxygen breathed at 21 atm pressure is not a hypoxic gas. It would be, however, if you tried to breathe it at the surface!
So yes, as you descend on a functioning constant PO2 rebreather the inspired fraction of oxygen falls, but the inspired pressure of oxygen remains constant.
Simon M