ComputerJoe:
... you plan a dive and set the o2 feed rate to provide you a 1.4 partial pressure at your normal breathing rate ...
You plan the depth of your dive, and choose a nitrox mix which at that depth will give you the desired pO2.
You then choose the flow rate based on the mix you're using.
On a semi-closed circuit rebreather that rate is greater than the amount of gas you metabolize, so the counterlung(s) slowly get filled until the overpressure relieve valve activates. The excess gas gets dispelled, hence semi-closed circuit.
... your body uses more o2 because the dive becomes more physically challenging, would not your PP o2 drop? I doubt if it would drop from the 1.4 pp o2 planned to LESS than .21 pp o2
How much O2 you metabolize depends on a variety of factors, but basically you're correct. As you exert yourself, you'll use up more O2. How much more varies. However you need to remember that the pO2 depends not just on the amount of O2 in the loop, but on the ambient pressure. Since you're talking about a nitrox mix (fO2), the pO2 will drop with decreased depth.
Say you plan a dive with Nitrox32, the pO2 at the surface is .32, at 10m it'll be .64, at 20m it'll be .96 etc. Now that's the supply gas in the tank, it'll always be a bit less in the loop. If you hit a stong current at 10-15m, your pO2 can drop to dangerous levels.
Every manual and text book I've read warns about the increased risks of shallow depth or surface swimming on rebreathers. There have been plenty of hypoxia accidents in less than 10m of depth, even fatalities.
This I think is locical because lowering the Partial Pressure is what you are supposed to do when you go from seditentary to more active dive modes.
While this is what happens, I don't understand why you're supposed to do it, or would want to.
The more you breath, the more gas goes into and through your body.
As O2 and N2 are the only major two gases (let's hope you're not overbreathing your scrubber and there is no CO2 in the loop), when the amount of O2 drops the amount of N2 rises, lowering your NDLs. Remeber you're calculating fO2 for NDL/deco calculations. The SCR doesn't replenish the loop with O2 like a CCR does, it replenishes with the premixed gas.
Ron Micjan wrote an excellent
article explaining partial pressure. I highly recommend it.
