wedivebc:
Yes but we all know that practice is dangerous and foolhardy. The risk of spiking your po2 should you require a fush or even clear your mask you could tox out very easily.
Dangerous? Foolhardy? I'd argue it's a lot safer from an oxtox perspective to dive to 150' on 25-26% Nitrox than it is to dive to the "approved" limit of 132 ft on 32%. If you have to flush the loop, at 150 ft on 25% your spike will be to no more than a PPO2 of 1.38. With 26% it would be 1.44 and both are much lower than the 1.6 spike you would get at 130 ft on 32%. With either 25% or 26% the loop FO2 for the normal portions of the dive will be around 21-22% with a PPO2 of 1.16-1.22 at 150' and it is essentially an "air" dive.
The critical limiting factor at 140-150 ft is instead the flow rate and you will need to moniter the loop FO2 so that you do not pull the FO2 below 21%. This is a potentially serious consideration if you are using an orifice intended for 32%, and perhaps not even an option if your O2 consumption is high in the first place. If your activity level is higher than planned, it may cause you to have to flush the loop and possibly abort the dive.
If you think about it, the 132 MOD for a 32% nitrox SCR rebreather is based on the idea that the loop O2 is going to drop about 4% as the PPO2 is then reduced to 1.4 at 132 ft rather than remaining at 1.6 for the entire deep portion of the dive. Afterall, no one is going to advocate a continuous 1.6 PPO2 for the working portion of the dive.
Going slightly deeper on 32% (loop FO2 of 28%) does elevate the O2 exposure as it would give a PPO2 of 1.47 and a potential spike of 1.68. But 1.5 for brief periods is not problematic and a spike to 1.68, while higher than the recommended 1.6, is not going to produce an immediate O2 hit and in many cases could be avoided by ascending 8-10 ft. before clearning your mask, etc. Is there more care and planning required? Yes. Does more consideration need to be given to O2 exposure? Definitely. Is it dangerous or foolhardy? Not anymore than choosing a rebreather over open circuit in the first place given the larger number of things that can go wrong, the greater knowledge required to operate it, or greater number of factors and operating limitations that need to be considered.
What needs to be stated to any diver considering a rebreather is not some type of "safety terrorism" comment but rather the fact that diving a rebreather demands more than just taking a course and learning the procedures specific to a particular rebreather. It also demands that you thourougly understand what is happening with the unit, that you fully master the planning involved and that you fully and clearly understand the impact a specific situation will have on the system as a whole.
If a diver is not comfortable with math, physics or technical concepts, a rebreather, whether it is SCR or CCR, is not for them.