That's what I had originally thought, but I'm now struggling to see how having two O2 monitoring computers is beneficial, as if both computers are deriving their information from the same O2 cells, then surely they'll both just say the same thing?
Therefore under what circumstances is having a secondary O2 monitoring computer beneficial?
I think you now have the answer to this question. But, to put it succinctly, the benefit of a secondary O2 monitoring computer is for the case when the controller itself dies. E.g. the battery dies.
That said, I doin't know if that is a benefit on a Poseidon VII or not. Can you fly one of those manually, if the controller dies? I have a feeling not. I just spent a week diving with a buddy that was on one of those and listening to him describe all the different ways that it sucks. But, maybe you CAN fly it manually - in which case the secondary with O2 monitoring would be handy.
Also, earlier, you were talking about monitoring the O2 cells but not the ppO2. Those are really the same thing. If the Petrel EXT is readying the O2 sensors, then it knows (and will display) the ppO2 for each sensor.
The shearwater is intended to have 3 cells connected not 1. You can use one is SCR mode I think
The Shearwaters with Fischer cable will monitor 1, 2, or 3 cells. You do a calibrate on it and it will detect how many sensors you have connected to it and set itself accordingly.
This is actually a source of frustration for me and I have emailed Shearwater an enhancement request. I want my NERD to monitor 3 cells, but only use 1 slot on the Center Row of the display. I want it to display the single ppO2 value that it is using for deco calculations. I presume that it does some kind of averaging of the sensor values to come up with that single ppO2 value. I want to display just that, with some yellow or red color coding to alert me if a sensor value is out of range or has been voted out.
As it is, if it is calibrated to 3 sensors, it forces you to have the entire Center Row of the display devoted to showing the individual values. On my Monitor (versus the Controller), I would like to use a couple of the spots on the Center Row for other data.