Hi Lance,
I have been adding O2 on the exhale lung from the beginning. It just made more sense to me to add the additional O2 in the exahale lung and give it a chance to properly mix with the loop prior to reaching the sensors and then my lungs. I do not have the extra sensor attached to my unit, but it seems to me that by rearranging your feed, the read out on your vr3 will be therefore be more accurate, and your controllers be giving you a more accurate reading of what you are actually breathing.
I also dive manually operating the O2 injection using the apecs as a backup to maintain PP, since in do not have the fourth sensor hook up I just dive using the vr3 set at a point of minus .1 of my pre set PP on my apecs to give a slight additional safety margin.
You will find that by switching your O2 add to the other lung will take a few dives to have your muscle memory readjusted but not very long. It will stop your vr3 from going nuts on you everytime you add.
Hey Lance I like the manifold you have shown on previous posts for the dill side. Where was it available from, I presently am using one from M & J but it does not have the four outlets that yours has on top for the BOV, ADV, Wing and offboard supply I like your set up and would be interested in that manifold to eliminate 2 swivels which I have to use on the one I currently have.