This is a little off track, but it falls along a similar line of PO2 spikes.

On my Optima there is a tube in the center of the canister that directs O2 from the solenoid to the far end of the cartridge. Without the tube there is a PO2 spike at every injection but it settles very quickly. With the tube in place there is no spike. My instructor told me that it made no difference in what I was actually inhaling just that it was injecting closer to the cells.

My manual add is in the exhale CL and effects the PO2 readings a little slower than the solenoid without the tube and faster than the solenoid with the tube.

I don't have a VR3 and a 4th cell.