Early (AP) Inspirations where often modified to take a 4th cell which could be plugged into a VR3.
The early Inspirations where only PO2 controllers (no decompression information). The 4th cell allowed a user to run the rebreather manually in the event the controller(s) failed, and it gave you 'real' decompression information based on the loop PO2. The negative was you did need to remember to calibrate the fourth cell, in a separate routine.
This did mean you had two fully independent systems;
- The CCR managing PO2 using 3 cell voting logic.
- The VR3 running the decompression, with the possibility to run the rebreather manually, if the CCR controller failed.
This solution has dropped a little out of favour. Most modern (including the AP range) of rebreathers have built in dive computers giving realtime information for the decompression.
The only potential issue is a full loss of the rebreather electronics. Most attempt to solve this by dual batteries running each controller/dive computer separately. It does still leave you with the issue of cell failure, hence the 3 cell voting logic approach.
Plugging a dive computer into the cells via a fisher connector does give you a fully independent display, calculating decompression requirements and displaying real time PO2. The weakness, as has been pointed out, is that you are reliant on the same cells. I guess there is always the possibility, space permitting, that an additional cell could be fitted as an independent cell. However, I would suggest the cells, even if shared with the primary controller, are a safer solution than one single cell. The only potential issue is if a failure in the main controller places an additional load on the sensor signals. This would distort the reading, giving erroneous information.