Interesting Dave - I like the idea but I also like having the 4th cell as a completely independent backup - It's not something to plan a whole dive around but if the whole system goes down for some reason this is the get out of jail card.

Its a bit like the alpinist idea - or to give it an aeronautical flavor - any landing you can walk away from is a good landing - so I fugere if you can get out of the water kicking even a bit bent from and "event" then you are out of the water.

I used to dive an inspo and have had both handsets die inside a wreck once - NOT HAPPY - but the 4th cell was enough to know the PPo2 was breathable flying manually - enough to get out of jail!

I have been thinking about the fischer bulkhead connector as well - but I heard the other day that besides the self imposed psycological problem of drilling a hole in a nearly 'unobtainable if you bugger it up' -centre section - the material from which it is made may tends to flake and chip rather than make a nice hole - that wouldn't be good - Im sure there is a way to do it though.

Anyway I thought that if you can get a fischer bulkhead connector in then it would be nice to mount a small K1 sensor inside the plastic ring at the inhale port. That way you can actually get the sensor out of the inside of the CL to clean it or whatever - and it would be small enough not to bugger the gas flow. I thought you could mount it in a piece of small diameter pvc tubing - maybe this should be split to facilitate removal / insertion.

Now I have those cooper hoses on my rig it is much more difficult to put in a T piece

Pondering........

Steve L