That is a cool feature. Those Divesoft guys are pretty smart.
Just a note on this. I had an interesting experience that I thought others would find helpful (especially if you dive a rEvo configured like mine).
In
most cases,
@tbone1004 is correct that the computer would average the two cells if only two are connected.
I dive a rEvo with three cells connected to my Petrel controller and two connected to a NERD2.
In October, I had one of the cells fail in the NERD2 during a dive. Not a big deal at all.
The thing I noticed is that my TTS was much greater on the NERD than the Petrel so I started to investigate to figure out why. It makes sense that it would be since the one cell was reading low, but it was a bigger difference than intuitively made sense to me and it was growing quicker than made sense to me.
On a Shearwater, in one of the screens, you can see what PO2 the computer is using for tissue calculations and it is usually the average of the cells connected (as
@tbone1004 said).
In this case, it was the using the PO2 of the lowest cell for tissue calculations and NOT the average.
It obviously didn't know which one was "right" or "wrong". I knew because I had the PO2 of the three other cells on the Petrel to use as reference and the likelihood of 4 cells being bad (more on that in a minute) and one correct is very, very low. Especially since I perform checks every time I assemble my unit.
After returning home, I referenced the NERD2 manual and found this for the two cell mode:
'PPO2 display will alternate with the message “VOTING FAILED”. The lower PPO2 value will be used for decompression calculations. The higher PPO2 value will be used for CNS calculations."
That makes total sense -- yet another reason I like diving Shearwater computers.
Sorry for the somewhat off-topic post but I thought folks would find it interesting -- especially if you are using a computer with two cells connected.
Back on topic:
Because I have five cells and I like to always have a spare (or two), I tend to rotate my cells out and use the spare (so it doesn't age out beyond usefulness) and then get a new spare. I don't do it on a set schedule. It usually ends up being 12-18 months but is really based upon how the cells are behaving. If you average that to 15 months, it means one cell is getting replaced about every 3 months for a 5 cell system like I use.
- brett