The weak link in the Rebreather technology is the cells. IMHO, the most important thing for any Rebreather diver to understand is how the cells work and their failure points.
You need to check your cells during your dive and you need to check the cell linearity ideally before every dive but at least periodically. I'll not get into the debate over how to do it but on the Meg you can do it by pressurizing the inner chamber with 2 - 3 psig of pure O2 and reading your ppO2 on the handsets.
The down side to this is that you are putting pressure against the membrane face of the sensor and risk rupturing it since they were not designed to take pressure against the face. OTOH, 2 lbs. is not much pressure.....YMMV
As others have stated. 9 - 12 mv in air is normal and 40's to 50's in pure O2 is also normal. I recently had a cell that would calibrate fine and gave the correct mv's in air and in O2 and tested fine for linearity. It would go erratic mid dive and stay that way until it dried out the next day. Then it would test good again. Dive Rite Express replaced it *
even though it tested good*, sent it back to the manufacturer and sure enough, per the manufacturer there was a problem with it. Kudos to Mark for that.
Mikes point is well taken here. Cell issues are the most dangerous aspect of Rebreather diving and the most likley thing to "get" you.
Quote: (Originally Posted by
barney)
........You also need to watch the cells during the dive, and not just on calibration. We recently went through a few cells that calibrated fine, but eventually (within 1 month) went a little squirelly. ..... This cell was only about 6 weeks old.
The bottom line is never-ever-ever-ever blindly trust those fuqing O2 cells!