O2 cells falling apart - bad batch?

dave t

Adding three new cells from the same batch or three new cells from different batches, adding three cells of different ages and different batches at different times. None of this is going to guarantee healthy cells and non failures. One method is potentially no better than the other, The MOST important method is to learn to spot a dodgy cell be it brand new or 12month old. Its down to us, the divers to understand how these cells work and how they fail. Complacency should not set in just because you believe your method of cell rotation is the best!!

just my opinion

Dave

AD_ward9

Correct, but it does reduce considerably the risk of all three failing at the same time, with the same fault.

I agree with you: good cell rotation should not cause complacency, but then the Rebreather design should also do it can to check the cells, for the different failure modes.

Alex

dave t

how much more likely is that than having say cells of differet ages and different batches failing from either premature old age or one (or two) of them is from a bad batch anyway?

AD_ward9

Having two AI PSR-11-39 cells from a set of four, from batches months apart, failing at within 4 hours of each other with the same fault is extremely unlikely. Less than 1 in 10^12 events, not just 10^9 hours. Having two cells fail within 4 hours of each other from the same batch is "possible", in safety terms.

Alex

dave t

Alex
in your opinion what checks should the controller make on the sensors both at start-up and whilst diving?

AD_ward9

Good question. Quick summary, of just my 2c is below:

Startup:
Check for correct cell fitted (test the cell by driving it for a few microseconds)
Check for correct cell source current (ditto)
mV value in expected range

Predive checks
Check if cells in cal, or calibrate using group tracking. I am in the cal in air only camp on this, I am afraid, to establish a clear reference point. High O2 checks are to check the validity of the cal.
Check O2 cell response time for rising and falling by doing O2 check first, dil check second.
Check for O2 not ceiling to >0.95

During first descent of the dive
Run the Rebreather as a pure O2 unit above 6m, and check output, so no ceiling to 1.6.
Then change to PPO2 set point below 4m.

If inconsistent during dive:
Check for cell current output by driving it with a load.
Use methods described in docs we published (no patents, deliberately), to track cell with highest normalised output (post cal), but eliminate any with the wrong current sourcing.

Alex

Sutty

Alex, can you give more details of the bits I've highlighted?

Thanks,
Neil

__________________
Never forget that life is a finite resource.

AD_ward9

The PSR-11-39-DL is designed to allow the electronics to check it by driving it, as well as listening to it. It has a 100 Ohm precision load resistor and no internal compensation.

This allows the circuitry to apply a voltage via a DAC onto the sensor to check what it does when subject to different loads (treat the DAC as a terminating voltage, so varing it varies the current out from the cell). This allows the true source impedance of the cell to be tested. Further information is available in Vol 3 of the FMECA on our website, and in the non-compliance report on our website here then look under the compliance and FMECA sections. This method allows the electronics to check for current limiting faults even with a PPO2 of 0.21, and also, very importantly, to identify leaking cells (this is one of the few failure modes where the output voltage from the cell rises above the correct level).

Cheers,

Alex