Cell replacement policy.

silent running

Hello Rich, what do you mean that cells "aren't designed to be used in the way we use them"? Which kind of cells and why not? -Andy

prharris

TBH, it was the other way around. I'm still inexperienced with the Shearwater and the cell failed high underwater, after calibration at 1 bar and pre-dive checks, however that could have been mine or a Shearwater problem during calibration.
The mV output - Shearwater or metered - just confirmed that there was a disparity between 2 cells and the third - as stated earlier (I think) all cells were of the same batch, so a 20% or so difference would seem to indicate a problems with that cell.

I'm now faced with a choice about which policy I go for - if I follow the all cells being similar(ish) option, then the failure of one cell could be an indicator that all 3 are nearing the end of their useful life, however the all cells being individual doctrine would say this is not so.

As the cells went in at the end of last July, they are coming to the end of their year anyway and my life is worth more than the cost of the cells, so perhaps the best cousre of action would be to swap all three, then start the one cell at a time replacement method after 3-4 months.

The checking by as meany methods as possible during preparation and use and letting this override the time-based replacement goes without saying.

r
Paul

Drmike

I dont care if my cells are 1 day old or 100 years old - age isnt relevant. Age is used as an INDIRECT measurement method guess end of cell life. Why use that when you have a real time DIRECT measurement method? All thats relevant to me is are the cells current limited now and are they working now.

If I was in your shoes Id likely replace just the one dodgy cell - maybe 2 but Id keep one older cell in there maybe 2 (as long as it/they working correctly) for as long as it/they are working correctly. Change all 3 at once and you will have 3 'unknowns' in the equation rather than just 1 or 2.

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Wreck diving is a sport
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Mdemon

Totally agree. The old ones especially seem to die very suddenly.

They also seem to absolutely hate heat - so leaving the unit in the car on a hot day is something I try to avoid. I mention this as some of those which have failed on me were fine the night before, but 0.01 on the boat/in the water. The common denominator is that they were in very hot conditions and/or had been in the car.

kieranu

I'm fairly new to this game but this seems like basic common sense so I actually agree with Mike on this one As long as the cells aren't current-limited and hold calibration consistently (I switched one out recently because it consistently started reading about .1 less than the other two on a couple of dive days even though it didn't show to be current limited in other tests) and you check them regularly (like, every dive), if it ain't broke, why fix it. You may end up switching a 12 month old one for a brand spanking new one, only for your new shiny cell to cark it after 3 months.

kieranu

[I mention this as some of those which have failed on me were fine the night before, but 0.01 on the boat/in the water. The common denominator is that they were in very hot conditions and/or had been in the car.[/quote]
Had that once too though I couldn't quite say if it was from being exposed to heat on that occasion or the effect of it having been exposed previously perhaps

prharris

Very good points from all of you - thanks.
Which I why I asked the question really.

I shall do as DrMike suggested and swap 1 cell, then monitor them all against each other.

This was my first incidence of a cell failure, other than in an analyser, I have always swapped them after a year and been lucky enough not to have a failure.

Thanks again
Paul

Drmike

ok Ive given some advice now can someone tell me where to dump this mountain of sh1t ive got? LOL

__________________
Cave diving is a sport
Wreck diving is a sport
Diving in general is a sport

'Rebreather diving' is not a sport
its the delusional obsession with a highly dangerous and often inappropriate piece of equipment

dave t

Paul can you give details of how your cell failed please

RichClark

I don't believe that the O2 cells (sensors) that we use [In my case R-22D] are specifically designed for rebreather use or for sub-aqua use. It's simply convenient at the technology exists and we're able to leverage the available technology to measure PPO2 on our respective hardware.

Because it seems to work, doesn't mean that the design, architecture and manufacturing process are ideal for our circumstances. An example of this has recently been highlighted by Dr Mike - cells falling apart after flood, dissolving the glue used in manufacture.

Conversely, just because they're not specifically designed for our use (rebreather diving) doesn't mean we should use them; it just means that we shouldn't have blind faith and have the necessary protocols in order to test the cells, before, during and after diving.

All IMHO of course

Cheers

Rich

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