Cell replacement policy.

prharris

I have had a cell fail high, the mV reading are:

11.4 9.2 and 8.6 ( love my Shearwater ),

and I was wondering about staggering cell replacement.
This (the new set) will be the 3rd set of cells and in the past I have always replaced all 3 in one go.
However, I have seen the argument (here) that it might me an idea to replace cells one or two at a time so that the cells are all different ages, as long as thay are calibrating ok.

As I have the Sheawater, there is a certain amount of buffering, so I son't se the 'dancing digits' that Gordon described as an indicator of cell health, however I think the the nV display on the S/water more than makes up for that.

It's not even an issue of me being tight, as I have the 3 new cells sat here, I just wondered what current thinking on the best procedure was.

TIA
Paul

RichClark

Hi Paul,

I change mine when they start to respond slowly or are current limited. So far I've had a cell fail at 7 months, 11 months and the last one is just starting to respond a little slower (14 months) now, so I'll swap it out soon.

Even though you don't have the 'dancing digits' as an indication, I find the HUD excellent at spotting cells that are about to misbehave.

I love the Mv readings that the Shearwater gives, but I don't think its an accurate way to determining cell life. I've had once cell fail reading high (current limited to 1.4ish) and the other low (current limited to 1.36)

Cheers

Rich

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schford

I would not change all 3 cells at the same time. But do use the same batch if that makes sense. Just change them out one at a time..

Stuart

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Gilles

1) Ideally one has a cell pot and checks them before each dive.

2) Alternatively, one spikes them with O2 @6m at the beginning as a pre-dive check.

3) More practically, one uses a 1.4 dil (or somewhat and safely higher) and checks on bottom using it, and then checks again with O2 at 4m at end of dive. This (bottom checking) is more practical if using larger offboard gas supply.

4) Rudimentry logging of mV's using atmospheric air and O2 can help predict the onset of non linearity, but is quite tedious.

Changing one at a time, putting dodgiest one in position 3, best one in #1.

18 months is the longest cell I've had (was still linear but getting slow).

I'd like to get a good cell pot system, and still shopping around. The following seems promising (don't forget the 'r' on narked ).

PRODUCT

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diverreb

Changing one cell every three or four months is another school of thought...

Keeping the last one or two out as spares. You figure an average cell life of about a year and it costs the same as replacing all three at once if you do use four months and it costs an extra $80.00 or so a year if you go three months, but at any give time after the rotation starts two of the cells will be less than six or eight months old at all times, depending on a three month or four month rotation.

Richie

hoppyinca

1) that would be nice, hope to make one soon...

i have decided to leave my cells installed when not in use as the discussions Ihave heard/ had/ read have lead me to beleive that no matter what your storage method they last "about" a year. that being written, if I do take them out, I will ensure that they are always installed in the same position, makes trend analy of the logs I keep much easier...

my 2 cents...

hoppyinca

Drmike

You can and perople have. I have, others I know have - toxing has occured due to it


all cells have individual behavior - yes
cells generally last about 12 minths - yes

er hang on?
Thats contradictory!!!

Cells become current limited at around the same rate - true
soooooooo cells dont have individual behavior when it comes to current limitation


You could ask my buddy Fred what he thinks - only, to put it bluntly, you cant cos hes dead
(O2 tox brought on through all same current limited cells)

-There is no benefit in changing all 3 cells at once
-There is a potential risk in changing all cells at once (batch issues or current limiting issues that could and HAS lead to death)

Of course if you dont check and change cells regularly your gonna die what ever you do.

But personally this 12 month life thinging is irrelevant to me. I check my cells before and after every dive. I stagger the replacements and I only replace those cells that begin to show current limiting to unacceptable levels (say 1.5). Blindly changing all cells every 12 months and assuming that they will be fine is not as safe as my method.

Cells can and DO fail earlier (or later)

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silent running

Wow Mike, is this the first time anybody has ventured the opinion that Fred Evan's death was most likely due to multiple current limited cells? I don't doubt it and suspected as much when the details of the accident were reported, but I had no personal knowledge of the situation.

It's very clear to me that dating cells is only one part of validating them. I've gotten bad ones out of the box and also been amazed by how long they can last. I always have at least 2 new spares and all of them get checked for MV output a week before the plane leaves. Then it's the 1.6 6M test on site. I guess the only thing I haven't seen yet is a good cell within it's 12 month span failing suddenly, say from 1 dive to another. Anybody have an opinion on how often this might occur?

Drmike

I didnt inspect his gear myself but discussed at length with the guy who did. My understanding/recollection from our discussion was that after the dive one cell had totally failed and the other 2 were current limited. I dont recall if they were same age but suspect so. Maybe Steve knows more/can clarify if my recollection is faulty

__________________
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

silent running

One failed, 2 current limited from same batch... The mere idea that this had anything to do with a clerical/stocking error or a manufacturing flaw makes my f****** blood boil. What the f*** do O2 cell manufacturers think people are doing with them, using them as paper weights?