CCR O2 cell replacement

[Garry Whyke]

Hi guys,

How often do you change your cells on your rebreather?

I was taught every 12 months as part of the annual service.

AP apparently say 12-18 months.

E-mails from the 'Inspiration group discussion e-mail' are indicating that changing all three cells together is a dangerous practice due to some experiences of bad batches and all 3 failing simultaneously. They recommend a 6 mtrs O2 flush and only change them as they become slow to recover???

This will be my first service of the unit, so if anyone out there has any views, they would be much appreciated.

........... Garry

[Nigel Hewitt]

How often do you change your cells on your rebreather?I treat cells suspiciously and they come out after 12 months anyway. In the big scheme of diving they are not that expensive. I've just paid six cells worth for a weekend course and I still need accommodation and food.

They get tested with an O2 flush at 6m, be it a deco stop or just a safety, on most dives and I want to see greater than 1.3 on all of them - say 1.4 and a bit. There are probably much smarter testing systems but I feel that deals with most problems. I don't bother with staggered changing which sounds good but produces unmatched voltages so you aren't going to spot a slow cell as it calibrates so easily. I feel that is a more probable fault than being shipeds a set of identically dud cells that passed the manufacturers tests.

[Vic]

How often do you change your cells on your rebreather?

Every 15 months. That seems to be about right for the amount I use them.

I was taught every 12 months as part of the annual service.

"Annual service". Now there's a thought :-)

AP apparently say 12-18 months.

It's generally a good idea to stick to what AP say. They're rarely wrong.

E-mails from the 'Inspiration group discussion e-mail' are indicating that changing all three cells together is a dangerous practice due to some experiences of bad batches and all 3 failing simultaneously.

Yep. That's never happened to me - indeed, it only seems to happen to people who also want to change minds on other aspects of CCR diving as well.

They recommend a 6 mtrs O2 flush and only change them as they become slow to recover???

The O2 flush is a good test of cell linearity - as your cells start to age, they'll get to 1.1, 1.2, even 1.3 bar, but fail to measure beyond this. This is a hyperoxia risk, so checking that your cells read the higher ppO2 is a safety thang.

This will be my first service of the unit, so if anyone out there has any views, they would be much appreciated.

My advice, as ever, is : make sure you understand the unit well enough to make your own decisions on all this. If you don't - get reading :-)

Vic.

[Mike Rowley]

Hi guys,

How often do you change your cells on your rebreather?

I was taught every 12 months as part of the annual service.

AP apparently say 12-18 months.

E-mails from the 'Inspiration group discussion e-mail' are indicating that changing all three cells together is a dangerous practice due to some experiences of bad batches and all 3 failing simultaneously. They recommend a 6 mtrs O2 flush and only change them as they become slow to recover???

This will be my first service of the unit, so if anyone out there has any views, they would be much appreciated.

........... Garry

Hi Gary

BSAC recommendation is to change sensors at intervals of not more than 18 months, in line with APD advice. Cells may last longer depending on a number of variables but most engineers will tell you that when dealing with life support systems it is not a good policy to wait for indications of failure. Predictive maintenance is a good policy. Trust me, I'm an egineer!:D

The BSAC Inspiration course teaches a linearity check either during initial descent or after reaching target depth on every dive. The logic behind this is
to ensure the sensors are going to be linear to 0.1 - 0.2 bar above set point for the dive. Some people advocate that it is best to do a linearity check at the end of the dive when the sensors are moist and have been used for a period of time. It is personal choice, I prefer to do mine at the beginning so I know they are likely to be good for that dive.

If you change sensors singularly you run the risk of having one or two sensors reading incorrectly. With majority voting logic you have to flush to a known PO2 to find the problem sensor/s. If the known gas produces a PO2 below setpoint you only know which sensor/s is reading linear at that lower PO2. This does not guarantee linear readings at higher PO2 values.

BSAC recommendation is to change all three sensors together. Yes, sensors have been known to faill early or straight out of the packet but it doesn't happen so often and at least you know the sensor history with less chance of confusion when all three are changed similalneously.

Cheers

Mike Rowley
BSAC Chief Technical Examiner

[Garry Whyke]

Hi Mike,

I'll be taking your advice re: cells, cheers.

I'm a little hazy re the linearity check though. Its a phrase that doesn't ring a bell.

One skill we did on the course (which I presume is what you mean) was the dil flush to assess cell integrity at depth, and comparing this against the expected PPO2 during the flush. As mentioned earlier this only checks the low PPO2 reading.

Although it was mentioned on the course, practically we didn't do 02 flushes to check higher PPO2's (at least I can't remember doing them anyway).

Flushes to check cell integrity certainly wasn't stipulated on all our dives.

Hmmmm food for thought ehh!

Thanks for all the comments.

................. Garry

[Gareth]

Garry

When Mike says do a linearity test he means both high PO2 & low.

In truth the low PO2 test you do on the surface, as part of your regular checks, just add air.

The real concern is the high PO2 test. The problem is, that as the cells degrade (age) their response at the higher PO2 degrades.

Here is an over the top simplified 1 cell rebreather explination
So, what actually happens is that although you can calibrate the unit on the surface, the calibration can only reach a PO2 of 1. You calibrate using 100% O2 & assume the mV reading indicates 1PO2 (rather the computer assumes!) The cell has actually degraded & is unable to read higher than a P02 of 1 (possible of 0.8). This means when you dive & switch to high set point (1.3 PO2), the unit injects O2 but never sees the PO2 rise above 1, & just keeps injecting!

Now. On an Inspiration you have the additional cells all cross checking, which normally means that you start to get cell warnings during a dive, rather than a catastrophic failure as shown above. In addition the limiting will occur at a higher PO2 than that stated above i.e 1.3 (1.2). The effect of degrading cells is the same the loop PO2 is higher than is indicated! When this limiting occurs lower than your set point the unit will just keep injecting O2 - remember them saying listen to the solenoid during the course (if it stops its bad news, if fires continuously its bad news).

So simplified.
O2 cells degrade with age (& use). The diving enviroment is not kind to O2 cells (high moisture, variations in temperature, etc). As they degrade their speed of response slows. There ability to read high PO2 decreases & stops!
End result if not changed, O2 toxing diver!
The Inspiration, gives you some warning - cell errors, but a manual check is much better than relying on the computer!
A test at 6m means that 100% O2 in the loop will give a PO2 reading of 1.6 (or close). If you do this at the start of the dive you can have some confidence that the unit will control properly because the cells will read above your working set point.
If you do this at the end of the dive you can have some confidence that when you use the unit again the cells will read above the set point (just not as certain as if you had tested at the start of the dive).

Sorry if this is grandmother & eggs. But it is very important that you understand what is happening to the cells, & why experienced users, (& I am far from one of these,) worry about checking the cells & changing them at the recommended interval.

(Sorry Mike if I stole your thunder)

Gareth
(Rebreather novice, finding his way in the black art of rebreathers, rather like a blind man in a room when they've moved all the furniture around).

[Garry Whyke]

Hi Gareth,

Good explanation, thanks.

.............. Garry

[Mike Rowley]

Hi Mike,

I'll be taking your advice re: cells, cheers.

I'm a little hazy re the linearity check though. Its a phrase that doesn't ring a bell.

One skill we did on the course (which I presume is what you mean) was the dil flush to assess cell integrity at depth, and comparing this against the expected PPO2 during the flush. As mentioned earlier this only checks the low PPO2 reading.

Although it was mentioned on the course, practically we didn't do 02 flushes to check higher PPO2's (at least I can't remember doing them anyway).

Flushes to check cell integrity certainly wasn't stipulated on all our dives.

Hmmmm food for thought ehh!

Thanks for all the comments.

................. Garry

Hi Garry

Sorry, wasn't being rude but had to leave for LIDs on Wednesday afternoon to do some crossover sessions for BSAC Technical Centres so didn't see your response and question until this morning.:o

Gareth has aswered your question very ably so that leave me to say thanks to Gareth.:)

Quite worrying that you didn't cover linearity checks in your training though, maybe you should mention it to your instructor sometime.

Cheers

Mike

[Garry Whyke]

Hi Mike,

It looks as though (from Gareths e-mail) that we did do the excersises in the course but the phrase 'linearity check' didn't stick in the old grey matter.

The course I did only resulted in 1 dil and 1 O2 flush to check high / low PPO2. We did do many dil flushes which were mainly excercise related (E.g. CO2 hit etc..) but not as a cell assessment, and we didn't do them on all dives. What this mean't to me was that the key elements I took away from the course were mainly those skills drilled in on every / most dives.

Please note, as well as BSAC AI I am also TDI Trimix, I did however find this course more challenging than the Trimix. It is not that CCR is technically difficult (compared to Trimix), its just that its ALL new. You really do start to learn again.

From a learning perspective, the skills that have stuck are the ones we did on ALL dives. Those that we only did rarely weren't as deeply engrained.

I am looking forward to seeing the BSAC CCR sylabus and seeing how it compares and how this course is designed to strengthen the learning process.

Cheers

............... Garry

[Drogon]

Hi guys,

How often do you change your cells on your rebreather?

I was taught every 12 months as part of the annual service.

AP apparently say 12-18 months.

E-mails from the 'Inspiration group discussion e-mail' are indicating that changing all three cells together is a dangerous practice due to some experiences of bad batches and all 3 failing simultaneously. They recommend a 6 mtrs O2 flush and only change them as they become slow to recover???

This will be my first service of the unit, so if anyone out there has any views, they would be much appreciated.

........... Garry


Just who are "they" ?

Be very careful when you read things like this online. Go back to the list, and re-read the replies very carefully. You'll find that it's a very very small number of people who are doing the "every 6 month" change thing *not* the entire list of over 1300 people!

Re-read the manual, and go back to the notes you took when doing your course and make your own mind up.

I'm going to change my Inspiration cells every 12 months (maybe a little longer, depending on what I've planned, etc. but certianly within 18 months!), and I've been doing it this way for over 7 years now and not had a problem doing it this way.

Gordon

[Garry Whyke]

Hi Gordon,

Thanks for that, although I didn't say it was the view of the full listing??

Don't forget the opposing argument which that just because this appears to be the view of a small number who are prepared to document their thoughts(although it came across much stronger than just a view), it doesn't necessarily mean that the ones not documenting their views oppose it?? (oops sorry, being argumentative here!).

It has certainly been worthwhile documenting it on this forum though, as I've had some excellent feedback and for some reason I tend to get a more level headed response? Go figure!

Anyway, I'll be sticking to the 12 month 3 cell change routine as per your advice and other comments made.

Cheers

............ Garry

[Nigel Hewitt]

Just who are "they" ?

Be very careful when you read things like this online. Hey I took advice on line.
Well it was on the internet so it's got to be right.
"Don't compact the scrubber" it only puts up the Work-Of-Breathing.

So I didn't.
Result?
A CO2 hit.

I have only been more scared once in my life and that involved being wired to a big box with two handles on it marked Sternum and Axis for the trolley ride from hell across a hospital campus. Like we teach in Sunday School "Always read the Maker's instructions".

[Beanie]

If in doubt RTFM :)