How many sensors should I replace on rEvo after 8 months of not diving?

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stuartv

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The official rEvo factory recommendation is to replace 1 cell every 6 months.

That recommendation is the same whether you are using 3, 4, or 5 cells in your unit. So, if you are using 5 cells, you could potentially have a cell that is almost 2.5 years old in use just before replacement.

But, if you are only using 3 cells, it's still every 6 months, so your oldest cell would be up to 18 months old.

That doesn't really make much sense, to me. If it's okay to have a cell in their for 2.5 years, why can't you do the same thing in a 3-cell unit? Or, if an 18 month old cell in a 3 sensor unit needs to be replaced, then why don't you need to replace the 18 month old cells in a 5 sensor unit?

I run 5 cells, I have a calendar reminder that goes off every 10 weeks to remind me to order 1 new sensor. 1 every 10 weeks means I replace all 5 over the course of (just under) 1 year. I never have one that is in use more than 12 months and am unlikely to have any 2 from the same manufacturing batch.

I could see making that interval as long as every 4 months, so the oldest (of 5 cells) would be almost 20 months. But, I personally would not go any longer than that.

They're $90. If you can't afford $90 for a new sensor every 2 or 3 months, then you probably should forego diving a CCR... before using old, depleted sorb kills you.


So, the direct answer to the OP is, replace somewhere between 2 and 5 of your sensors. Personally, I would probably replace 2 immediately, and then the remaining 3, 1 every 2 months. In 6 months, you'd have all new sensors, with only 2 purchased at the same time. After that, switch to replacing 1 every 10 weeks to 4 months - depending on your personal risk tolerance.

Meanwhile, hold onto the 2 you take out. They would be useable as spares, in case one of the new ones craps out prematurely.
 

Dsix36

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I think the entire CCR industry will recommend to bin any sensor after 1 year from the time it has been opened. Unless someone has stopped covering their butts. Using one after a year is pushing the safety limits and will be entirely a personal decision.. Telling someone to use them longer than manufacturers recommendations is just short of asking to be sued.
 

stuartv

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I think the entire CCR industry will recommend to bin any sensor after 1 year from the time it has been opened. Unless someone has stopped covering their butts. Using one after a year is pushing the safety limits and will be entirely a personal decision.. Telling someone to use them longer than manufacturers recommendations is just short of asking to be sued.

I emailed back and forth with rEvo HQ about this question just 2 or 3 weeks ago.

I was pretty shocked when they confirmed in email that they recommend replacing one every 6 months, even on a unit with 5 cells.
 

Heat Miser

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I emailed back and forth with rEvo HQ about this question just 2 or 3 weeks ago.

I was pretty shocked when they confirmed in email that they recommend replacing one every 6 months, even on a unit with 5 cells.
I wasn't going to say anything earlier but if we are going to keep repeating it shouldn't that be "at least every 6 months" There is an obvious difference revo Policy 3 "at least every 6 months" works out to be at an expected 3.65 cells per year. Every 6 months works out to be expected 2 cells per year.
 

Wibble

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You should replace one cell as it's over 6 months old.

Posted a few days ago...
It is a quite straightforward process:
  • If any cells are failing, replace them immediately.
  • On each build, if the youngest cell is >=6months, then replace the oldest cell or one that’s on the edge of the bounds (mV in air or mV at 100% calibration).
Worked example: all 5 cells are new on 1/2020 and they all continue to work.
7/2020 - cell #1 replaced (cells #2, #3, #4 & #5 are now 6mo old)
1/2021 - cell #2 replaced (cells #3, #4 & #5 are now 12mo old)
7/2021 - cell #3 replaced (cells #4 & #5 are now 18mo old)
1/2022 - cell #4 replaced (cells #5 is now 24mo old)
7/2022 - cell #5 replaced (cell #1 is 24mo old, etc...)
1/2023 - the oldest cell, #1 is replaced, now #2 is 24mo old
... etc.


To repeat, but lets have a cell fail at 9 mo...
7/2020 - cell #1 replaced (cells #2, #3, #4 & #5 are now 6mo old)
9/2020 - cell 5 fails. It's replaced.
3/2021 - cell #2 replaced (cells #3, #4 are now 12mo old)
9/2021 - cell #3 replaced (cell #5 is now 18mo old)
3/2022 - cell #4 replaced (cell #1 is now 21mo old)
9/2022 - the oldest cell, #2 is replaced, now #3 is 21mo old
... etc.

Thus, in theory, it’s possible to have ONE cell that is 24 months old. In reality this is seldom the case. Howeever, the reality is no cell will get to 24mo simply because one of the others would have been replaced.
 

stuartv

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I wasn't going to say anything earlier but if we are going to keep repeating it shouldn't that be "at least every 6 months" There is an obvious difference revo Policy 3 "at least every 6 months" works out to be at an expected 3.65 cells per year. Every 6 months works out to be expected 2 cells per year.

I noticed that in their table. I don't know how the policy translates to 3.65 cells per year.

You should replace one cell as it's over 6 months old.

Posted a few days ago...

Following that exactly would mean that if you didn't dive for 5 years, you'd still only replace 1 cell in the first 6 months after you started diving again. That does not seem like what is intended.

Following the worked example, which shows starting with 5 new ones and, after 12 months, having replaced 2 seems like a better approach than just replacing 1 and going another 6 months.
 

Wibble

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Following that exactly would mean that if you didn't dive for 5 years, you'd still only replace 1 cell in the first 6 months after you started diving again. That does not seem like what is intended.
They’d all be dead by then, so replace them all as duff :cool:

Personally I’d stick to the spirit of the rules, replacing one every six months, so five years, replace the lot.

If it was, say, 13 months AND the cells all calibrated within expected mV then replace 2 cells. Would be prudent to have another one or two in stock.
 

Tassi Devil Diver

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I replace a cell every 6 months as per rEvo cell replacement policy 3, replacing a cell in January and July each year. The three newest cells go on my predator controller (position 1,2 and 3) and the two oldest on my backup PO2 monitor, currently a Dream (position 4 and 5).

I have found the rEvo B cells (manufactured by Vadagraph from what I understand) to be rock solid, to get the rEvo B label the cells are tested for performance by rEvo. I have never had a cell fail in 18 months before it is rotated off the controller, I have never had a cell fail after 24 months (cell position 4). I have had one cell fail after around month 27 (cell position 5) and 2 fail in month 29 (cell position 5), the cells that failed at 29 months I disconnect and dived with 1 cell on the dream until it was time to add the new cell a month later, the cell that failed at 27 months I replaced with a cell that was cycled out earlier that I found to be still functioning well.

I monitor my cells closely check for linearity every time I calibrate on my bench at home, I spike my cells to 1.6+ as I descend on most dives, by the time I get to my target depth I have flushed the loop with dill to check that P02 of dil corresponds with expected P02 before bringing the unit up to set point. I spike my P02 before I leave the bottom and let it decay as I ascend. Then at the 6m deco stop I get my P02 up to as close as 1.6 as possible as a linearity check. I also put all my cells through a cell checker every 6 months (including the new cell) when it comes to putting a new cell in the unit, I also test the next three oldest cells that have been cycled out of the unit, cells that are 3, 3.5 and 4 years, as I like to track the cells until they are completely dead. I have found around 25% of cell that are 3+ to be current limited, around 50% of cells that 3.5+ to be current limited to totally dead, for 4+ cells 100% are current limited or dead.

Until I see evidence that I need to replace cells sooner, I will stick with the rEvo 6 month cell replacement procedure. A fellow rEvo diver has been using the AST cells and has had some fail within 12 months, if these cells were the only ones available, I would certainly revise my cell replacement timing.
 

broncobowsher

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The same number of cells as if you were diving for the past 8 months.
 

Heat Miser

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I wasn't going to say anything earlier but if we are going to keep repeating it shouldn't that be "at least every 6 months" There is an obvious difference revo Policy 3 "at least every 6 months" works out to be at an expected 3.65 cells per year. Every 6 months works out to be expected 2 cells per year.
I noticed that in their table. I don't know how the policy translates to 3.65 cells per year.

They are estimating that you will replace some cells within less than 6 month interval because of current limiting tests (O2 flushes at 6-7m).

That is why I say "at least every 6 months" clearly you wouldn't just replace every 6 months blindly - if something is broken you fix it earlier.

It's the same thing as @Wibble says
  • If any cells are failing, replace them immediately.
  • On each build, if the youngest cell is >=6months, then replace the oldest cell or one that’s on the edge of the bounds (mV in air or mV at 100% calibration).
Wibbles strategy doesn't work out to be 1 cell every 6 months, using the revo data (in the paper) its estimated to 3.65 cells every year.

The problem with calling Wibbles/Revo strategy a once every 6 months replacement strategy (in my mind), is that people get lazy and just put in one new cell every 6 months. And non-rEvo divers say that stupid you have 2.5 year old cells in your rebreather. That's clearly not the policy.
 
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