Where should I start to approach the rebreather world

[Tassi Devil Diver]

I'm supposed to do MOD 2 with Rubens down in Tassie next month, are you guys going to let him in? @Tassi Devil Diver

Ha I think this is the third time he has tried to get in over the past 18 month.

 

[tbone1004]

@Wibble please explain how your logic works? If you follow the Revo recommendation of replace them when they die instead of on some arbitrary time interval *which I fully agree with*, you have 5 cells that are going to fail at some point vs. 2 or 3 from another unit, it is impossible that you are replacing less cells per year than a unit with less cells.
I fully agree with most of what's in those papers you linked in terms of how we check cells and how we determine when they're dead, but I don't believe that having 5 cells makes it inherently better. I also don't think that 3 cell voting logic is a good thing, but I think 5 is excessive and that there are better ways to skin that cat.
I haven't refuted anything about the twin cans, the RMS, or anything else, just the quantity of cells. I will never dive a Revo, it does not work for my diving and it never will, but that doesn't mean it isn't a good unit.

 

[Wibble]

This is a common, errm, discussion around Revo cell management.

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. Hoewever, the reality is no cell will get to 24mo simply because one of the others would have been replaced.


3 cell boxes need all three replaced each year — because there’s no built in redundancy. Some people, perish the thought, replace them from the same batch. Others replace them every 4 months from different batches.

Just to reiterate; having 5 cells means I have the information to decide what to do in the event of a cell failure -- for example you could continue the dive. On the surface and with another dive looming, you may move the dodgy cell to, say position 5 so it's on the Nerd not Petrel controller.

On your three cell boxes, you're basically stuffed and need to get out and replace the cell ASAP; if you've no replacement you can sit out the next dive and wonder why you only have 3 cells.

 

[tbone1004]

@Wibble if you start a dive without all of the cells, then you are starting with a malfunctioning rebreather. My mCCR has 3 cells in it and if one dies, it is removed and I conduct the dive on 2 cells. I use a Divesoft Freedom and I can manually disable a cell in the middle of the dive if it cooks itself, no issue. Carry one spare cell and you are fine. I don't call the dive if a single cell fails on the mCCR. I keep 1 spare, when one cell dies, I put the spare in and get another spare. I don't follow any time schedule and do not pay attention to the dates on my cells. The dates are in the spreadsheet I use to track the cell health, but I don't care if it's 30 days old or 30 months old, if it is still behaving linearly and isn't current limited, then I don't mess with it. I do not like moving cells around either, they get put in a slot and stay there until they die.
If you have Shearwater electronics, you can't do override the voting logic and that is very scary to me and people have died because of it. This alone is a HUGE benefit of the Divesoft electronics platform. If you have a cell fail that is on the solenoid side with a Shearwater controller you really need to call the dive.

 

[broncobowsher]

As for rEvo weight, Micro FT with a pair of 3L steel bottles, no scrubber, no loop. As I carry it onto an airplane as my carry on to go into the overhead bin. 52 pounds. If you need to convert that to some international units, 2.2 kg per pound. I typically add about 4 pounds of lead for a drysuit dive.

Bailout is very flexible ranging from a single AL40 of air for a recreational dive to a couple of AL80s as deco gets added.

 

[rddvet]

@Wibble if you start a dive without all of the cells, then you are starting with a malfunctioning rebreather. My mCCR has 3 cells in it and if one dies, it is removed and I conduct the dive on 2 cells. I use a Divesoft Freedom and I can manually disable a cell in the middle of the dive if it cooks itself, no issue. Carry one spare cell and you are fine. I don't call the dive if a single cell fails on the mCCR. I keep 1 spare, when one cell dies, I put the spare in and get another spare. I don't follow any time schedule and do not pay attention to the dates on my cells. The dates are in the spreadsheet I use to track the cell health, but I don't care if it's 30 days old or 30 months old, if it is still behaving linearly and isn't current limited, then I don't mess with it. I do not like moving cells around either, they get put in a slot and stay there until they die.
If you have Shearwater electronics, you can't do override the voting logic and that is very scary to me and people have died because of it. This alone is a HUGE benefit of the Divesoft electronics platform. If you have a cell fail that is on the solenoid side with a Shearwater controller you really need to call the dive.

I agree with this, though 30 months old is probabaly a stretch for me. But I don't understand this fascination with replacing cells based on dates, especially in today's times when good cells can be hard to find due to shortages. We have tests that can tell you if your cell is healthy or not. Do a linearity and current limiting test or buy a cell checker if you want to be fancy. But just because a perfectly fine cell is X amount of months old doesn't mean it will perform worse than a brand new cell. The cells from Oxycheq are crap. Cells were in short supply and I had to change one cell to the Oxycheq ones. I would have been better sticking with an imperfect cell than a "brand new" Oxycheq.

 

[Degenerate]

Most rebreathers use a single large >3kg+ / >5pound scrubber and have to throw the lot away when you refill it. The Revo uses two 1.3kg/2.8pound scrubbers in series and you normally only ever re-fill a single scrubber, switching the second lower scrubber to the top (first position) and re-fill only the expired scrubber.

It's not like I'll throw out my entire scrubber after one dive unless it's a pretty long/deep/cold one.
Easily do two 50-60m dives with 40-50 min bottom time on a fill of sorb, can you do that on the Revo and only use up one of your two scrubbers?
Because if not, then you are definitely not "saving a fortune" on sorb.

Or to phrase it another way, would you feel comfortable doing the two dives I mentioned above with just one of your scrubbers packed with sorb?
No, you wouldn't because you would most likely have a very bad time somewhere during dive #2.

If it's in one or two scrubbers should not make much of a difference, X kilos of sorb can absorb Y amount of co2, no?

 

[barth]

I check all my divegear at the surface. But that doesn’t mean that it will work from the start till the end of the dive.

I do understand that there are people who are using cells for a long time. But there are also manufacturers who tell users not use cells longer than 12 month and don’t use cells older than 15 month.

There is even an agency who is telling don’t use cells older than 12 month…

The older a cell gets the higher the risk will be for a failure.

A common myth and misinformation.

5 cells are less susceptible to a single or dual cell failure. Most importantly to have two fully independent computer systems that are not cross-connected, one with three cells and the wholly independent backup (nerd or Revo Dreams) with two cells.

I experienced a two cell failure on my MOD1 and it was exceedingly clear to see that two cells were slow to respond. A JJ doesn’t have this as the two monitors (Petrel controller + flashing HUD or Nerd) are interconnected to the same three cells so will both show the same wrong information in case of a double cell failure. There are cell failure modes that could render you 'blind' as the monitors are connected together at the cells; the Revo has two completely independent monitoring systems.

As all three-cell rebreathers are more susceptible to single cell failures, you end up replacing perfectly good cells as a risk-mitigation strategy. Revos allow the perfectly good cells to continue, as per the papers linked here and here.


Whilst we’re on the topic of costs, let’s talk about your single scrubber. How do you know when it’s expired? Aside from runtime, you’ve no way to determine this so you have to be far more conservative unless you "push the scrubber" and get a potential breakthough.

A Revo on the other hand has two scrubbers and a very effective "RMS" (Revo Monitoring System) temperature-sensing monitoring system that determines where the reaction front is and can be used to diagnose scrubber problems (only the AP Inspiration "Temp Stick" is similar, but that is a single scrubber). The RMS determines the amount of scrubber time left, but most importantly, tells you when the second scrubber's being used, such as from channeling.

Most rebreathers use a single large >3kg+ / >5pound scrubber and have to throw the lot away when you refill it. The Revo uses two 1.3kg/2.8pound scrubbers in series and you normally only ever re-fill a single scrubber, switching the second lower scrubber to the top (first position) and re-fill only the expired scrubber.

Therefore a Revo saves you a fortune in scrubber lime compared with all other rebreathers.

A Revo is also far more resilient to scrubber breakthrough, channeling, as there's two independent scrubbers in series. The exhaust gas going through one scrubber, then is mixed before going through the second scrubber. The RMS will warn you when the reaction front reaches the second scrubber -- as that's independent of the first, the system is exceedingly accurate.


Very happy to continue to refute the Revo bashing. Seems common to attack something that's different.

How old we’re the cells which you were using when you had a 2 cell failure ( manufacturing date till date of dive) ?

 

[Heat Miser]

I agree with this, though 30 months old is probabaly a stretch for me. But I don't understand this fascination with replacing cells based on dates, especially in today's times when good cells can be hard to find due to shortages. We have tests that can tell you if your cell is healthy or not. Do a linearity and current limiting test or buy a cell checker if you want to be fancy. But just because a perfectly fine cell is X amount of months old doesn't mean it will perform worse than a brand new cell. The cells from Oxycheq are crap. Cells were in short supply and I had to change one cell to the Oxycheq ones. I would have been better sticking with an imperfect cell than a "brand new" Oxycheq.

The revo system doesn't require you to replace the oldest cell, specifically, assuming nothing has failed a current limiting test. after 6 months you 1) replace the weakest cell i.e. the one then reacts the slowest or 2) the one that seems to be the closest to being current limiting or if neither 1) or 2) can be determined 3) the oldest cell in your system.

You can let good cells stay in your revo. That's the whole point of the system. @tbone1004 30 month old cell can stay provided its stronger than the other 3 newer cells in the rEvo. On average rEvo say it would be expected to work out to 3.65 cells a year on a 5 cell rEvo, so yes that is 0.65 cells per year more than a 3 cell system replaced annually.

However you don't need to carry spare cells.

By the way I have found cells seem to be stronger pre-covid than new ones I'm buying I have 2 (1.7 year old cells) in my 5 cell rEvo.

 

[tbone1004]

However you don't need to carry spare cells.

If you are OK with diving a non-fully functioning unit...