Well, the vast majority of instructors, manufacturers, and actual industry experts all agree a certain set of rules apply.
You follow a different set of rules. One that, you might say, deviates.
If it quacks like a duck, man...
Rebreather - Cost of Ownership
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Well, the vast majority of instructors, manufacturers, and actual industry experts all agree a certain set of rules apply.
You follow a different set of rules. One that, you might say, deviates.
If it quacks like a duck, man...
Now and again you read incident reports which state cells were two or three years old. I don’t like the money I spend on cells but there is no excuse. Same with sorb. If in doubt change the consumables.
On the subject of overall cost for the OP, it sounds like you are just trying to justify it on economic grounds. You don’t need to do that, IF you can afford the unit and fixed overhead then go for it, whether OC is cheaper doesn’t matter, CCR really is better.
On the subject of overall cost for the OP, it sounds like you are just trying to justify it on economic grounds. You don’t need to do that, IF you can afford the unit and fixed overhead then go for it, whether OC is cheaper doesn’t matter, CCR really is better.
Jack Hammer
Contributor
@tbone1004
Tom, I'm a bit surprised to read you are ignoring cell expiration dates. Especially so by saying the Medical industry does it all the time. Very rarely as a whole are medical cells pushed above a PPO2 over 1.0. This is very different from what a rebreather frequently sees in the water, higher PPO2s and moisture. I hope you'll reconsider your opinion on that matter.
Tom, I'm a bit surprised to read you are ignoring cell expiration dates. Especially so by saying the Medical industry does it all the time. Very rarely as a whole are medical cells pushed above a PPO2 over 1.0. This is very different from what a rebreather frequently sees in the water, higher PPO2s and moisture. I hope you'll reconsider your opinion on that matter.
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I never said the medical industry ignores the expiration dates, in fact I said quite the opposite. It is highly likely that the expiration dates are quite conservative because they're going into the medical industry.
I track my cells on every dive and know what they're doing. When they go beyond my accepted limits for linearity or exhibit any semblance of current limiting, they get chucked. I believe that since those are the criteria that we have to use to decide if a cell is bad regardless of whether it is the first dive, or the thousandth dive, then that's the criteria that I use to change cells. I'm willing to bet that the divers that died with 2-3yr old cells weren't actually doing proper checks for linearity or current limiting. Sure the cells may have been old, but the cells are just as capable of exhibiting that behavior on their first dive, and if you decide that 100% of the fault was cell failure, then there is no way that all 3 cells passed their 1.6 check at 20ft to check for linearity and current limiting and magically all croaked mid-dive.
Personally right now being a newer CCR diver I'm planning to follow the guidelines set by my two instructors and change cells every 12 months and not go beyond the manufacturer's recommend expiration date. But honestly, I don't like it and I really see tbone's point of view. Based on my medical knowledge and regularly being the one using and maintaining medical equipment (that is the exact same equipment as the human side of medicine), I see getting rid of cells prior to any abberation as wasteful. There's a very serious necessity to cover your butt in the medical field, and maunfacturers of any consumable equipment really pads expiration dates and manufacturer recommendations heavily to cover themselves. There is study data that they use to come to the decision as to when to expire products, which is often arbitrarily assigned to be earlier than what the data shows in order to pad safety. In very large hospital and university settings, they absolutely follow the guidelines to the t to protect themselves. Having knowledge of what happens inside of outpatient facilities and surgery centers, I can say that not all areas of human medicine follow manufacturer's guidelines as big universities and large public and private hospitals do. I've had many conversations with technical services doctors, sales reps, and technicians for anesthesia and patient monitoring companies, and often when discussing recommendations vs real world usage, their recommendations vary greatly than what they deem as realistically safe.
Having said that, I have alot of employees and clients that rely on me and family that doesn't want me gone, that makes me currently follow the guidelines set by the cell manufacturers and my instructors. It's a little tough to separate the knowledge that I have from outside of the diving world from what's recommended in diving, and not be tempted to keep using that cell that's working perfectly. I recently changed cells, and they totally suck compared to my old ones that were still working perfectly. I also don't want to be "that guy" that gets posted about that died because he used an outdated cell. I have heard some of the cases of deaths associated with divers using out of date cells. To be honest, I have questions that will never get answered such as how often or how did they test their cells for linearity and limiting. Maybe there's things I don't know about those cases, but I question what else was going on there, So I will keeo following guidelines, but I do think Tom is not wrong either. We're still somewhat in the infancy of using a medical device in a diving environment. I don't disagree that we're still learning how to utilize cells for diving, somewhat like Tom eluded. There's a good chance I'll always adhere to the guidelines, but I don't know that in another 10-15 years the guidelines aren't going to change and support the idea that it's not necessary to change cells unless there's an obvious issue or change in the cell when tested. I could be wrong, but I don't think Tom's ideas are foolish. Not the norm, and not supported by those that make the recommendations, but not necessarily 100% wrong
Having said that, I have alot of employees and clients that rely on me and family that doesn't want me gone, that makes me currently follow the guidelines set by the cell manufacturers and my instructors. It's a little tough to separate the knowledge that I have from outside of the diving world from what's recommended in diving, and not be tempted to keep using that cell that's working perfectly. I recently changed cells, and they totally suck compared to my old ones that were still working perfectly. I also don't want to be "that guy" that gets posted about that died because he used an outdated cell. I have heard some of the cases of deaths associated with divers using out of date cells. To be honest, I have questions that will never get answered such as how often or how did they test their cells for linearity and limiting. Maybe there's things I don't know about those cases, but I question what else was going on there, So I will keeo following guidelines, but I do think Tom is not wrong either. We're still somewhat in the infancy of using a medical device in a diving environment. I don't disagree that we're still learning how to utilize cells for diving, somewhat like Tom eluded. There's a good chance I'll always adhere to the guidelines, but I don't know that in another 10-15 years the guidelines aren't going to change and support the idea that it's not necessary to change cells unless there's an obvious issue or change in the cell when tested. I could be wrong, but I don't think Tom's ideas are foolish. Not the norm, and not supported by those that make the recommendations, but not necessarily 100% wrong
broncobowsher
Contributor
I think some people need to read up on cells becoming current limited and what happens with a set of current limited cells.
For those of you that have not done all the proper rebreather training here is the short answer. The cells start dying by loosing the upper end of the output. They won't read over an amount of O2. Your PPO2 display will stop reading the higher levels of O2. You can (or the controller trying to keep you alive) add oxygen trying to reach a setpoint. Since the cell is limited you can go as high as pure O2 and the display/controller will be reading low and you need even more O2. Ready for your Ox hit? It's coming fast.
And waiting until a cell starts behaving poorly before replacing it is IDIOTIC! Wait until one cell is trying to kill you before replacing it in order to delay spending a few dollars. Let me guess, you buy all at the same time and same batch as well? They have all been through the exact same stress living next to each other. So they can all fail together. Not seeing that one cell that is trying to kill you. Please, we already have enough accident reports that were created this way. If you want to be a statistic, drive around without a seatbelt. It's completely safe, until it isn't.
Proper rebreather ownership requires spending money. It is a rather predictable amount. Spend it or sell it. Tech diving is not for the cheap. It is a rather spendy hobby no matter how you do it. It is also a place where being cheap can kill you for no other reason than you were trying to be super cheap about it.
For those of you that have not done all the proper rebreather training here is the short answer. The cells start dying by loosing the upper end of the output. They won't read over an amount of O2. Your PPO2 display will stop reading the higher levels of O2. You can (or the controller trying to keep you alive) add oxygen trying to reach a setpoint. Since the cell is limited you can go as high as pure O2 and the display/controller will be reading low and you need even more O2. Ready for your Ox hit? It's coming fast.
And waiting until a cell starts behaving poorly before replacing it is IDIOTIC! Wait until one cell is trying to kill you before replacing it in order to delay spending a few dollars. Let me guess, you buy all at the same time and same batch as well? They have all been through the exact same stress living next to each other. So they can all fail together. Not seeing that one cell that is trying to kill you. Please, we already have enough accident reports that were created this way. If you want to be a statistic, drive around without a seatbelt. It's completely safe, until it isn't.
Proper rebreather ownership requires spending money. It is a rather predictable amount. Spend it or sell it. Tech diving is not for the cheap. It is a rather spendy hobby no matter how you do it. It is also a place where being cheap can kill you for no other reason than you were trying to be super cheap about it.
lermontov
Contributor
this is worth a read
In the grand scheme of things, cells are cheap.
Imagine this...
You die diving. It gets posted here and everywhere else. The Powers That Be start their research and find you were diving expired cells. The crapstorm of mudslinging that ensues is unparalleled about how your complacency cost your family everything. Even your family starts to believe it. "If Johnny had only paid the $80 for a new cell he'd still be with us today."
Right or wrong, screw that. Just buy new cells. Geesh, if I die diving, it's not going to be blamed on me being cheap.
Imagine this...
You die diving. It gets posted here and everywhere else. The Powers That Be start their research and find you were diving expired cells. The crapstorm of mudslinging that ensues is unparalleled about how your complacency cost your family everything. Even your family starts to believe it. "If Johnny had only paid the $80 for a new cell he'd still be with us today."
Right or wrong, screw that. Just buy new cells. Geesh, if I die diving, it's not going to be blamed on me being cheap.
completely agree with the sentiment, and that’s why I just replace as instructed to. My only point is that Tom gets written off as completely wrong, when in reality there is a good bit of info out there (not necessarily from the diving side) that supports his thoughts. It’s obviously a hot button topic, but there’s evidence that shows we still just don’t know.
Disclaimer: Everything I am about to say is completely wrong in details, but correct in principle. MTBF (mean time between failures) analysis is a super complicated exercise, and analog vs mechanical vs hybrid vs whatever is a brain destroying game. That being said....
A manufacturer tests their cells in the environment they are being planned for (O2 cells generally in the manufacturing environment for gas monitoring of processes, and medical). They calculate a MTBF of, lets say, 18 months. They then add in all the regulatory fudge factors and come up with an "acceptable" number, let's say 12 months.
This number means that, in any given cell, the chances of a failure "without cause" is, say, 1%. However, that curve is a bell (for mechanical stuff) and a logarithmic curve (for electronic stuff), so the ends get interesting fast. This means that the odds of a failure after 12 months may be 1%, after 18 months might be 90%, might be 10%. All depends on how the test parameters have been selected.
Now, most CCR monitoring systems rely on the idea that only one cell is likely to "just die" in a dive without any indications beforehand. Since sensor redundancy is NOT a simple matter of "2 sensors is twice as good as 1", the likelihood of 2 sensors failing during a dive might be 1%, once you go over the dates it will ramp up really fast to some point where it is "mathematically certain". All it will take is an overly-hot car and overly cold water temps or some such, and bang goes the sensor/s.
In terms of current limiting and those failure modes, I am sure that sensors can last long past their dates. It's ALL the other failure modes that become a major factor. I won't dive a sensor past the date on it, luckily my unit only uses two, the folks with rEvo Dreams running 5 sensors must get Xmas cards from DGX.
A manufacturer tests their cells in the environment they are being planned for (O2 cells generally in the manufacturing environment for gas monitoring of processes, and medical). They calculate a MTBF of, lets say, 18 months. They then add in all the regulatory fudge factors and come up with an "acceptable" number, let's say 12 months.
This number means that, in any given cell, the chances of a failure "without cause" is, say, 1%. However, that curve is a bell (for mechanical stuff) and a logarithmic curve (for electronic stuff), so the ends get interesting fast. This means that the odds of a failure after 12 months may be 1%, after 18 months might be 90%, might be 10%. All depends on how the test parameters have been selected.
Now, most CCR monitoring systems rely on the idea that only one cell is likely to "just die" in a dive without any indications beforehand. Since sensor redundancy is NOT a simple matter of "2 sensors is twice as good as 1", the likelihood of 2 sensors failing during a dive might be 1%, once you go over the dates it will ramp up really fast to some point where it is "mathematically certain". All it will take is an overly-hot car and overly cold water temps or some such, and bang goes the sensor/s.
In terms of current limiting and those failure modes, I am sure that sensors can last long past their dates. It's ALL the other failure modes that become a major factor. I won't dive a sensor past the date on it, luckily my unit only uses two, the folks with rEvo Dreams running 5 sensors must get Xmas cards from DGX.
