Where should I start to approach the rebreather world

[Brad_Horn]

It SEEMS, from what I read and hear people talk about, having a CO2 hit is one of the biggest risks in CCR diving.

The data would not appear to fully support this.

Based on the rebreather fatality tracking that Deep Life publish for roughly 600 fatalities and considering the issue totally unit agnostic, the biggest known risk to consider when designing rebreathers would appear to be Hypoxia by a rough factor of two over Hypercapnia.

 

[Wibble]

Not when the voting logic fails. Both will think you are running whatever ppO2 it has voted for and you have no ability to correct it. The spare perdix is the one that you have to rely on.

Just FYI, the wrist-mounted Petrel is the controller (in standard configuration). This has it's own separate CAN-bus connection to cells 1, 2 and 3, RMS, solenoid controller, etc. The Nerd has a completely separate connection to cells 4 and 5 using it's own path and cabling (might well be analogue not CAN-bus). There's a spare cable where you could connect cell 6. The two computers aren't connected together.

For a simultanious current-limited failure of cells #1 and #2 (e.g. it's reading low on both), the controller should vote out cell #3 and kill you with hypoxia.

This being a Revo, you've cells #3, #4 and #5 to check cells #1 & #2 against.

However, being a stock Revo, there's no way of just turning off the gas through the solenoid**. So assuming the controller's pumping in oxygen, the only recourse is to:

• Feather the oxygen valve (risking a valve leak on the first stage)
• Turn the oxygen off and connect your rich bailout gas to the off-board connector and run it manually (you've still got three working cells)
• Turn the oxygen off and run it on diluent as an SCR
• If the failed cells are reading lower than setpoint-high, but not below 0.7 (setpoint-low), then change the setpoint to 0.7 and run the unit manually (the preferred option)
• Bail out. You've got enough gas, use it.

Plenty of options really.

The main thing we keep pointing out is you know what's going on by having 5 cells and two computers. The more common failure mode would be one of the computers packs up -- damn, didn't charge the Nerd or change the Petrel battery and it shuts down. You'd then just run it on the second computer to get out of dodge -- minimum of two cells, just like an Inspiration, JJ or any other with a broken cell.

The Revo does not have a single redundant component. Not the scrubber, not the electronics, nothing is redundant.

Disagree. The two computers + cells are not connected. They're in the same box, with the same cell tray, but there's two separate cables out of the tray for the Petrel and Nerd.

Some people connect one of the cells to both computers. Don't like that as you're then connecting the electronics together and could have a full system failure. Although extremely unlikely, it's best to keep them as designed.


The scrubbers are two in series. Benefits done to death in the above posts. The Revo's never claimed to be an in-water changeable dual scrubber redundant system like the (sorry, forgot the name - Boris? made from purest unobtainium).



** One person's customised their unit by re-routing the oxygen hose to add a flow-stop. It works but adds another hose, connections and a flow stop to the list of things that could fail.

 

[tbone1004]

@Wibble you don’t get to disagree about redundancy of electronics, it is not an opinion. If anything in the SOLO side fails, the unit is not functional in its intended configuration because the solenoid will no longer work properly, ergo not redundant. It’s simply not up for debate.
The Revo system with 5 cells is insignificantly better than 3 cells which is marginally better than 2 which is infinitely better than 1, but because the entire system relies on 3 to function, it is less good than the 2x2 on the Liberty.
From a configuration standpoint a standard rebreather is 3x1. A Revo is 3x1 +2x1. A Liberty is 2x2. 2x2 wins from an engineering standpoint and that is also not up for debate because it is a fact. When the Revo first came out they chose To use a system that couldn’t support redundant electronics and they continue to do so, but 5 is truly only marginally better than 3, it’s not significant.

 

[Wibble]

The data would not appear to fully support this.

Based on the rebreather fatality tracking that Deep Life publish for roughly 600 fatalities and considering the issue totally unit agnostic, the biggest known risk to consider when designing rebreathers would appear to be Hypoxia by a rough factor of two over Hypercapnia.

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[Wibble]

@Wibble you don’t get to disagree about redundancy of electronics, it is not an opinion. If anything in the SOLO side fails, the unit is not functional in its intended configuration because the solenoid will no longer work properly, ergo not redundant. It’s simply not up for debate.
The Revo system with 5 cells is insignificantly better than 3 cells which is marginally better than 2 which is infinitely better than 1, but because the entire system relies on 3 to function, it is less good than the 2x2 on the Liberty.
From a configuration standpoint a standard rebreather is 3x1. A Revo is 3x1 +2x1. A Liberty is 2x2. 2x2 wins from an engineering standpoint and that is also not up for debate because it is a fact. When the Revo first came out they chose To use a system that couldn’t support redundant electronics and they continue to do so, but 5 is truly only marginally better than 3, it’s not significant.

The solenoid has its own battery and is also on the CAN-bus.

The unit comes from a manual background, so has an orifice (leaky valve) meaning the O2 levels slowly diminish. Many people (me included) run them as manual units using the controller as a backup, hence it's a "hybrid electronic CCR". If it injects it means I've not been taking notice of my PPO2 and need to be punished with the hiss of shame.

Summary: Petrel controller with its own battery. That's connected to the CAN-bus talking to the cell tray, the two RMS sensors and the solenoid controller. The solenoid has its own separate battery so it doesn't deplete the Petrel (not that it works like that anyway). The Nerd is completely separate from all of that with its own with a rechargable battery and two dedicated cells (and a third cable).

In fact, in extremis, one could have a full Petrel failure (above water) and connect one of the Petrel cells, #3? to the Nerd and run it as a manual rebreather. That is a violation of the principle of two separate monitors though.

In my mind, and we must agree to differ here, 5 cells = great, 4 cells = good, 3 cells = OK. That there's 3, 4 or 5 has no consequence for costs; I really couldn't care less if it's slightly more expensive than a 3 cell box, I only care about more cells meaning better reliability -- not just cells but with two separate computers.

 

[broncobowsher]

If I recall correctly, on the Liberty, with all they claim to be running 2 separate controllers...
I recall there is a single board that connects the 2 separate systems and actually makes them combined. And when that packs up (which I hear does happen) neither controller will work. The rebreather is dead.

Going back about 18 pages to where all this started...
I was shopping for a rebreather. Didn't know of anyone using real ones. Had not found that crowd yet. Only seen a couple in person. The Liberty was new. Read everything they claimed. Wow, this thing sounds like it fixes issues with everything else. Got to DEMA, saw it in person. Holy crap, it doesn't fix everything, it just adds multiple layers of complexity. Not the kind of complexity you really want. It went from near top choice before seeing it in person to not even on the radar.

 

[stuartv]

@stuartv voting logic has nothing to do with your brain on a shearwater. It is controlled solely by the computer with no override other than manual setpoint control Within the range you are allowed, nor does it allow you to have accurate decompression calculations because it is using the wrong ppO2. The revo gives you the ability to see a ppO2 reading from an independent system but you are still required to have an off board decompression plan because the onboard shearwater is now useless. In this regard the Liberty is vastly superior because you can disable the cells which allows the unit to function normally with regards to decompression and solenoid function.

Liberty allows you to disable the cells that your brain votes out as invalid, they go away, that is vastly superior than having to continuously override a system that is actively malfunctioning. If your brain is the final decision maker, then disabling a bad cell is superior. The revo has independent monitoring systems but it is not redundant because the Dreams can't do anything for the units function. That said, if your brain is the final decision maker, why do you need 5 cells on 3 independent systems to tell you what's going on? Your brain knows what is going on, you need to be able to tell the computer what to do. Shearwater does not allow you to do that which requires you to manually change the setpoint as low as it will let you and/or shutoff the feed to the solenoid so it doesn't tox you, AND requires you to have offboard decompression. With the Revo you have the Controller, HUD, Dream1, Dream 2, and a separate computer to track constant ppO2 for decompression to safely conduct a dive. With the Liberty you need one controller, one backup controller, and one HUD to have the same function.

I have mentioned many times having a NERD2 as a monitor. I don't know why you would insist on using an example of a rEvo with Dreams so that you can support your argument. You make your whole argument invalid when you base it on a different configuration than the one under discussion.

A rEvo with a Predator/Petrel controller and a NERD2 monitor does indeed allow me to have accurate deco calculations even in the scenario where 2 cells have become current-limited at the same time and voting logic causes 1 good cell to get voted out.

It is true (as far as I know) that IF 2 cells on the rEvo controller both go out at the same time - and in the same way - so that they are wrong, but the controller decides they are right and votes out the one good one, I would have no control beyond setting the controller to the low SP. So? I do not see how that means that the Liberty is "vastly superior". Mainly because the chances of that happening are so infinitesimal, and the consequences are so benign (when I have a completely independent monitoring system that still gives me real-time deco calcs), that I think it is a complete exaggeration to say that that makes the Liberty "vastly superior". The only real consequence, if that were to happen to a rEvo (with NERD) is that the user would have to run the unit manually for the rest of the dive - and even then, that is only to avoid continuing to dive at a SP of 0.7.

I change my SP to its low setting and fly my unit manually, with accurate real-time deco calcs - using my NERD.

rEvos are available from the factory as a manual unit, in this configuration so you can't even keep saying it's a "malfunctioning unit" or other such nonsense.

With 5 sensors, any 2 can pack off at any time. If they are both on the controller, then, IF they both pack off the same way - i.e. they both become current limited at the same time, to the same amount - then in that one scenario, I would have to change to low SP, fly manually, and follow my NERD for deco calcs. That is pretty dang low risk and low effort if it happens. And, as a matter of fact, I have had 2 pack off on my controller before (when I flooded my inhale lung due to my own stupidity) and I did just that. Finished the dive flying manually, using my NERD for deco calcs.

I also had the solenoid pack off while diving the Jodrey once. I had to fly it manually for the rest of the dive on that one, too. Flying it manually is no big deal. It's a lot easier, with the CMF, than flying a unit manually that is electronic only.

Saying that the Liberty is vastly superior because, in the same situation, you don't have to fly it manually (but you ARE letting it be controlled based on only 1 cell) does not compute, to me.

How much is "one load" and what is the water temp?

Call me crazy but I would do 6 hours on my 6.5lb Meg axial scrubber in warm water (Mexico, Bikini, Truk, etc).

I would not do 3 hour dives in the ocean though. Way too much can change topside in 3hrs. A thunderstorm could be especially hazardous for instance.

"One load" means one scrubber can full. Discussing apples to apples would mean a can that holds approximately the same amount as what the 2 rEvo baskets hold, combined.

I wouldn't call you crazy at all. But, I would observe that in warm water like that, I would also be pretty likely to use the same amount of sorb, except that when you are diving on the "last" part of the sorb in your 6.5# scrubber, I will have swapped out a basket for a fresh one. So, when we're all done, we'll have used the same amount of sorb. But, during those last (say) couple of hours, you'd be diving with whatever margin you have - presumably not much - and I'd be diving with a new basket of sorb in the bottom of my unit, giving me a much greater margin of safety. After the dive, I'd toss my top basket and move the bottom one to the top to use on my next dive. You'd toss your whole can full. So, we burned the same amount. But I had a better margin of safety on the last part of our diving.

As for 3 hour dives in the ocean, all I can say is that I haven't done one. My ocean dives of any length have been wreck dives. None shallow enough to do a long dive with no deco. As you know, once you start racking up deco, the dive gets kind of self-limited. 25 minutes on a 250 wreck gives an hour or so of deco and are limited to about that by available BO. Longer time on a shallower wreck ends up giving a similar total run time. So, my ocean dives have been 107 minutes, at the longest.

 

[stuartv]

Not when the voting logic fails. Both will think you are running whatever ppO2 it has voted for and you have no ability to correct it. The spare perdix is the one that you have to rely on. Even if the nerd is on 2 separate cells, both of those must be correct in order to have accurate decompression. This should have been discussed During your MOD1 course as it is a failure mode that you can’t correct. They likely told you to just bail out, but if you are going to stay on the loop then you must have off board. Your only recourse is to actually bail out and tell the unit that you bailed out, but you can't run decompression off of any of the integrated computers. With the Divesoft computers you just disable the cells you don’t want and you can stay on the loop.

The Revo does not have a single redundant component. Not the scrubber, not the electronics, nothing is redundant. Redundancy in systems is the duplication of a system to allow complete failure of the primary system without losing system function. The Liberty is the only rebreather out there that actually has redundant electronics, everything is truly duplicated. Is it something that needs to be redundant? Different discussion, but it is important to note that while the Revo may have independent monitoring, but in this case 5 cells is absolutely inferior to 2x2 because the 4 cells can be read by either side of the system AND the most important part is that they can be manually disabled by the diver. This is a vast improvement over any other system on the market and is the reason that I have a Divesoft Freedom on my mCCR's

Again, you are simply incorrect. We're talking about a rEvo with a Predator/Petrel controller and a NERD2 monitor. And we are talking about your worst-case scenario that you raised of 2 cells that both become current-limited at the same time - to about the same value.

In that case, you will still have 3 working cells. If 2 of them are on the controller, no problem at all. If both of the bad ones are on the controller, then you fly manually and your NERD2 is your totally redundant deco calculator.

If, somehow, you had 4 cells die and the only good one was on the controller, you could still set it to low SP, change the NERD - IN THE WATER - to internal, fixed SP, and fly the unit manually, using the NERD to calculate deco.

Give me a realistic example where the rEvo 5-cell setup results in any problem worse than "fly the unit manually" and we can explore this topic further. So far, the closest you have come is the (one time ever?) scenario where 2 cells fail at the same time and are both current limited to the same(-ish) values, resulting in the controller voting out the one good cell.

For that, you change to low SP and fly manually by the NERD. If that's the worst you got, I don't see a problem. I would actually PREFER that to a system that flew itself on auto-pilot based on only 1 sensor.

And I would definitely prefer that to a system where the ONLY sign of the worst-case scenario you brought up is that one cell shows as being voted out. As you'd have in a 3-cell system. On my rEvo, if that happened, I would see 3 cell values flashing in red - 2 on my NERD and 1 on the controller - all showing the same hyperoxic value. Immediately cluing me in that my unit is pumping me towards OxTox Land.

@Wibble you don’t get to disagree about redundancy of electronics, it is not an opinion. If anything in the SOLO side fails, the unit is not functional in its intended configuration because the solenoid will no longer work properly, ergo not redundant. It’s simply not up for debate.
The Revo system with 5 cells is insignificantly better than 3 cells which is marginally better than 2 which is infinitely better than 1, but because the entire system relies on 3 to function, it is less good than the 2x2 on the Liberty.
From a configuration standpoint a standard rebreather is 3x1. A Revo is 3x1 +2x1. A Liberty is 2x2. 2x2 wins from an engineering standpoint and that is also not up for debate because it is a fact. When the Revo first came out they chose To use a system that couldn’t support redundant electronics and they continue to do so, but 5 is truly only marginally better than 3, it’s not significant.

You keep making up what is "intended". The rEvo is intended to be run manually. You have the option when you order it to add a solenoid and controller. If the SOLO side fails, then you're no longer running in an optional configuration. You're now running in the original intended configuration.

Besides that, you're really just using semantics to try to make the rEvo sound somehow unacceptable as a unit that requires you to fly it manually in some very unlikely failure scenarios. That just seems silly.

Now that we've established that 5 is better than 3, it's funny that NOW a small difference is "not significant" when some other differences you mentioned earlier that are also marginal, were deemed to make a unit "vastly superior".

 

[stuartv]

If I recall correctly, on the Liberty, with all they claim to be running 2 separate controllers...
I recall there is a single board that connects the 2 separate systems and actually makes them combined. And when that packs up (which I hear does happen) neither controller will work. The rebreather is dead.

Going back about 18 pages to where all this started...
I was shopping for a rebreather. Didn't know of anyone using real ones. Had not found that crowd yet. Only seen a couple in person. The Liberty was new. Read everything they claimed. Wow, this thing sounds like it fixes issues with everything else. Got to DEMA, saw it in person. Holy crap, it doesn't fix everything, it just adds multiple layers of complexity. Not the kind of complexity you really want. It went from near top choice before seeing it in person to not even on the radar.

I don't know the details on that on the Liberty, but it is something I have suspected. I have seen the same thing on "redundant" controllers in computer systems. One place that I worked had a very large array of storage with mirroring that stored fingerprints. All "fully redundant", etc.. Until the day the actual controller went bad and corrupted everything and the IT admin staff had to spend 24 hours rebuilding the entire thing from backups, meanwhile having literally lost 24 hours worth of fingerprint submissions.

That is what I like about the completely independent monitoring that my rEvo with NERD2 monitor and separate cells gives me. My brain is the only link between the Controller and its sensors and the Monitor and its sensors. As long as my brain does not completely fail, either side of the rEvo can completely fail and I can still finish my dive safely.

If what you said about the Liberty is true, then it cannot make that same claim. Meaning that it has one board that can fail and render the whole system unusable?

Is there any other unit that can have one complete side (controller or monitor) fail and still let you finish safely on the loop (obviously, barring something like SCR mode)? Probably there are completely manual units that can. Is there any other unit with a solenoid that can?

The closest I can think of is if a 3-cell unit has only 2 cells fail, you could fly it manually (on low SP) and use another computer, operating in fixed SP mode, to get out. I would not say that is nearly as good as a having a computer that is still getting real-time sensor data and doing valid deco calcs. I would suck if you got rusty on flying manually and then had to depend on a computer running in fixed SP mode, and ended up getting bent because you didn't fly it manually well enough...

 

[rjack321]

"One load" means one scrubber can full. Discussing apples to apples would mean a can that holds approximately the same amount as what the 2 rEvo baskets hold, combined.

I wouldn't call you crazy at all. But, I would observe that in warm water like that, I would also be pretty likely to use the same amount of sorb, except that when you are diving on the "last" part of the sorb in your 6.5# scrubber, I will have swapped out a basket for a fresh one. So, when we're all done, we'll have used the same amount of sorb. But, during those last (say) couple of hours, you'd be diving with whatever margin you have - presumably not much - and I'd be diving with a new basket of sorb in the bottom of my unit, giving me a much greater margin of safety. After the dive, I'd toss my top basket and move the bottom one to the top to use on my next dive. You'd toss your whole can full. So, we burned the same amount. But I had a better margin of safety on the last part of our diving.

As for 3 hour dives in the ocean, all I can say is that I haven't done one. My ocean dives of any length have been wreck dives. None shallow enough to do a long dive with no deco. As you know, once you start racking up deco, the dive gets kind of self-limited. 25 minutes on a 250 wreck gives an hour or so of deco and are limited to about that by available BO. Longer time on a shallower wreck ends up giving a similar total run time. So, my ocean dives have been 107 minutes, at the longest.

So you haven't even done a 2 hour dive and you're arguing about the "best" way to (somehow) do 2x 3hr dives? You sure have a lot to say about dives you're not actually doing. And you claimed I would be crazy for doing 6hrs on 6lbs of sorb already, but now you're admitting its not nuts at all.

For the record, I did 6x 3+ hour dives and 1x 2hr dive last week with @girlwithbigtanks. An entire week's worth of diving longer than your lifetime longest. Neither of us were diving Revo's... Sorb was about $3.50/lb in MX, we probably "wasted" 8lbs worth in the week, $28 of the cost of our $2000 cave diving trip (~%1).