Where should I start to approach the rebreather world

[Wibble]

I would not replace my scrubber after the first dive, I would absolutely use the same sorb for dive #2.
You are not saving anything on sorb in this scenario..

You are deliberately running your scrubber for over 6 hours? (Your example is two three hour dives).

That’s brave.

Most rational people would dump the scrubber after a three hour dive for peace of mind.

 

[vioch]

Actual rational people know how to estimate real absorbent scrubber capacity using their O2 consumption.

 

[Wibble]

To state the obvious, the downstream canister is not virgin sorb after this dive, and I would dispute a claim that rotating this canister to the top for the next dive would give you another 3h30m, simply because while the rMS may be indicating a depletion front, it is certainly not indicating a transition point to absolutely fresh sorb. How do we know this? The Revo itself shows that the difference between RST and RCT is not two to one, but only an extra fraction of an identical can of sorb. Part of that absence of a doubled RST is conservatism on Revo's part. But part of it is a recognition that there is probably a small amount of CO2 extraction that is going on in the downstream canister. Otherwise you'd only need one.

Every time my Revo has been filled (barring one following the winter+Covid layoff) only one scrubber has been filled, recycling the other. Thus the "slightly used" second scrubber is always recycled.

The manufacturer's recommendations -- for all rebreathers -- is always conservative. The Inspiration is either 2 or 3 hours depending on the scrubber size. Revo is 3h45.

Following two dives this weekend totalling 3h20, the scrubber monitoring system says there's still 2h55 to recycle (meaning swap one scrubber out) or 3h40 to end (meaning replace both). Water temperature was a balmy 16C/61F.

BTW the monitoring system never says more than 3h45 (the manufacturer's limits).

This isn't willy waving; am actually trying to provide supporting evidence.

 

[Nico-ITA]

The discussion seems to me to be moving now towards JJ vs REVO or in any case the pros and cons of the two machines.

Coming back to my initial question "Where should I start to approach the rebreather world?", I understand that the best thing is to talk as much as possible with rebreather users, in particular with those in the area where I live, so as to understand also which are the most popular machines nearby and now I will try to do this.

I take this opportunity to thank everyone for their contribution

 

[Tassi Devil Diver]

I understand that the best thing is to talk as much as possible with rebreather users, in particular with those in the area where I live

That is a good place to start. When I started down the rebreather track there were very few rebreather divers in my region, a meg and the occasional inspo. A mate and I went with the rEvo did our MOD1 together, now 8 years on there are 8 rEvo divers in my region. It is great to be able to troubleshoot with each other when the occasional issue arises swap components in and out etc.

 

[stuartv]

I'll agree and disagree on sorb usage @stuartv, I suppose it all boils down to the dives/profiles.
Yes, when I dump my sorb there is definitely a little bit of unused sorb in there, at least I hope so because I don't plan my dives to push to the absolute limit of what the sorb can handle, after all it is cheap and not worth taking a hit to save a couple cents.
I also think that the majority of divers who use a CCR with a single scrubber design end up dumping good sorb because again, sorb is cheap and it is not worth taking a hit to save a buck or two.

Again, I am sure the Revo is a nice unit but I don't think I'll be seeing one here in the EU anytime soon.

You know rEvo are made in Belgium, right?

The first part that I quoted really proves my point. You are always dumping some unused sorb. Pretty much every time you dump your scrubber, you are throwing away SOME unused sorb.

As @Wibble has alluded, on a rEvo with rMS, you virtually never throw away any unused sorb. Because you pretty much always use the top until it is fully consumed, throw it away, move the bottom to the top, and insert a freshly packed cartridge in the bottom.

You can compare doing 2 dives and then throwing away your sorb and say you've used as much as the rEvo guy, but you're ignoring that the rEvo guy will have 1 basket (half a full load) still in hand and ready to put in the top and use next time. (if I understood one of your examples correctly)

@stuartv traveling so I’ll get caught up later. Point I want to make is that yes, people have died with Shearwaters due to voting logic. No shearwater configuration uses more than 3 cells on any individual computer so having 5 cells has no bearing on the voting logic.

Having a completely independent monitoring system with its own 2 cells absolutely does have bearing on the voting logic. With only 3 cells in the unit, if 2 of them become current-limited at the same time (per the example of a death due to voting logic), then all your electronics will TELL you that you have 1 bad cell (the one that is actually good). The voting logic will then kill you, if you let it.

With 5 cells, when the controller says that I have 1 bad, and it's now operating on the other 2 (which are really the bad ones), the 2 showing on my NERD will show me that I'm starting to go hyperoxic and I will be able to quickly confirm that my voting logic is trying to kill me and stop it.

Because your brain is the final gate in the voting logic. With only 3 cells, you have only one small warning sign to tell you that your voting logic is trying to kill you - i.e. the 1 cell that has been voted out. Hopefully, you didn't just believe your controller (that that one cell is bad) and you did a dil flush (or whatever is appropriate for you at the time) to discover that it's really the other 2 cells that are wrong.

With 5 cells, you get THREE great big flashing red set of numbers (2 of them, in your face) to tell you your voting logic is trying to kill you. You don't have to do anything to know that your unit is about to kill you if you don't fix it.

Or dive a unit that lets you manually override the voting logic by disabling cells a la divesoft. 5 cells doesn’t make a unit any safer than 3 which in this case is not much better than 2. The Liberty gets a pass with 4 because each side has 2 which gives true redundancy but the O2ptima does not because it uses 3 on controller and 3 on readout but they don’t talk to each other.

You describe the Liberty as if it does better than a 5-cell rEvo. The rEvo gives you the same true redundancy, while ALSO giving you a side with 3 cells for the controller to work with. How can you possibly conclude that is not BETTER than having 2 sides with only 2 cells each?

Look at my previous example: 2 cells bad and 1 good one voted out. Monitor has 2 independent cells that immediately tell you when your controller starts to take you hyperoxic. How can you say that is not safer than only having 3 cells?

No, I'll just leave it and do dive #2 on the same sorb.

Still waiting for you to pull off two 50 min BT dives at 60m on half the amount of sorb that I do.

I'm still unclear. 50min at 60m yields a nearly 3 hour dive, for me. Does it not, for you?

You said you wouldn't use your sorb all the way to the last bit. So, forget "bottom time" and let's stick to talking about what actually matters, which is total run time.

Would you do 2 x 3 hour dives to 60m on one load of sorb?

If yes, then you're crazy. If no, then you would use 2 loads of sorb, yes?

On the rEvo, Wibble (or I) would do 1 dive for 3 hours, then re-pack just one cassette to do the 2nd dive. After the 2nd dive, we would have 1 cassette (the bottom one from the 2nd dive) that we would pack away and use in the top on the next dive. So, total sorb actually consumed and tossed after the 2 dives? 1 total load. Meanwhile, you're either "braver than I am", or you've consumed twice as much.

And to me, this is the crux of the problem in this silly debate. This is a very dangerous thought to propagate. What it implies, but doesn't state, is that two of these half-sized sorb canisters could therefore give you 7 hours of diving. Of course, that is very unlikely in anything but the warmest of shallow water dives (low gas density/max efficiency) of low physical effort.
To state the obvious, the downstream canister is not virgin sorb after this dive, and I would dispute a claim that rotating this canister to the top for the next dive would give you another 3h30m, simply because while the rMS may be indicating a depletion front, it is certainly not indicating a transition point to absolutely fresh sorb. How do we know this? The Revo itself shows that the difference between RST and RCT is not two to one, but only an extra fraction of an identical can of sorb. Part of that absence of a doubled RST is conservatism on Revo's part. But part of it is a recognition that there is probably a small amount of CO2 extraction that is going on in the downstream canister. Otherwise you'd only need one.
A single 5" high axial scrubber? Uh, no thank you.

Why is that wrong or dangerous?

rMS has been shown empirically to be pretty darn accurate.

Normal usage for me would be 3.5 hours on the top scrubber basket, then rotate and re-pack 1 basket (because the rMS would show that I have not run out of Cycle Time yet - meaning I'm still in the top basket). Then do another 3.5 hours. That is presuming it's water that is not TOO cold. My personal experience is water that is above 50F would likely be warm enough to do 3.5 hours without getting into the second basket. That also has some presumption that the water may be 50 on the bottom, but would be warmer during deco. 50F from top to bottom definitely might mean getting into the bottom basket during a 3.5 hour dive.

End result, I've done 7 hours of diving and I still have the bottom basket from the last dive which I'll put in the top for the next time I dive, so net usage/consumption is 1 complete load used for 7 hours (2 x 3.5) diving.

Yes, I know that the reaction front is a cone and that, after 3.5 hours, there will likely be some amount of consumption of sorb in the bottom basket. Fortunately, once that basket goes in the top and there's new sorb in the bottom, all that means is that the top may get consumed faster than it would if it was brand new. And if, for some reason, my expectation that I would still get 3.5 hours on the top basket during its second dive proved to be false, well then, my bottom basket of brand new sorb will be MORE than adequate to handle it. Still no concern for my safety.

For the record, I emailed rEvo HQ and asked if it was true that there has never been a report of a rEvo diver with a CO2 hit. This was their response:

"Correct, with good sorb and within the limits, no breakthrough reported.
Attention: there have been cases of auto intoxication. Similar like in OC
But that is actually rebreather independent"

(I take "auto intoxication" to mean that the diver worked so hard they built up CO2 in their tissues and took a hit, despite the scrubber working correctly. I have emailed back to confirm that)


It seems like this is all going around in circles. First, some are saying "the rEvo doesn't save you any sorb". Then, when it is explained how it does save sorb, the critique changes to "sorb is cheap. It's not worth gambling to save a few pennies."

The split scrubber does save sorb. Quite a bit of sorb. With rMS it's not gambling.

My first rEvo was 10 years old. The rMS took a crap shortly after I started diving it. Replacing sorb on the factory schedule, instead of when rMS said it was depleted, and not KNOWING where my reaction front in the scrubber was were definite added (admittedly minor) stressors in my CCR diving. I fixed it and have had working rMS ever since. It is very comforting to KNOW my scrubber is working and has plenty of life left.

If you have never experienced diving a CCR with the confidence that comes from KNOWING that your scrubber is working (no breakthrough, no bypass, and plenty of time left to complete your planned dive), then maybe you just don't have the perspective to really appreciate what the rEvo scrubber design and rMS offer.

You know what they say, don't knock it 'til ya' tried it!

 

[tbone1004]

@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.

 

[rjack321]

Would you do 2 x 3 hour dives to 60m on one load of sorb?

If yes, then you're crazy. If no, then you would use 2 loads of sorb, yes?

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.

 

[Wibble]

...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.

There's the Petrel controller and the Nerd backup. Both are within seconds of each other on a dive (always the Petrel clearing deco first as one's arms are a few inches lower than one's mouth).

Thus you've the Nerd backup for your deco plan.

If diving deeper, there's the spare Perdix set to 90:90 on the other wrist. That's there for bailout as the Nerd's awkward to see off the loop, not impossible though.

The revo has independent monitoring systems but it is not redundant because the Dreams can't do anything for the units function.

Petrel controller and Nerd backup. Separate electronics. Dreams are old-school.

 

[tbone1004]

There's the Petrel controller and the Nerd backup. Both are within seconds of each other on a dive (always the Petrel clearing deco first as one's arms are a few inches lower than one's mouth).

Thus you've the Nerd backup for your deco plan.

If diving deeper, there's the spare Perdix set to 90:90 on the other wrist. That's there for bailout as the Nerd's awkward to see off the loop, not impossible though.


Petrel controller and Nerd backup. Separate electronics. Dreams are old-school.

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