Measure scrubber runtime?

[Gareth J]

Well technically you could do a 3 hour dive at 60ft running a 1.3 and bail with a spare air. A 2l would be a ton of gas. Makes me wonder about rigging up a 2l/3l diluent with an h valve for some sort of redundancy for a shallow reef drift with no overhead/entanglement hazards

You have to be very careful. My unit has a 3litre DIL (and a 3litre O2).
200bar x 3litre = 600litre

15lpm x 3bar(i.e. 20m) = 45lpm

600/45 = 13 minutes

HOWEVER, if you have a CO2 issue or are stressed, you will have an elevated breathing rate, say 3x normal.

13/3 = 4 minutes.

In addition, the on board is providing DIL and buoyancy. So it is an ever reducing supply.

I have used off board 3litre cylinders in the past for bailout DIL, and O2. But its another set of cylinders to maintain (although there is some benefit if they are valved for the inspiration because they allow you to take three cylinders of O2 on Exped's). Currently I have reverted to ALI80's. They cover everything, from air DIL dives with OC buddies, to mixed gas dives. (One is out of test - didn't do any mixed gas stuff during Covid).

This is one of the reasons I so seldom dive with only onboard DIL. At 10m, it's not so much of an issue. At 20m the risk is far too high if you find yourself dealing with a CO2 problem.

 

[Gareth J]

I'd be curious to know what agency teaches no off-board bailout (except for SSA).

My instructor held TDI and IANTD tickets, it was the first BSAC pilot course, I could have had a TDI and an IANTD MOD1 ticket as well if I had wished, we covered everything in the IANTD and TDI syllabus and more. Relying on the on board bailout had significant restrictions. From memory, that included no decompression (not difficult on shallow CCR), under 20m, and with a full DIL.
In fact one of the main points that came out of the pilot was they where a little heavy on telling us we where all going to die. In the old BSAC tradition, no lesson was complete unless they had told you you where going to die , multiple times during each presentation. After the first day there where more than a few who were regretting buying a unit. They also rather over did the pilot with independent secondary PO2 displays, i.e VR3's, which gives away how long ago it was! I think it was the only course at the time that required recovering a CCR diver from the bottom, with the tow, AV and recovery onto beach or boat.
We also had an SCR pilot running at the same time - that gives you another indication of how far back.

The card I use now is the BSAC MOD3 ticket.

 

[stuartv]

The problem is, the rMS is "allowing" divers to push their scrubbers way past the manufacturer's recommended duration. Ergo, according to your own logic, the diver would in general have to immediately abort their dive if the rMS crapped out on them. Which might be fine if they have planned their dive with this in mind, but if they had not (I know divers who are ridiculously light on bailout), then a whole game of unknowns would suddenly open up to them...
I would treat the rMS exactly as a pO2 sensor in this respect (except that you cannot flush a rMS and you don't have redundancy).

What you are actually communicating here is that you don't understand how rMS works.

I know a rebreather instructor (not mine, I just happened to be his "buddy" that time) who pulled this prank on me (i.e. he dove with on-board "bailout"...to >130 ft, something we had not planned). Needless to say, that was my last dive with him, and I certainly had no intention to bail him out if he had run into trouble.
15 m wouldn't have changed my attitude: you dive without a bailout cylinder "with" me, you are on your own, as I have no way to predict what your next move is.
If your buddy AND you ever are in trouble, it will be too late to regret not having brought a bailout cylinder.
It always amazes me when someone seems to believe that planning for a good day (anyone remembers TrueDive?) is actually planning...

Crikey, I hope I never end up as your insta-buddy! "He did something I thought was stupid, so I chose to let him drown."

Yes. You have now built in your experience of the working rMS to trust that a fresh scrubber will actually last twice as long as the recommended duration (to oversimplify, I assume a factor 2).
But the original conservative numbers were so that if you had one bad canister (channeling, say) out of 2, you still would have been fine.
Now, if your rMS dies in the middle of a dive which is longer than the original recommended duration before rotation (or worse, for dumping both canisters), you have to assume that no canister will channel. If you are cautious, you should tell yourself: OK, now that I have no way to monitor the health of my scrubbers, I have to bet that they are both healthy otherwise, if one channels, I am already past the safety margin on a single canister.
It is your decision to reason differently.
It is a dangerous one.

Do you assume that your single-scrubber CCR is going to channel? No. You dive on the assumption it's not going to channel - but you watch out for if it does. Why is it different if you have rMS and it fails? FYI (since you don't understand how rMS works), when rMS fails, the result is that it stops telling you how long you have remaining - either on the top scrubber basket, if you're still only using the top one - or on the whole stack, if you've gotten into the second basket. rMS failure does NOT mean that it starts giving you incorrect "scrubber remaining" times. At least, I have never heard of one failing in that way. So, rMS failure is kind of like if you're using wireless air integration and it fails. You know how much you had before it failed. That doesn't mean you now assume that you are suddenly on the verge of running out. For rMS, it's even less critical than an AI failure. With AI failure, if you run out of gas sooner than expected, you are out of gas (a major emergency). With rMS, if you run out of "scrubber" sooner than expected, you have bail out to switch to. It's a contingency you prepare for because it could happen on any CCR, at any time (rMS or not). You could have breakthrough any time, on any unit, for a variety of reasons, including channeling. So, the possibility that you COULD experience breakthrough is not a reason you would AUTOMATICALLY turn your dive. If you're 1 hour into a 2 hour dive and rMS was telling you that you have 3:30 left on your scrubber, but then it dies, most of us would be perfectly comfortable to continue the dive unchanged. That is also based on the diver knowing what exposure his current load of sorb has had and believing, based on that and experience, that the rMS was correct before it died. It's not simply blindly trusting the rMS.

I am shocked that you would do a dive without a buddy briefing, especially to 40m, with an unfamiliar buddy.
I would also suggest a 40m+ dive is significantly different to a 15m dive.
I don't know about the last dive, I wouldn't have done the dive.

All the agencies have the same recommendation for my unit. Dives beyond 20m, recommend an independent bailout.

The first dives on my MOD1 course were all without independent bailout. Bailouts where the first exercise we did on the course - after all - "if in doubt bailout" is the mantra at MOD1. Independent bailout cylinders were introduced on preparation for the 20m dives on the course (2nd or third day). Which was quite amusing, both myself and the other student where mixed gas divers. With the unfamiliar harness, we both faffed a bit with the unclipping and clipping of the stages. The instructor was amused to note we both continued to remove and replace the stages until we could do it with our eyes closed in mid water. She, expected it, but it did amuse her. Mind you, I think we clocked twice the normal in water time for the course. We took ever opportunity, to spend as much time in the water as we could.

Out of interest, how big are the Revo on board cylinders? I'm not familiar with Revo.

Gareth

You can fit various sizes to a rEvo. I have personally used 2 and 3L steels and also a pair of AL19s on my rEvos. Really, any cylinder that is no bigger in diameter than a 2 or 3L steel can be used on any rEvo and cylinders that are no bigger in diameter than an AL19 can be used on a rEvo Mini or Standard. Supposedly, the AL19 is just a little too fat to fit on a rEvo Micro (though I think you probably could make them work, if you wanted to). In other words, there is not really a specific limit on the length of the cylinders you can use. And, of course, if you really wanted to, you could, umm, "redesign" some things and connect off-board cylinders of any size you wanted.

 

[fsardone]

From the report

Conclusion: When operated at even shallow depth, temperature sticks provided timely warning of significant CO2 breakthrough in the scenarios examined. They are much less accurate during simulated exercise at surface pressure.

The rEvo warned conservatively in all five tests (approximately 60 minutes prior). Inspiration warnings immediately preceded breakthrough in six of eight tests, but were marginally late in one test and 13 minutes late in another.

You are quoting out of contest with the results that people get a wrong impression.

Marginally late sounds like marginally dead.

The study conclusions are:

These data represent the first publicly reported demonstration that temperature sticks can reliably warn indirectly of CO2 breakthrough before it occurs during simulation of a common rebreather diving scenario (resting decompression in 19°C temperate water). This was usually also true even during moderate exercise at shallow depths; conditions which, based on our tests at surface pressure, we incorrectly predicted would significantly confound temperature stick accuracy. However, despite this positive result, one cannot draw confident conclusions about temperature stick performance in conditions beyond those tested in this study. The possibility cannot be excluded that factors such as colder or warmer water, greater levels of exercise, greater pressures and different gases may change their accuracy.

 

[uwxplorer]

What you are actually communicating here is that you don't understand how rMS works.

I can certainly agree with this statement, if you add "the way we rMS-believers do"

Crikey, I hope I never end up as your insta-buddy! "He did something I thought was stupid, so I chose to let him drown."

I have learned from my rescue diver training that a panicked diver is not only a danger to himself, but also to his rescuer. I had no way to predict the behavior of this diver in case things turned South, so yes you can bet all bells went off in my hindbrain. Read the story of the diver who witnessed two buddies fighting each other for gas on a bailout event, or those where one of the buddies comes back alone with a story full of holes...
Read again: the guy told me I WAS his buddy, and I had HIS bailout!
But this way out of the original OP's topic.

 

[DS79]

[My point about the Revo is that it's pretty much a binary measurement when the second scrubber begins to be used and warms up. This gives an absolute reading that half of the scrubber has been used (actually slightly more as it's measuring the second scrubber, but round it to half). This means that the measurements can be very accurately gauged. A bonus is that provided the second scrubber's not been used, it can be swapped over with another, so only needs 1.35kg/4lb of lime for a fill.[/QUOTE]

Absolutely.
By the way not only AMV and Temperature affect the duration of your scrubber. It's also depth (actually pressure, actually gas density). I learned, that this is due to how the gas molecules get in contact/ can react depends on their density, so it's not only temperature as a result from depth but also the density itself.

Maybe also interesting to non-rEvo divers: I do cold water dives (10-4° celsius) to around 40m and with rather gentle exertion levels (no current). My AMV is a bit below average (around 10-15l/min) RMS gives me on average around 5-6hrs on a 1.35kg (3lb) scrubber. That's in theory 10-12hrs on both scrubbers together (6lb) if rotated according to manufacturer's recommendation in between. That means I use less than 0.25kg sorb/hr.
In warm waters, ít should be considerably more duration.
Obviously, you shouldn't make out of my example, that you can use your 3kg sorb for 10+ hrs if you don't have a detection system and you don't have redundancy on your particular model (on the rEvo you usually only use your first of the two scubbers and then rotate them, so that there is always an entirely fresh scrubber with 3lb sorb in the loop).

 

[davidvan]

I don't get this comment about the rEvo rMS being "binary".
The rMS has 4 temperature probes per scrubber (ie 8 in total).. it's not just detecting when the second scrubber warms up.. it's detecting at a very fine-grained level and can very quickly detect changes in consumption. It's constantly updating its estimated time remaining based on current usage.

 

[Heat Miser]

I don't get this comment about the rEvo rMS being "binary".

I'm guessing but I think by saying "binary" they are referring to the RST calc being conservative and a not a "predictor" of when the scrubber runs out, but a predictor of when the scrubber in the bottom lung gets to an "absolute reading that half of the scrubber has been used"

I just follow the manual i.e. stop diving before RCT goes to zero (red). so I can cycle.

I've only seen the RCT go to zero (red) once, on a long 2 1/2 hour dive where, mis-estimated scrubber time swimming (finning) back into stronger current to reach car park. Rationalised I was with buddy, both has full bail-out S80's, no overhead environment and was nearly home, with no deco. The RST fell quickly finished 20 mins after the RCT went to zero, threw both scrubber sorb out after dive (didn't cycle).

I've read threads where people have pushed it much further, but why I ask.