Prebreathing Survey

What is your prebreathing procedure?

  • None

    Votes: 16 18.8%
  • 5 minutes, wearing unit

    Votes: 19 22.4%
  • 5 minutes, before donning unit

    Votes: 16 18.8%
  • Less than 5 minutes, wearing unit

    Votes: 30 35.3%
  • Less than 5 minutes, before donning unit

    Votes: 4 4.7%

  • Total voters
    85

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First, the rMS should in principle detect channeling: by definition, channeling would allow most of the gas to flow unfiltered, stopping or at least reducing the CO2 conversion reaction and therefore resulting in a drop in temperature. I was planning on testing this in my unit at home with a straw inserted in the canister, but never got around to doing it, and now that my rMS is dead, I can't.
I recall (other forums) some chanting the merits of the rMS which had (supposedly) saved their life or their buddy's life, by reporting a drop in RCT (remaining cycle time, i,e. when do you need to dump your top canister). Upon checking their unit, they discovered a flood. So there might be some merit to it as a warning system, in principle.
HOWEVER, I have experienced (and reported) many occurrences of the rMS telling me on one dive that my RCT was dangerously low, while I knew by experience (and according to rEvo non-rMS recommendation) that my scrubber should be fine, ignored that warning, only to see the RCT jump back to normal on the next dive. In other words, I was correct in suspecting the rMS figures to be bogus and ignoring its dire warnings. This were never challenging dives, so I was not taking any risk by ignoring the "warning".
Similarly, I have started a standard 1 hr dive with plenty of RCT remaining (shallower than the previous one, which allowed me to take the RCT prediction as a safe margin of what was left), only to see the RCT dropping precipitously in the middle of the dive. Keep in mind that the rEvo has two scrubbers back to back, so getting a RCT of 0 is fine, since the reaction is supposed to now take place in the second one (and is indicated as a RST, or remaining scrubber time).
Should I have suspected channeling? There was certainly no flooding that I could detect, and as a precautionary measure, I just dumped both scrubbers after the dive, but you get the point: sometimes the rMS would sound alarming when, if the scrubbers were packed right and the unit tight, there was no reason to suspect that there would be any risk of overloading the scrubber, or some other times, the rMS was clearly taking a lot of time to make its algorithm converge, after throwing some initial alarming warnings.
Since what the rMS is doing is a trade secret (but that apparently this is not an issue to get a CE rating), there is no way to know what rEvo is cooking up with the temperature sensors (not mentioning the information on sex and body mass) to come up with these numbers.
I have suggested that a mere reaction front information would be much more informative than this "prediction", like a tank gauge is in many ways more reliable than the estimation of "miles remaining" in a car (since it makes no assumption about driving and traffic patterns).

Anyway, to go back to the original topic, rMS or not, proper packing and unit maintenance, as well as scrupulous pre-dive checklist are probably more important to a precisely timed prebreath.
 
Wow! That sounds really complex. I appreciate the explanation, and I don't mean to sound snarky, but that makes me love my JJ even more! :D

So the failure mode is that the unit is telling you that a dangerous situation exists, but you could ignore that warning because you knew that it was probably an error? And what about channeling? I mean, I guess you would have a CO2 hit before this was an issue, but wouldn't channeling result in the rMS UNDERreporting the stack usage? It seems there is a need to explain away alerts, which to me sounds more dangerous than just not having the alert in the first place.

And all of this is just to save sorb, right? Is there a scenario where you are doing a dive and you thumb it because of an unanticipated drop in RCT? I mean, I guess saving sorb is a worthy goal, and for people who dive a lot more than me, it might make sense. For my diving, I just dump the scrubber after 3 hours of run time. If I was doing long deco, I guess I could get the expedition scrubber, but the stock axial scrubber works fine for me.
 
Just wait for the happy rMS users to chime in for a contrasted (or muddier) perspective...

The rMS is sold as a safety feature, in practice allowing (inviting really) to go beyond the ultra-conservative recommendations in terms of scrubber usage (to be used when you don't have it). In my experience (and I have talked with local divers who have acknowledged the same type of experience), in some cases, the "prediction" doesn't make much sense (see examples above).
In a sense, it reduces the safety margin (untouched scrubber) as it is supposed to precisely quantify it (rather than trying to encompass all scenarios into a single recommendation).
What happens if you are 3 hrs halfway into a cave dive and the rMS suddently tells you there is no time left while everything seems right ortherwise? Do you bailout and put your life in danger because of reduced resources, or do you ask yourself: what was this guy on the internet saying already he had experienced with the rMS?
When a piece of electronics you don't know the internals starts spitting out nonsense occasionally, it seems sensible to ignore the piece of electronics altogether or at the very least take its reading with a grain of salt (or sorb?).
Mind you, I am not advocating ignoring strange O2 readings by a cell, as their failure modes are pretty well known, and checks are straightforward.
In the case of scrubber duration, I have only two things to fall back on:
- recommended usage (conservative, but safe).
- practical experience (usually less conservative, but still safe).
 
Hey, I'm not really arguing against it. More experienced divers than I have designed and used this system. Just trying to learn.

From an outsiders point of view it seems like a lot of complexity that can easily fail (both you and Stuart apparently have dead systems now, right?). And when it does work, you are telling me that you have to know when to ignore the alert. And all of this is really just to save sorb, right?

I pack my scrubber, I change it after 3 hours.
 
I pack my scrubber, I change it after 3 hours.

How big is it? And what's the water temp?

I assume you are never doing 4 hrs dives...
 
How big is it? And what's the water temp?

I assume you are never doing 4 hrs dives...

You assume correctly, but if I was, I would get the radial scrubber!

Stock JJ axial scrubber is 5.5 lbs, typical water temperature around here in the ocean is in the 40's and 50s.
 
You assume correctly, but if I was, I would get the radial scrubber!

Stock JJ axial scrubber is 5.5 lbs, typical water temperature around here in the ocean is in the 40's and 50s.

I would have no problem using that around here (46 to 55F) quite a bit longer than 3hrs. I use my 6lb axial regularly for 4hrs in those temps and have done 4.5 on occasion. I would do 5 if the last hour was just sedentary deco. In FL I could go even longer on 6lbs of sorb but I run out of steam
 
I would have no problem using that around here (46 to 55F) quite a bit longer than 3hrs. I use my 6lb axial regularly for 4hrs in those temps and have done 4.5 on occasion. I would do 5 if the last hour was just sedentary deco. In FL I could go even longer on 6lbs of sorb but I run out of steam

This is not a recommendation, but people quite routinely dive JJ's standard axial scrubber for 4 hours in 3-4C water without issues. It has enough capacity to do that.
There is a difference between doing one long dive with a freshly packed scrubber and planning repetitive dives with the scrubber drying in the garage between the dives.
I have understood that people use the duals scrubbers and rMS to push scrubber capacity also for repetitive diving.
 
I would have no problem using that around here (46 to 55F) quite a bit longer than 3hrs. I use my 6lb axial regularly for 4hrs in those temps and have done 4.5 on occasion. I would do 5 if the last hour was just sedentary deco. In FL I could go even longer on 6lbs of sorb but I run out of steam

Yup, I have been told that by divers that I respect...

It might be fine, for some divers. I just don’t do it. Simply not necessary for me, wouldn’t really change my diving.
 
https://www.shearwater.com/products/peregrine/

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