rMS RCT question

uwxplorer

uwxplorer

rEvo carrier
This weekend, I had to do an unplanned dive to retrieve our anchor and while I jumped in the water with plenty of margin for that dive (RCT of 1:20 at 71 ft), I apparently ran into the red zone (0:30 at 66 ft) even though I only spend 5 min at the max depth of 100 ft (where the anchor was wrapped and twisted around a rock and where I may have slightly exerted myself).
I only noticed a RCT of 0:40 when I was on my (slow) way up, and it slowly increased back to a more comfortable value until I reached the surface. I discovered the 0:30 back on the boat (which took sometimes since the boat has drifted when I surfaced under my SMB and took ages before it finally noticed where I was, 1/4 mile away, but that is another story).

My question is not about the behavior of the rMS, as I think that this is in line with expectation, but rather what I should do with my bottom scrubber. I think I remember that there was a discussion somewhere, sometime ago (which I can't find) where it was discussed by PR what to do (or not) when the recommended min RCT of 0:45 was reached.
Obviously, I can just move that scrubber in the top position and put a new one in the bottom position and see what happens during the dive. However, I am not sure whether the rMS will actually detect that the current bottom scrubber may have already part of its capacity used up and therefore deliver a reliable prediction on the remaining scrubber time.

Knowledgeable comments would be much appreciated.

PS: I could have asked rEvo the question directly, but thought that info might of general relevance, as I can't imagine that nobody else will or has ever run into this situation.
 
However, I am not sure whether the rMS will actually detect that the current bottom scrubber may have already part of its capacity used up and therefore deliver a reliable prediction on the remaining scrubber time.
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The system will detect the loss of temperature when the top sorb will (or will not in that case) react. So I guess the RCT/RST will fall, it will give you less dive time than predicted than at the beginning of the dive.
 
silentbob said:
That should answer your question. In short the RMS will have tracked any usage of the second scrubber, so rotate it to the number 1 position and continue as normal.
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I don't think that's correct. The rMS does not keep track of what happened to a canister, it just updates an estimate during the dive, AFAIK.
However, it might be indeed a good idea to do what you are suggesting. This would need two things:

1) a way to identify which canister is used in what position. Since the probe communicates with the rMS chip, this might be already possible without user input.

2) a way for the user to tell the rMS software that nothing unbeknownst to the rMS has happened to the canister (such as using the canister further with rMS tracking turned off, for instance).

With these features, the rMS tracking system could reduce the predicted RCT of a top canister which might have been slightly used before while in the bottom position.

That might add too much complexity to only avoid a "holy cow, my RCT just dropped from 2:30 to 0:45!" moment.

I'll note that the addendum to the manual (about short SI features of the rMS) promised by PR has not made it to the website yet...
 
uwxplorer said:
I'll note that the addendum to the manual (about short SI features of the rMS) promised by PR has not made it to the website yet...
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my error: the manual was already uploaded in august last year, but the link to the revised manual was.. to the old manual

fixed now

and for your question.. you never came into RST, so you just cycle your scrubbers now
 
Okay, so help me out. Because I'm a bit confused now.

Last week I did 4:30 on my rEvo and showed 1:30 left on RCT. But only showed I think :30 on RST. So, when I finished the dive, I dumped both scrubbers. I'm using indicating sorb and the top scrubber was about 80% dark blue (meaning it was mostly used up). However, the bottom scrubber was bleach white, not a single spec of blue. Have I missed something?

Admittedly, there's probably a lot I don't understand about the RMS. I've only just started diving it. (2 dives so far)
 
Do you mean 30 left in RCT (remaining cycle time) and 1.30 left in RST (remaining scrubber time)? I have had this too. Your RCT time allows you to use a full top scrubber so 4-6hrs before cycle in my experience. Once you go past the RCT time and into the RST time is always much shorter. I think this is because once you go into RST you have already started using a small portion of the second scrubber eg the top temp probe which is down inside the second scrubber has detected the reaction, and by this time the top part of the scrubber is already used up, and the last temp probe is also inside the scrubber and not at the very end of the bottom scrubber so once you pass the last probe the RST would be 0. I have occasionally gone into RST with less than 1 hour remaining showing, then cycle that bottom scrubber to the top and new scrubber in the bottom and next dive had 3-4 hrs RCT calculated by the RMS on the same scrubber which showed less than an hour on RST last time. Always have a new scrubber at the bottom backing your top scrubber up.

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Absolutely. RCT is remaining cycle time. The bottom scrubber is hardly used at RCT 0.05. RST indicates total scrubber approximate remaining time. Of course recommended to start fresh if not sure of sorb for any reason or deeper dives.

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uwxplorer said:
Am I reading: even if your RCT was 0:05, you would just discard the top scrubber, swap the bottom one, put a new one at the bottom, and off you go?
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That's what I would do, but I wanted to see if any of the sorb was Dark Blue/Purple (Indicating usage) so I dumped it just for curiosity's sake.
 
paulraymaekers said:
that does not seem right... you know you have to look at the log screen of the previous dives hé, not what is displayed on the other screens ?
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Why does the SW still displays any rMS data on the main screen during SI when this has no practical meaning? As you have noticed time and again, this is confusing for newcomers and useless for the rest of users. Once a user switches to setpoint 0.19, don't display any rMS info, period. Only when the user switches back to setpoint 0.7 before prebreathing the unit does it make sense to report any kind of prediction (and even then, it is in general misleading for quite some time after short SI, as we have discussed elsewhere, which is what new users are in general doing if they have no prior CCR experience).
 
uwxplorer said:
Why does the SW still displays any rMS data on the main screen during SI when this has no practical meaning? As you have noticed time and again, this is confusing for newcomers and useless for the rest of users. Once a user switches to setpoint 0.19, don't display any rMS info, period. Only when the user switches back to setpoint 0.7 before prebreathing the unit does it make sense to report any kind of prediction (and even then, it is in general misleading for quite some time after short SI, as we have discussed elsewhere, which is what new users are in general doing if they have no prior CCR experience).
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I think as long as you understand warm up times and the planning screen (which you really need to if you're using the RMS for planning) it's a non-issue. I personally like to see the rct rising during the beginning parts of my dive. It gives me a warm fuzzy feeling that things are working right. I wouldn't want it to go from zero to fully warmed up remaining time as it may never reach that due to some potential issue (for example wet/channeling scrubber).
 
Tmccar1 said:
I think as long as you understand warm up times and the planning screen (which you really need to if you're using the RMS for planning) it's a non-issue. I personally like to see the rct rising during the beginning parts of my dive. It gives me a warm fuzzy feeling that things are working right. I wouldn't want it to go from zero to fully warmed up remaining time as it may never reach that due to some potential issue (for example wet/channeling scrubber).
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That's not what I am recommending. Of course, being able to monitor the rMS evolution during the dive is important. I actually wished it would not take that many button presses to check the temperature profile. And BTW, since SW has introduced a bar graph for tissue loading, maybe this idea could be extended to the temperature profile display?
I am just suggesting to remove the rMS "prediction" during surface interval. Basically, as soon as you switched to setpoint 0.19 (not when the SW decides you have ended the dive, as there are cases of last minute underwater swim to the boat where you might want to monitor your rMS in the last few yards), the display should go back to ?? or some alternate display (like "cooling down" or something similar).
My experience as a recreational rebreather trainee is that during short SI (~1 hr) typical on dive boats, the temp probes do not have time to cool down. Therefore when you prep your unit for the next dive, prebreathing generally results in the following sequence:

1) the main screen RCT is usually much higher than the min RCT (hidden in the Scrubber View screen). This is as it should, since it has been last estimated at the surface (if not on the boat) but is not what the user is interested in.
2) it first does not change for a couple of minutes and then starts dropping. In fact, I have once experienced it going down to zero before it started climbing back up.
3) the above step depends critically on your prebreath duration:
a) If you follow the initial manual instructions, you would start prebreathing for a couple of minutes or less, and seeing that you have more than 45 min+ of RCT, you would consider it done (the instructions have been modified to reflect this).
b) If you are a paranoid newbie, you will want to prebreath for 5 min and that's when you will see the drop in RCT (possibly down to zero). I would suggest this is the best approach as you may want to check that your sorb bed was not compromised by whatever shocks the unit may have experienced during the SI (for instance in rough seas).
c) You may well have to prebreath your unit more than 5 min for the RCT to climb back up above 45 min.

This is all OK in principle, but this behavior of the display is puzzling for a novice to say the least, not withstanding that it provides useless information (since we don't know how the RCT is calculated and this evolution is a transient during a regime that has no relevance for diving).
What I very humbly suggest is that the rMS display keeps showing "??" during the post-SI interval prebreath until the RCT decrease phase is finished (which is easily detected by the software) and the RCT increases back.


PS: the new rMS manual still speaks of a "Surface Mode" on page 12 and says that there should be a "? Warm up" Display. That is true for the first dive of the day, not a short SI. I'd suggest modifying this sentence to reflect the difference.
 
uwxplorer said:
This is all OK in principle, but this behavior of the display is puzzling for a novice to say the least....
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Xavier, I do understand that the rMS is not perfect 'perfect', probably nothing can reach absolute perfection, but I think the consensus amongst users is that it is already 'amazingly' good..

However, do understand that changes that seem so 'easily' in your understanding, are maybe not always that easy to implement, and need a lot of study before: in what way a simple change can influence other parts in a program etc.

It's not that we don't listen, but more the consideration of putting our energy, time and money in things that maybe matter more. I assume you know what the cost is for programming and checking etc etc..
 
Fair enough.
I'd be curious to know how many rMS users (after their first dive), prebreath their units until the main screen RCT drops down to a minimum before it climbs up again.
Maybe I should start a poll?
:-)
 
I don't recall ever seeing this happen. But I always turn off the shear water during surface interval. My process for checking rms scrubber time. Before dive day check rms log from previous dives for rct. Check dive times I also write on tape on side of scrubber. Now I can be pretty confident I have enough time for the 2 dives before cycling. Pre breathe before 1st dive, watch to see 45 mins on front screen. Go dive. Occasionally monitor rct on front screen during dive. End of dive surface, exit water and check display on front screen is still showing enough time for next dive. Turn off shear water. If it's off you won't see any weird surface rms predictions, probably why I have not seen it. Surface interval approx 1hr-2hr. Turn on shear water, pre breathe till 45 shown on display - this generally takes longer 2nd dive but still only 2-3mins while you are getting every thing else in place. Go on 2nd dive.

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2) it first does not change for a couple of minutes and then starts dropping. In fact, I have once experienced it going down to zero before it started climbing back up.
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I have experienced this, where I had a short surface interval between dives (around 45min) and did not turn the shearwater off. I ended up doing close to a 10 minute pre-breath before RCT climbed up to 0:45.
 
Semaj said:
I have experienced this, where I had a short surface interval between dives (around 45min) and did not turn the shearwater off. I ended up doing close to a 10 minute pre-breath before RCT climbed up to 0:45.
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Glad I am not alone in having experienced this, although I don't think it has anything to do with keeping the SW on during SI (I don't). Indeed the prebreath time needed to "reset" the rMS prediction can be quite long. Usually I am stopping after ~5 min, since it is the recommended time without rMS. The rMS prediction stabilizes during the beginning of the dive anyway.
 
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