Prebreathing Survey

[Dr Simon Mitchell]

Hello,

I have uploaded the study that prompted this thread so that you can read it yourselves. I think it is interesting (and scary in some ways)!

To be very clear, our finding was that you can't rely on a prebreathe to identify problems with CO2 scrubbing. But we never intended that this be interpreted to mean a prebreathe has no value. The prebreathe has other purposes which have been articulated by various posters on this thread. We should always do prebreathes.

Simon M

 

[doctormike]

Hello,

I have uploaded the study that prompted this thread so that you can read it yourselves. I think it is interesting (and scary in some ways)!

To be very clear, our finding was that you can't rely on a prebreathe to identify problems with CO2 scrubbing. But we never intended that this be interpreted to mean a prebreathe has no value. The prebreathe has other purposes which have been articulated by various posters on this thread. We should always do prebreathes.

Simon M

Dr. Mitchell...

Thank you so much for posting!

So what do you do yourself? How many minutes to confirm that a rebreather can hold a safe PO2, etc...

Or is it unit specific?

 

[_Ralph]

I dive a manual unit.

After verification that gas is on, the leaky/manual add valves work, loop hoses are correct, my cells give me a tolerable voltage and the display is displaying, that is about as much as I can do ...

Either I have useful scrubber material and did install my canister, or I did not do one or more of the two things.

_R

 

[kensuf]

You could use a clothes pin to pinch your nose. If you use your left hand, you won’t be able to access your turn indicators, so you’d fit right in here in Florida.

Just push the hazard light button and get on with life. That's how we do it in the 'duh.

 

[uwxplorer]

rEvo diver here: when my rMS system worked (whatever that means, which wasn't for long anyway, in the grand scheme of things), I had to prebreath a couple of minutes for the temp probes to start registering a temperature increase (and the secret rEvo sauce to announce the magic 45 min of RCT or Remaining Cycle Time, the time left before dumping the top canister) That was BEFORE the first dive.
After the first dive, and a surface interval of ~1 hr, that changed to MORE than 5 min, because of residual scrubber temperature. I believe I clocked over 7 or 8 min once, before the display swung from 4 hrs+ RCT to "??" to 0 and then slowly back up to 45+ min.
After a while and the usual demeaning comments from rEvo about my lack of understanding of the system when I described the previous phenomenon (note than none of what I just described had ever been taught me, let alone documented before I brought it up), I concluded that I was better off not bothering about any display, but instead focusing on careful packing (easy with the rEvo), and verifying that I had a positive stereo check, good negative check, tight cover seal. In brief that I did a careful built. I still waited for the 45+ min RCT message when the rMS worked, but now prebreath for only a couple of min regardless, just to make sure that nothing funky is going on, period.
The prebreath paper told me that I should be skeptical about the outcome of any subjective assessment of scrubber function from simply breathing off the loop at the surface. Instead, I keep an inner eye on my breathing pattern and other symptoms when diving. And dump my top scrubber after ~3 hrs of dive time (in 50-60 °F water).

 

[stuartv]

rEvo diver here: when my rMS system worked (whatever that means, which wasn't for long anyway, in the grand scheme of things), I had to prebreath a couple of minutes for the temp probes to start registering a temperature increase (and the secret rEvo sauce to announce the magic 45 min of RCT or Remaining Cycle Time, the time left before dumping the top canister) That was BEFORE the first dive.
After the first dive, and a surface interval of ~1 hr, that changed to MORE than 5 min, because of residual scrubber temperature. I believe I clocked over 7 or 8 min once, before the display swung from 4 hrs+ RCT to "??" to 0 and then slowly back up to 45+ min.

That is discussed in detail in the document here:

http://www.revo-rebreathers.com/wp-...se-the-scrubber-monitoring-system-V-11.17.pdf

Section 6, on page 19.

 

[doctormike]

I know that we aren't supposed to agency bash, and I hope that this doesn't come off as unit bashing. I respect rEvo divers and instructors, and I'm sure that it's a fine CCR. But this is not the first time that I have heard about complex problems with the rMS system.

If it's really this finicky, is it a big advantage over (1) doing a good job packing your scrubber, (2) monitoring yourself for any symptoms during a dive, and (3) not pushing the time limits on the stack? Asking for a friend.

 

[stuartv]

I know that we aren't supposed to agency bash, and I hope that this doesn't come off as unit bashing. I respect rEvo divers and instructors, and I'm sure that it's a fine CCR. But this is not the first time that I have heard about complex problems with the rMS system.

If it's really this finicky, is it a big advantage over (1) doing a good job packing your scrubber, (2) monitoring yourself for any symptoms during a dive, and (3) not pushing the time limits on the stack? Asking for a friend.

It's no substitute for packing properly or self-monitoring. rMS won't detect if you have channeling.

But, I think the spec is 4 hours on the scrubber (roughly). However, in reality, it seems like everyone knows that in warm water it can go MUCH, MUCH longer. It seems to me that rMS simply gives a pretty safe and reliable way to use more of the scrubber. In other words, people know it lasts longer than 4 hours. They're going to use it for more than 4 hours (e.g. on a cave dive that is longer than 4 hours). rMS lets them do that with some actual data versus non-rMS systems where people just dive it "knowing" that it will last longer than 4 hours (and hoping they are right).

 

[doctormike]

Yup. I was told (not gonna say by who, but someone that I really respect) that those scrubber times are VERY conservative, especially if you dive in water warmer than the 4 degrees C that is used for testing. I actually started a thread here a while ago about why manufacturers don't do a second test in warmer water, and the upshot is that it probably isn't cost effective for the small companies that most CCR manufacturers are (I think that Innerspace does this).

Me, I'm just more comfortable sticking with the published limits. I don't know how much verification goes into the rMS system in the real world, but I'm assuming that it's well thought out. Based on the things that I have read here, i don't know how "reliable" it is, but I guess if it's only going to give you a false positive that's safer than relying on an all clear from a poorly functioning system.

I'm fine with getting less potential mileage out of my stack just because of the type and amount of diving I'm able to do.

 

[stuartv]

Yup. I was told (not gonna say by who, but someone that I really respect) that those scrubber times are VERY conservative, especially if you dive in water warmer than the 4 degrees C that is used for testing. I actually started a thread here a while ago about why manufacturers don't do a second test in warmer water, and the upshot is that it probably isn't cost effective for the small companies that most CCR manufacturers are (I think that Innerspace does this).

Me, I'm just more comfortable sticking with the published limits. I don't know how much verification goes into the rMS system in the real world, but I'm assuming that it's well thought out. Based on the things that I have read here, i don't know how "reliable" it is, but I guess if it's only going to give you a false positive that's safer than relying on an all clear from a poorly functioning system.

I'm fine with getting less potential mileage out of my stack just because of the type and amount of diving I'm able to do.

In his posts here, CO2 monitoring (and the X-CCR), and here, CO2 monitoring (and the X-CCR), Dr Simon Mitchell mentions the rEvo and tempstick monitoring for predicting scrubber breakthrough. His post gives me some bit of confidence that the rEvo rMS is reasonably reliable - WHEN it is actually giving you a prediction. rMS may not be reliable in terms of always giving you a prediction - like mine, which is currently broken. But, when it IS giving the appearance of functioning and giving a "remaining scrubber time", I feel pretty good about it not telling me the scrubber is still good when I'm actually about to experience breakthrough.

In other words, the manufacturer spec may be 4 hours, but if I've got 2 hours on the scrubber and the rMS is telling me I still have 4 hours of remaining scrubber time, I would feel pretty okay about planning another 2 hours of diving without changing the sorb. Without the rMS, if I had 2 hours one day and was contemplating 2 hours the next day, I would probably go ahead and change the sorb.