Does Pre-breathing Your Rebreather Matter?

[dreamdive]

I was taught that it matters. Why? Oh....well....(so I have been told) it will identify a CO2 leak. "Better to pass out on the boat platform than in the water". Sounds like good advice, BUT...there is a recent study that questions this. I am not advocating that you do or do not pre-breathe after reading this. But I thought this was quite interesting and not too many of us know about it.

I summarized the study on our website: Add Helium - The Rebreather Epicenter.

You should read the study yourself however, and the link is provided.

Safe diving.

 

[PfcAJ]

That study is quite the jaw dropper.

On my rb course we all got in the water (steps at OG, flanked by 2 buddies and the instructor, fairly safe) and swam HARD against the wall with an empty scrubber. One of the 3 of us didn't feel anything and stopped the excerzise after like 5mins. That's 5mins of working, not even just sitting chillin out.

pretty wild IMO.

 

[dreamdive]

That study is quite the jaw dropper.

On my rb course we all got in the water (steps at OG, flanked by 2 buddies and the instructor, fairly safe) and swam HARD against the wall with an empty scrubber. One of the 3 of us didn't feel anything and stopped the excerzise after like 5mins. That's 5mins of working, not even just sitting chillin out.

pretty wild IMO.

I know - got me thinking, too. You had first hand experience but most take what he/she learns as gospel. This is just another example of why we need to keep an open mind and keep questioning what we think we know.

 

[Dr. Lecter]

Having couch-dove my revo without scrubbers (as part of my own familiarization process, not part of a class), I'm not shocked by these results. On the other hand, the one time I had a CO2 issue at depth (240' open water in Hawaii on a DPV, sudden and overwhelming sense of imminent doom followed by BO right as the breathing was picking up when it shouldn't have been), it was in retrospect something I should have caught during my pre-breathe.

I'd been off-island for about a month, with my scrubbers sealed up in a Pelican case inside the house - less than an hour on the top scrubber, no time on the bottom when I put them away. Came back, built everything by my checklist, but pre-breathed for almost 10 minutes because I'd never left a scrubber in storage that long and I was soloing off a rec charter with plenty of time/space to gear up. Around 7-8 minutes in, I thought I felt some tingling in my face, as if I'd been breathing too fast/shallow for a while. Dismissed it because it was mild, seemed to go away when I focused on the sensation, and I wanted to splash. Tested the unit as-built once I got it home, and 15-20 minutes into the couch dive (right around when things went sideways on the actual dive) the facial tingling got much more pronounced.

tl;dr: pre-breathe may take longer than we currently teach as standard and what it reveals may be quite subtle, but I'm a firm believer in doing them carefully.

 

[descent]

I'd been off-island for about a month, with my scrubbers sealed up in a Pelican case inside the house - less than an hour on the top scrubber, no time on the bottom when I put them away. ...

Did you find settling or gaps in either scrubber when you took them apart?

Had the old reaction front slowly burned through the first scrubber in storage while you were traveling?

 

[Dr. Lecter]

Did you find settling or gaps in either scrubber when you took them apart?

Nope, both looked just as I'd have expected in terms of packing--evenly dense throughout and the screen ~1/2 cm below the top edge of the cassette. Which is unsurprising; I'm fairly anal-retentive about such things and a revo axial scrubber cassette is probably one of the easiest to pack scrubbers out there.

Had the old reaction front slowly burned through the first scrubber in storage while you were traveling?

Not that I saw. The top scrubber's reaction front, judging by how far down clumping extended, was not even 2/3s of the way through; the bottom scrubber seemed fresh when dumped out, without reacted clumps. The only thing I can figure is that both of them dried out enough to significantly hinder the reactivity of the sorb--I no longer trust that Pelican case to be air/moisture tight, and seal things in bags before sticking them in a sealed container.

 

[kwinter]

Keep in mind that the study is really about human behavior and the perception of a problem rather than the chemistry and physiology of whether the problem exists. Still, it is an eye opener (despite the source).


iPhone. iTypo. iApologize.

 

[nickbutcher]

Simon Mitchell presented this at last year's Eurotek conference here in the UK.

This came as no surprise to me; I dropped the 5-minute pre-breathe thing shortly after starting with a CCR and the only pre-breathe I do now is to check that the solenoid is firing and maintaining low setpoint.

 

[dreamdive]

Hello Nick,

Yes, Dr. Mitchell was one of the co-authors listed on the article where this came from.
I applaud the science community for conducting a study that is currently applicable to us divers. Like in any other field, a lot of studies are "far out there" and not currently applicable. Although they may lay the foundation for later, I am always grateful getting my hands on something that I can use now.

Regards,

Claudia Roussos MD