Pre-breathing procedure

[LFMarm]

Hi all,

I am trying to understand in detail the sequence of pre-breathing actions and their reason. Acknowledging that pre-breathing has been proven ineffective for hypercapnia prevention (article from @Dr Simon Mitchell in 2015; also discussed at length here Does Pre-breathing Your Rebreather Matter?), I have tried to understand the procedure recommended by Edd Sorenson ():

1. After building the rebreather do a positive on pure O2: you have 100% O2 in the loop as you have just calibrated the sensors; you go to the dive site with pure O2
2. Before geting in the water turn OFF the oxygen and ON the dil — Breathing 3 times from the loop and exhaling from the nose (not in the loop) should bring the ppO2 down to that of the dil (e.g., 0.32 on 32/00) as the ADV is compensating the mix lost at every breath by adding diluent
3. Turn the O2 ON take 3 breaths exhaling from the nose and check that O2 is raising to the set point (e.g., 0.7) as the solenoid is now able to inject O2

By doing this you

• Check calibration of sensors on O2 (Steps 1 and 2)
• Analyse the dil (Step 2)
• Verify that the dil is added correctly by the (Step 2)
• Verify that the O2 is added correctly (Step 3)

I would love to check with experienced CCR divers (1) if my understanding of the procedure is correct, and (2) if you do something different and why.

 

[Vicko]

I don't waste my time or o2 on the o2 positive test, i leave that for the 6m if I haven't done it in a few weeks, but other than that yeah that's what I expect from a 3-5 minute prebreath.

 

[beldridg]

I would caution about the Dil test. If your Dil is hypoxic (or anywhere near it), I would not use that procedure.

I also think there is a difference between tests when building the unit and the pre-jump tests (at least for me there is).

Once I turn on my O2, it doesn't get normally turned off until I'm done with the dive and off the loop. With all respect for Edd, I wouldn't do test #2 the way he does it.

- brett

 

[doctormike]

Seems a bit complex, although of course I also defer to Edd's vast experience. I guess you could have that procedure added to your pre-jump checklist, but for the average diver I would worry that would add an opportunity for an error.

I just do a 5 minute pre-breathe (sometimes less) before the first jump of the day, shorter one if I do another dive. I do this (1) IN my gear with everything fully donned, and (2) with my mask on. As Simon has shown, that's not to check the scrubber. I do it to make sure that the O2 injection system is working while I'm on the boat. Doing it kneeling in front of the unit, with no mask, as I see some people doing, is a bad idea. No mask means that you can entrain ambient gas (air), and that something that worked during your pre-breathe might not work when you are in your CCR (i.e. if in the donning process a connection is displaced or a tank rolls off, etc..).

As far as the four cited advantages, I have generally done the calibration on build, not sure that doing it again is really necessary. Dil is also analyzed as part of the build procedure. I check the ADV to see if I can add dil, and the pre-breathe confirms that the solenoid is working (or MAV / orifice, if you have an mCCR).

 

[bubblemonkey2]

Is there any particular time point(s) in that video you are referring to? It's quite long.

Do they really even touch on the 'known unknowns' or 'unknown unknowns' that might have actually caused a fatality?

Demonstrating that he can notice planned pO2/volume decreases while still and calm with minimum loop volume--I don't think anyone doubts that this is possible--was there anything else? How does this have anything to do with dozens of drownings?

(Explaining to us that air has 21% oxygen in it and that SCUBA gear has o-rings...really though?)

Were there any recommendations in there that I missed while skipping the long-play extended bro chat?

A lot of these super confident & creative guys (and their students) who depart from manufacturer/agency protocols aren't with us anymore.

Do you have reason to believe the official setup, checklist and use guidelines of your rebreather are somehow lacking?

 

[mr_v]

The procedure may depend on the unit, diver preferences. I follow the standard rEvo checklist. The procedure ensures that the loop isn't hypoxic and the sensors react to changes in PPO2.

 

[rddvet]

It was taught when I took the sw class with Edd. I never bought a SW (different story) but had I, I would not have followed his procedure

 

[Blugold]

Pre breathing sitting down doing nothing for a couple minutes likely will provide little feedback in terms of hypercapnia due to minimal CO2 building up. However pre breathing your unit under stress 5-10mins when your producing a lot of CO2, as you walk down to your dive site entrance or from your car aboard the boat, may provide some feedback.

I’m newly trained but this was a suggestion by my instructor. A diver passed out on land on their way to the dive site is better than one underwater. Given your surrounded by friends or fellow divers.

 

[Gareth J]

Pre breath also activates the chemical reaction. Today it was -2C, so I extended the pre breath to make sure the scrubber was warm and acting normally, before we splashed. Heat is part of the reaction process, if the scrubber is cold, it slows down the reaction. Once the reaction has started, it is generating heat. Which keeps the scrubber warm, even in very cold water. Entering the water with a cold scrubber, in cold water, increases the risk of break through (i.e. slows down the chemical reaction).
In addition, on a Inspiration, if you have a temp stick, you can see the reaction starting. Which is also a validation.

Unless you are working hard on the surface, the pre breath is unlikely to indicate you have a CO2 issue. (One of the advantages of the temp stick is you can see where the front is. If you have bad channelling, bad 'lime', poor packing, the temp stick may show multiple fronts or a vary deep section of the scrubber working.)

 

[AJ]

I pre breathe my unit to know it's not doing strange things. SO I monitor pO2 levels, but mostly how quick they react to changing O2 levels. I have no illusion I will spot any serious O2/CO2 problems on shore.