Pre-Breathing (The AP Diving Version)

scunick

I have send my unit back to AP for what I considered a fault on the temp stick. Beside the now standard "this units is perfect" response I also received the following ADVISE:

"What you are looking for on a CCR pre-breathe more than anything is to simply blow one good lung full of air into the loop, observe the reaction of the oxygen cells, ensuring the display drops and then rises again during O2 addition.

Nepa

I was told to do the pre-breathing as a precaution to see if there is anything wrong with the system. If people would start skipping the pre-breath then there probable would be more accidents with closed valves and etc.. dont know if its necessary to prebreath sodalime though

Allan_o

My personal reference does tell me that it is in fact not neccesary to prebreathe the unit to start the slime.
Given the Rebreather is working properly the slime will work, i dont know if the slime uses any time to start but in that case it reacts very quikly.

/Allan

chrismc

I dont think its neccesery to pre-breath sofnolime, after all whats the diffrence from properly breathing it?

However as mentioned a pre-breathe is important on your unit to ensure everything is working ok and responding the way it should. The same with open circuit. This makes sure you have assembled it correctly and hopefully means if theres anything wrong you will collpase before you get in the water e.g. missing seal o-ring - Co2

decoweenie

IMHO, that's the key point of the message.

If it takes a few minutes to get the sorb to work, it must mean that no CO2 scrubbing is taking place during this period. If that's true, why do you not feel any hypercapnic symptoms ?

However, pre-breath is more of a safety checking procedure to make sure that everything else is working correctly (i.e. controller adjusts SP, orifice not plugged, valves turned on, etc).

Again, IMHO.

__________________
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jradomski

I have seen enough traces from mass spec that shows, the scrubber is only partially active at the beginning.. It depends on the temperature and how much moisture is present.. The scrubber is removeing co2 but not all of it.. The drier the sorb is initially the slower the initial reaction is.. Martin is correct that co2 incontact with sorb with moisture immediately works, how much it can remove is what is in question..


Watch this the next time you refil your sorb..

do a prebreath and watch the po2 (use a sp of 0.70), when you first go on the loop, the controller has quite a bit of difficulty keeping it near the sp.. a 0.50-0.55 range is pretty reasonable, after a few minutes (in my climate its usually 2-3 minutes) the controller usually settles into around 0.65-0.68.... Why do you think this is the case?? The o2 cells dont need time to speed up reaction time, there is another gas in the loop.. You need at least 10% co2 in the loop on the surface to feel the symptoms quickly, and higher than that to have any real short term effect..


Now after doing a dive, with any reasonable surface interval (as long as you dont open up the rig to reduce the moisture), now do the same thing, on the second pre breath the delay time is almost non existent (unless the ambient temp is really cold)..

The limiting factor of the sorb reaction is how much moisture is available.. If there is sufficient for the co2 level produced, the stack is fully working, if not, it IS absorbing co2 but not all of it..

What are the consequences.. well if I'm skipping a pre breath in warm water and not going to bust my ass swimming, I probably wount have any appreciable rise in co2 levels... but now if I am in cold water and busting my ass initially, there is the POTENTIAL for a co2 buildup until enough moisture is present... Granted this delay is small but it COULD be a problem..

While the delay to get the scrubber FULLY functional is small, the prebreathe time allows you to evaluate the system and how it is responding.. Many problems can be spotted at this point before you hit the water..

__________________
Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

decoweenie

If you haven't breath the loop, and turned the controller on, it will inject a certain of O2 to create an "envelope" around the O2 cells to maintain SP.

As there is no gas movement, this envelope remains in place until you start breathing which immediately lowering the PO2. And the controller starts to inject to keep SP.

This cycles repeats until the majority of the loop is uniform at the same fiO2 (on surface). And you see the steady SP shown on the controller.

This "envelope" concept is how the Inspiration does the O2 calibration.

__________________
"...after a while you get bored offering advice to a bull that like to keep butting the fence with its head rather than walking through the open gate..."

- Rebreather World PM

lizardland

I APD are technically right, assuming that teh scrubber is filled right, the o-rings are sealing, that the lime is good, etc., then why pre-breathe? Personally, I'm a muppet and I don't trust myself to get everything right. I'd rather find something wrong on the surface than underwater so I'll continue a pre-breathe. If they were telling you to not do something which is harmful then that's understandable but I don't see the point in discouraging something that doing does no harm. It's usually the last thing I do, with the Rebreather on when I'm putting on my gloves, walking to water, calling the skipper a ****, etc., then I'm usually doing it all with the DSV in my gob.

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decoweenie

I agree that the APD message could be interpret wrongly from the way it is written, but I don't believe they are saying that pre-breath is useless from the point of checking the system.

__________________
"...after a while you get bored offering advice to a bull that like to keep butting the fence with its head rather than walking through the open gate..."

- Rebreather World PM

nigelh

I thought we all knew that sofnolime works from cold so it doesn't care...

I prebreath the thing because I don't trust myself, despite all the checks and double checks, to have put it together right. I want to watch the ppO2 plummet as I first suck new air in on the ADV and round the system (extra credit for being quick enough to hit <0.4bar and sound the beeper), I want to watch things come back up and taper in on 0.7bar. I want to breath the thing for long enough that if the scrubber O-ring is in the bottom of my tool box I'll feel breathless.

Cowards live longer.

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nigelh