What is the primary objective of pre-breathing a manual CCR?

[fullytek]

As a new CCR diver I am quite mindful of pre-dive checks.

Having read the article by Simon Mitchell I realise that prebreath would probably not alert me to issues with co2 scrubber.
But I still do it to: warmup the the lime, listen to the solenoid clicking away to keep 0.7 and kinda relax my mind and play with my computer ( check settings/gasses etc).

I did a test at home without any scrubber in the machine to get a feel of co2 in the loop. It took 3+ minutes to feel the hypercapnia symptoms. I held it for about 4min until it was fairly uncomfortable and the headache started.
It's at 1 ATA so not realistic for depth related co2 hit but I wanted a general taste of what it feels like.
Heartrate and breathing normalized within 4-5 mins but light headache persisted for 1h+.

I like the prebreathing, it's a nice procedure to slow down and relax your thoughts before going in.

That being said I do not dive on boats so it's not a rush to prebreath infront of a cave. I hate boats. I puke on boats.

So hear is an example of an experiment that supports the original assumption that a pre-breath may alert the diver to an issue with the scrubber. I think it's funny how 1 article produced by a questionable source is taken as gospel and so many divers are quick to use that as an excuse for skipping a critical safety check.

 

[foxdude]

So hear is an example of an experiment that supports the original assumption that a pre-breath may alert the diver to an issue with the scrubber. I think it's funny how 1 article produced by a questionable source is taken as gospel and so many divers are quick to use that as an excuse for skipping a critical safety check.

well i guess but that was with 0 scrubber inside. Just an empty cartrage so if it's channeling it probably will be more subtle.
You can try it yourself it's fairly safe experiment as health is concerned.

 

[foxdude]

FWIW, the rate of increase of CO2 partial pressure with a breakthrough is the same regardless of depth. (It's tied to your metabolism.)

ah yes true. But it increases your risk of oxygen toxicity exponentially so it's safer at home than at high ppo2.

 

[bletso]

The only reason for a pre-breathe is to check O2 injection and diluent injection system. My buddy didn't and isn't here anymore. Checking scrubber function is bosh, (Not enough time), making sure O2 injection working is absolute life saving / sustaining is an absolute must.

The time it takes for a pre-breathe before the water is all the time it takes to be dead in water if it isn't working. Simple as that, death or life, what do you prefer. Rebreathers can be very efficient self euthanasia machines.

 

[lostsheep]

So hear is an example of an experiment that supports the original assumption that a pre-breath may alert the diver to an issue with the scrubber. I think it's funny how 1 article produced by a questionable source is taken as gospel and so many divers are quick to use that as an excuse for skipping a critical safety check.

Pray tell, what do you find questionable about a peer reviewed paper published in a professional journal by actual experts?

Mitchell has done more to progress our knowledge using analytical methods than most of us combined ever will; if you’re going to come out swinging like that you better make it good.

ETA: the conclusion is not that one shouldn’t do a pre-breathe , the conclusion drawn is that a 5 minute pre breath is an insensitive test of co2 scrubber function.