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.

 

[Rebreather Rabbit]

During pre-breathe on my Spirit I'm just verifying that PO2 is stable and the unit is doing what it's supposed to (no sensor/monitor issues and no unexpected loop volume/gas pressure loss). Since it's mechanical there's very few things to monitor while doing the pre-breathe.

I'm fairly new (100 hours range), but I agree with other points already mentioned (5-minute won't reliably catch scrubber issues, tests prior to a pre-breathe will catch pretty much everything else, etc.).

 

[GF99/99]

I'm working on updating our dive manual on CCR diving, and I'm pretty ignorant on manual CCRs. My take on pre-breathing is it is primarily a function check of the solenoid to reach and maintain set point and verification that cells are behaving as expected in the .7 to .98 range. Secondarily it is a function check of MAV, ADV and BOV. I don't consider it a useful verification of the scrubber. There is plenty of evidence that the variability of individuals to tell if their scrubber is working or not in 5 minutes makes that a non-functional test.

For those of you that dive manual CCRs, what are you looking for during a pre-breathe? Is an IP check of the O2 side a critical step? How about leaky valves, is there something in particular that you are looking for from a functionality check?

How about hybrid systems? Anything unique to them that isn't captured in the eccr and mccr pre-breathe?

To me pre-breathing on an MCCR is the same as what you have listed but just remove the world solenoid and replace it with CMF/needle valve.

All you are really doing for a pre-breath regardless if its a manual or electric unit is checking that your unit is functioning properly while your on it. This is the key word while you’re on it. This differs from your CCR checklist because now you will find out if you forgot to plug things in or turn things on and you will see how the unit is acting as you breath on it.
Is your O2 and dil open
Is your computer working and batteries good
Is your HUD working and lining up with your P02
Is your p02 being maintained even
Are your cells working properly (flush some o2 through to see how your cells react/flush some dil through to see how they react)
Are your mV corresponding to the p02 read outs
Is your MAV’s working
Is your ADV working
Is your dump working

 

[DiveLikeAMuppet]

I pre-breathe my JJ for exactly five minutes, doing most of the checks in the thread, fully kitted up except for stages, with my hood and mask on.

I’m usually done with any checks by minute 3 - but stay on the loop for extra 2 minutes. Not to check for CO2 but because it forces a pause in the kitting up process and lets me think. Am I happy to dive? Have I missed anything? Kitting up on a boat can be really rushed and stressful close to slack window so forcing myself to sit for 5 minutes, insulated from most disruptions (hence the hood and mask) helps me focus on the dive.

After completing a pre-breathe I follow with with a final head to toe check.

So for me, the secondary purpose of timing a 5 minutes pre-breathe is honestly to just … slow down.

 

[happy-diver]

insulated from most disruptions

I can't talk and work, this stuff must have disrupted many dives if not killed people as they don't get to

slow down

A five minute curfew on talking for all dives

The head-space check, is that on those lists

 

[DiveLikeAMuppet]

I can't talk and work, this stuff must have dirupted many dives if not killed people as they don't get to



A five minute curfew on talking for all dives

The head-space check, is that on those lists

That’s exactly how people jump in with their controllers or O2 shut off. But the boat getting busy is inevitable, time and tide wait for no man or woman, deck space is at a premium, you just get used to it. If you don’t you need to limit diving to lakes quarries and caves .