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

[rddvet]

I don’t do a long prebreathe. I just breathe the p02 down a little, add a little dil to watch the change, then add 02 and watch the change. Probably takes under a minute and it’s done right before the dive. Usually walking to the water.
I do my other flow checks as a team in the water, but I’m also diving caves that you can stand or float on the surface unlike a boat dive

 

[mr_v]

If you look at net-new benefit of pre-breathe, it does only one thing that no other steps have done - it warms up the scrubber; that may be useful in cold climates. Depending on your build process, all other functions, like CMF and/or solenoid test, PPO2 stabilization in the loop, etc., should have been tested by the steps preceeding the pre-breathe.

 

[kensuf]

If you look at net-new benefit of pre-breathe, it does only one thing that no other steps have done - it warms up the scrubber; that may be useful in cold climates. Depending on your build process, all other functions, like CMF and/or solenoid test, PPO2 stabilization in the loop, etc., should have been tested by the steps preceeding the pre-breathe.

It's supposed to hit a heat index of 110 today. I'm not worried about warming up a scrubber.

 

[mr_v]

It's supposed to hit a heat index of 110 today. I'm not worried about warming up a scrubber.

I don't know how you folks cave dive in the summer. Just got back from EN and almost had a heat stroke while suiting up.

 

[kensuf]

I don't know how you folks cave dive in the summer. Just got back from EN and almost had a heat stroke while suiting up.

yeah, I spent yesterday there myself. The horseflies were the worst part of it.

 

[fullytek]

To Jon's point the pre-breath serves to varify the function of the lifesupport system in its entirety. Yes function checks are cool but then your girlfriend called you or you went to the bathroom. Or whatever. Did you turn that valve off ?
Did someone else turn it off for you well you were on the toilet updating your Instagram or surfing tinder?

The point is every legitimate agency teaches a pre-breath.

There is no good reason to not follow the processes that you were taught.


If the ceo of Astronomer had the time to run a company, raise a fiamily, cheat on his wife, and go to a Cold Play concert then you have time to do your ****** pre-breath

 

[rddvet]

I don't know how you folks cave dive in the summer. Just got back from EN and almost had a heat stroke while suiting up.

Adaptation via exposure. My gym is a metal warehouse with one huge fan on the ceiling and zero other form of ac. Diving in the summer is not bad once you’re heat adapted.
The bugs in the other hand suck. I have never seen so many gnats at Ginnie as I have this year. It’s like they moved from Madison to Ginnie. I specifically haven’t dove Madison yet because if the gnats at Ginnie are as bad as they are I don’t really want to find out what they’re like at Madison.

 

[mr_v]

To Jon's point the pre-breath serves to varify the function of the lifesupport system in its entirety. Yes function checks are cool but then your girlfriend called you or you went to the bathroom. Or whatever. Did you turn that valve off ?
Did someone else turn it off for you well you were on the toilet updating your Instagram or surfing tinder?

The point is every legitimate agency teaches a pre-breath.

There is no good reason to not follow the processes that you were taught.


If the ceo of Astronomer had the time to run a company, raise a fiamily, cheat on his wife, and go to a Cold Play concert then you have time to do your ****** pre-breath

To be fair, many legitimate agencies were not always right. It does not hurt to review the process to improve it.

There are several functional tests one can perform on a system:

1. Unit tests address functional capabilities of a specific item, e.g., pressing a MAV button and observing gas addition.
2. Sub-systems tests evaluate several components, e.g., a negative pressure tests checks all the seals against pressure drop.
3. Simulations test systems in hypothetical conditions, e.g., pre-breathing on the surface.
4. Full system test, e.g., testing buttons, PPO2, solenoid, CMF while submerged.

Many divers stop at the third step, but that's not enough because the system is not exposed to water. A unit that passes surface pre-breathe may fail once submerged due to electronics malfunction, e.g., computer flood or head connector short. If you re-run the same tests as you do during pre-breathe while submerged, you'll have a better chance of spotting a problem. It is easy when cave diving and a bit more sophisticated when dropping on a wreck but is doable.

As for the interruptions by social media, text messages, calls, bathroom breaks - that's just silly. If you have that level of ADD, you need to modify your SOP.

 

[foxdude]

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.

 

[inquis]

It's at 1 ATA so not realistic for depth related co2 hit

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