Prebreathing Survey

[NAND]

...just for couriosity sake:

'The RCT usually shows about 3hrs RCT left after the 2nd dive.
You take a break and 3-4 weeks later you build the unit for a dive.
The unit is now saying the RCT is about 90 mins.'

Does this mean you are packing the used scrubber as-it-is (moist and partly used) for a 4 weeks period in a plastic bag? Or are you using new sorb?

 

[Germie]

Most courses state 3 or 5 minute prebreath. And do it sitting, so when something happens you will not fall down.
There are some researches that states that prebreathing only helps to identify if the unit works (PO2 constant, solenoid firing, oxygen open, etc), but not to detect scrubber issues. To warm up the scrubber is no issue and not needed is stated in the newer researches.

I see with the Tempstick of my inspiration that used sorb will take a little bit longer to 'warm up'.
What I read here about the rms, I see same with the Tempstick. But you cannot predict scrubbertime with the Tempstick, you only see what is 'active' and this changes with depth.
Most times only 1 or 2 blocks are 'warm' after prebreathing. Then I start the dive. If I go to 10m, 2/3 of the stick is 'warm'. If you drop down to 60-100m, you see the first block or 2 blocks are 'cold' and the last blocks are 'warm'. When ascending, all blocks are 'warm'. If you stay at 10 m it takes some time to 'warm' all blocks.
If you take 2.5 hour used sorb after some days again, and prebreathe it again, you see it warms up a little bit more slowly, but the first blocks are 'warm'. When you start a shallow dive these first blocks 'cool down' quite fast. And not all of the sorb is 'warm' anymore as with fresh sorb.

It is known that scrubbertime also depends on depth, so I would not trust 90 minutes left because of a calculation at surface and a planned deep dive, I would replace it (but use it if I have planned a shallow houselake dive). Do 2 dives to 80m on the same scrubber is never a good idea for example. So is this what you see when using a partly used scrubber? In my eyes not strange that remaining divetime changes by depth.

 

[dsp]

agreed that the main reason would be to see if it maintains po2. at that time, my eyes are very focused on the huds, the displays, the gauges, the sensors and their mVs. i really doubt that i would get hypoxic and pass out so that i fall into the ground since the reason i'm doing this is to monitor my po2 for 5 minutes but whatever.
now for scurbbing purposes we all know that you won't catch it in 5 minutes but let me share an amazing event that happened to us a couple of years ago.
a very good friend and diver that i have huge respect for was diving with me. He had finished his checklist and did his 5 minute prebreath while walking with the unit and breathing from the loop. we enter the water and started the dive and 3 minutes into it he bails out and dive is over.
what happened: in the meg between a 2.7 and a 15 you can use the same can and scrubber can configuration but you have to change a spacer. the reason being that the 15 head is shorter. When he changed from a 2.7 to a 15 head that morning he forgot to change the spacer.
now, before you blame my friend, keep in mind that *all tests pass*. checklist, pos/neg on the whole as well as the head etc.
we believe that the prebreath saved him from far worse because during all that time that he was prebreathing and walking around with drysuit and bailouts he created plenty of co2 that was never scrubbed because the head was never attached to the scrubber cannister. even with this high load it took him approx 10 minutes until he felt the symptoms of hypercapnia and bailed out.

tl;dr. IMO yes prebreath at least 5 minutes and do physical work too. if you're monitoring your po2 don't worry too much about passing out on the ground (and you should be only monitoring your po2 all this time and hearing the solenoid), but physical work will make scrubbing problems evident earlier, hopefully before the bottom part of the dive.

 

[uwxplorer]

agreed that the main reason would be to see if it maintains po2.[snip] etc. [snip] hopefully before the bottom part of the dive.

How does it apply to a unit which doesn't have this possibility to built it up wrong (assuming there is such a unit)? Are you saying that pre-breathing the unit to the point of accumulating CO2 just beyond the threshold of detectability at 1 ata is sufficient for the user to get a rapid hit at the beginning of the dive (when things are hopefully recoverable)?
In that case, this is a conclusion and emphasis that should have been stressed out in the prebreath paper.
I must say that it does make a lot of sense.

 

[dsp]

How does it apply to a unit which doesn't have this possibility to built it up wrong (assuming there is such a unit)? Are you saying that pre-breathing the unit to the point of accumulating CO2 just beyond the threshold of detectability at 1 ata is sufficient for the user to get a rapid hit at the beginning of the dive (when things are hopefully recoverable)?
In that case, this is a conclusion and emphasis that should have been stressed out in the prebreath paper.
I must say that it does make a lot of sense.

i can only tell you that it makes sense to me and our team, and it a process that we were lucky to witness and also very lucky to have an expert diver on the wheel, who understood the symptoms of hypercapnia and didn't try to push anything by fooling himself. talking about the incident afterwards he mentioned "i felt something was wrong" during all the time that he walking to the shore.
As for any reasons of not doing a prebreath like that i can't think of any (?) . i mean apart from actually reducing your CE scrubber time to Tmax-10minutes or so which shouldn't be a problem cause you should never run a dive plan close to these limits anyway.
For me a huge takeout from all this was also that "something felt wrong". I don't interpret that as a feelings based decion that is arbitrary and maybe based on hocus pocus. I see it as admitting that your focus isn't 100% on what it should be, therefore you won't be on top of your game if you need to react to something unexpected, whatever the matter that caused your feeling is. I can't thank my instructor enough for introducing me to this way of thinking, which i have found valuable in other activities too. I'm also happy that this seems to be the mentality in the whole of our ccr diving community ("don't rush ccr", "call it anytime", etc) , maybe the agencies are doing a good job hammering it in ?