Off the top of my head, the CE test parameters call for something like 1.3L of CO2 per minute with a flow rate ("breathing rate") of between 25-40L of air in 39° water at 40m of depth. In reality, most of us aren't generating that much CO2 for the tested duration of the scrubber in that temperature of water.
Having said that, you should be relatively safe from the effects of breakthrough if you stay within the duration of your scrubber.
Given that the risks from CO2 include narcosis, blackout, heart attack, and death, I would be really hesitant to dive beyond the CE rated scrubber duration because of something I read from some random person on the internet.
EN14143 standards are 40 m depth, 4 degrees C (39.2 F) water temperature, 40 lpm breathing rate, and 1.6 liter of CO2 generation. It's easy to remember roughly as 40 m, 40 degrees F, and 40 lpm.
The 5.7 pound KISS classic scrubber using 5.7 pounds of Sofnolime 797 gave a scrubber duration of 2 hours and 37 minutes to 5 millibar of CO2, and only slightly longer at 2 hours and 50 minutes to 10 millibar of CO2. Once a scrubber starts to break through, it goes fast. As an aside, the NATO test standard STANAG number 1411 uses a CO 2 production rate of 3 lpm, which is way over the top.
The problem with the CE based durations are two fold in that you don't get the breakthrough along the side of an axial flow scrubber canister as quickly in warm water, and no body on the planet can sustain 40 lpm ventilation rates for 2-4 hours.
That makes them very conservative - almost to the point of non usability for real world purposes, and while that's convenient from a liability standpoint, it leaves individual CCR divers as more or less their own scrubber duration test dummies with not much to go on and that's unfortunate.
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I agree with you Ken that exceeding the rated scrubber duration has it's perils, but I suspect many, if not most, CCR divers are doing that based on their personal progressive experience with their scrubber under specific conditions.
We have for example done 4 hour swimming dives on our KISS sidekicks with a similar amount of 8-12 mesh sorb in 70 degree water with a light workload (normal 50 fpm swim rate interspersed with some pulling and slower swim rates in restrictions and small passages).
That's a fair bit more than the CE duration of 2 hours 37 minutes suggests, but we've worked up to that slowly and we're always cognizant of the caveats in the form of the specific conditions that accompanied these longer duration dives.