I don't have a JJ manual but for every manufacture I have actually seen, this isnt the case. They'll say their scrubber is good for X minutes at Y depth in Z temp. Often 40m on air at 4C with 1.6lpm CO2 production i.e. the CE test parameters. This is not diving (there's no ascent, nobody works that hard recreationally, 4C is rare).
The JJ test dives do incorporate ascents.
You are required to interpolate that to your diving.
Well, not sure what you mean by this. The manufacturers don't require you to interpolate anything, they just say that here is our test data that shows breakthrough times with this specific set of circumstances. Of course, they can't state anything regarding different circumstances that they haven't tested.
Interpolating to your own diving is not deviance - it might be art I'm not sure.
Look, I don't mean to be argumentative about this - pretty much every CCR diver in this forum is more experienced than me. But I'm not saying anything absurd here, or telling you how to dive.
Saying that the test data is way to conservative and you should just come up with your own breakthrough guidelines isn't the best message for a new CCR diver. Sure, maybe you have thousands of hours on a unit and you know that you routinely push it from X to Y with no hypercapnea, then you are comfortable running your scrubber for Y. But that's in your very specific circumstances with your personal metabolic profile - it's simply not generalizable. If it was, then the manufacturer would publish an algorithm that says plug in your water temperature and your workload and your own CO2 production rate and this will give you your real scrubber time. But they don't do that.
So maybe it's an art form, but it is deviating from published guidelines. I don't know about other CCRs but I can't find anything in the JJ manual that waffles on that. When I look at that CO2 breakthrough curve, it takes off pretty quickly soon after 180 minutes. Yeah, I'm sure that I could save some sorb by running it longer in warmer water, and if I was doing very long dives I might have to rethink all of this, switch to a non CE canister and decide what my particular risk tolerance was. But again, all I'm saying is that for my diving that I am doing now, with run times less than 3 hours, I'm fine dumping the sorb after that.
It's sort of like FDA clearance. The absence of evidence isn't the evidence of absence. Flonase is FDA approved down to 4 years of age, and Nasonex is FDA approved down to 2 years of age. Does that mean that if you give a 3 year old Nasonex you will be safe but if you give them Flonase they will have a problem? Probably not. What it means is that the manufacturer of Flonase didn't spend the money to establish that lower age approval because it probably wasn't worth it from a business standpoint.
At what point does breakthrough happen on the 2.5 kg scrubber in 25C water? Dunno, but if you have the money, you can run the same tests and find out.