If you do the calculations of total CO2 removal capacity, then convert that into metabolized oxygen, then convert that into the capacity of the O2 cylinder, you should get a number.
How useful, depends. Poseidon stipulates that the 3L O2 be filled to 135 bar. Reason? thats the amount of O2-CO2-scrubber that their scrubber can handle. Ive noticed that in colder water when working hard, I get a little over 180 min. Warm water, relaxed dive I get closer to 220 min.
When I change the O2 its time to change the scrubber. FWIW the Poseidon is also rated CE for 180 min.
The theoretical numbers include a lot of variables that may or may not apply to your actual diving, and they involve some assumptions that may or may not be accurate - but it's a best case starting point.
I think your observation on the Poseidon scrubber versus O2 limit often works that way in other designs as well, probably by intentional design. There may be some merit to having a "balanced" design where O2 capacity doesn't encourage the diver to exceed the scrubber limit.
The math and chemistry more or less support this on our KISS Sidekicks, in warm water at least.
For example, Intersorb 812's spec sheet indicates it will absorb 245 liters per kilogram of CO2 at a scrubber temp of 90 degrees, 40 Lpm inspiration rates and 1.6 Lpm CO2 production rates. Our KISS Sidekick scrubbers hold 2.5 kg of Intersorb 812, and that gives a theoretical scrubber limit of 612 Liters of CO2 absorbed (assuming an unlikely 100% efficiency). However, at a conservative 1:1 ratio that would require 612 liters of O2 to be metabolized in order to max out the scrubber. If you back off to a more likely ratio of .8 Lpm CO2 produced per 1.0 L of O2 metabolized, you'd need 765 liters of metabolized O2 to max out the scrubber.
Given that we dive a 2L, 205 bar O2 bottle with 410 liters of O2, and use perhaps another 80-100 L of O2 from the diluent, (call it 500 L of O2 total metabolized on the dive) we should run out of O2 before we exceed the scrubber capacity, even at the more conservative 1:1 ratio, assuming warm water and light levels of exertion.
In terms of actual O2 use, we've found that we average 1.0 to 1.25 Lpm on multi-level cave dives with set points in the 1.0-1.2 range, and an O2 flush to check for current limited sensors. That gives an average O2 duration of about 5.4 hours (1.25 Lpm) with either an AL 13 or Faber 15 O2 bottle filled to 3000 psi.
In practice we've done 4 hour swimming dives using Intersorb 812 on our Sidekicks in 70 degree water, and with light to moderate work load. That's well short of the theoretical maximum, but that's also as far as we are inclined to push it. It's probably worth noting that the scrubber in the Sidekick is also surrounded by the counter lung and even the exterior of the scrubber tends to stay slightly warmer than the water temperature.
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Spherasorb 408's spec sheet indicates it will absorb 184 Liters per kg, and given that our Sidekick Scubbers hold 2.4 kg of the bulkier 408 and that gives a theoretical capacity of 441 Liters of CO2.
At a 1:1 ratio, and considering the maximum possible O2 metabolized in the diluent on a multi-level dive to (conservative) to 500 L total, the O2 available in a 2L bottle is now slightly exceeding the scrubber duration. At a more likely 0.8 : 1.0 ratio, the O2 should still exceed the scrubber capacity on a warm water, light to moderate effort dive, but 8-12 mesh sorb adds a greater safety margin.
Obviously, in either case if you used a 3L bottle filled to capacity with a Sidekick, you're now in a position where the scrubber capacity will be the limiting factor on the dive, even in warm water. And in cold water, the limiting factor would almost certainly be the scrubber, given decreased efficiency of the reactions involved and greater potential for breakthrough.