That's all I'm asking for. The body of evidence. You don't need to do CE testing to get numbers. But if you are going to categorically state that a 2.4 kg scrubber will last for a minimum of 6 hours in 25C water, I personally would like something more reproducible than "many excellent divers do that, and they are fine". Because until you actually do a real world prospective diver study, you just don't have that data.
I'm not saying that such studies don't exist. I'm just asking for them. And if they don't exist, then my point stands.
Hello Mike and others,
Many factors are important in determining the duration of a scrubber, but arguably none moreso than exercise levels and therefore the amount of CO2 presented to it. To bring some objectively to this debate about duration, there are some great examples in our tempstick efficacy study that will be published in Diving and Hyperbaric Medicine in March
Silvanius M, Mitchell SJ, Pollock NW, Franberg O, Gensser M, Linden J, Mesley P, Gant N. The performance of temperature stick carbon dioxide absorbent monitoring devices in diving rebreathers.
Diving Hyperbaric Med. 49(1), 2019: In press.
We ran an Inspo Evo plus (2.6 Kg scubber - sofnolime 797) immersed in 20oC water at surface pressure on a test circuit that ventilated the rebreather and introduced CO2 to the exhaled gas to simulate two scenarios:
1. A diver exercising at 6 MET for the duration of the dive - this is pretty close to the CE test standard - maybe a little harder exercise-wise, until CO2 broke through the scrubber to 1 kPa (1% surface evquivalent CO2).
2. A diver exercising at 6 MET for 90 minutes, followed by a diver resting on deco (about 2 MET) until CO2 broke through the scrubber to 1 kPa. The latter was designed to be more in keeping with what happens on a real technical dive (hard work at first followed by a longer period of relative rest).
Everything was identical on all repetitions (6 on each protocol) except the simulated exercise.
The average duration over multiple repetitions on
protocol 1 was 3 hours, and on protocol 2 it was just under 8 hours.
Please understand that these tests were
NOT designed to establish expected durations for this scrubber in real diving. For example, the tests did not take place at pressure. But the data do illustrate the remarkable context sensitivity of scrubber duration where context in this case is exercise load. So, yes scrubbers probably can last 6 hours, but only if you are not working too hard!
The temp stick results (how good the temp sticks were at predicting CO2 breakthrough in inspo and revo rebreathers) will be published in the paper.
Simon
PS. Mike, you asked me what I do regarding prebreathing. I do a prebreathe. I do it for long enough to see that the controller establishes and begins to maintain surface PO2 setpoint. It usually takes a couple of minutes.