The US Navy was facing a serious problem: during dives in the Arctic, rebreathers repeatedly failed after a very short time - for no apparent reason. The Navy Experimental Diving Unit (NEDU) took up the investigation. This episode, described in John Clarke's book “Breakthrough: Revealing the Secrets of Rebreather Scrubber Canisters”, provides exciting insights into the operation and challenges of rebreather diving.
Before we solve the problem, I would like to ask you this question:
Imagine you go ice diving in the Engadine with your rebreather. The CCR stays in the car overnight and cools down to the ambient temperature of 0 °C. What action do you take for the dive?
a) None - I dive as usual.
b) I insulate the scrubber with a neoprene cover.
c) I pre-breathe the lime for 10 minutes.
d) I borrow a colleague's equipment and dive OC (i.e. with conventional equipment).
e) I regularly flush the breathing loop with Diluent.
To solve the Arctic problem, Clarke, the scientific director of NEDU at the time, used a computer model to simulate various circumstances. He had developed this in his spare time in order to test the function of lime scrubbers without costly, manned or unmanned experiments. In his book published in 2023, he describes the underlying model in detail and summarizes the current state of knowledge about scrubbers (see also the Dekoblog article from 6.2.25).
The assumption
From the very beginning, it was suspected that the scrubbers were no longer functioning sufficiently in the cold - in other words, they were no longer fully absorbing the CO₂. It is known that cold lime has a lower absorption capacity, as the chemical reaction between CO₂ and the lime is slowed down at low temperatures. This leads to an earlier CO₂-breakthrough, a critical increase in the partial pressure of CO₂ in the breathing gas of a rebreather above 0.5 kPa (5 mbar). This occurs when the lime is either completely saturated with CO₂ or no longer absorbs CO₂ effectively for other reasons.
To counteract this effect, it is recommended to bring the lime to operating temperature by pre-breathing before the dive. The colder the environment, the more important this measure is.
Confirmation...
It is known from NATO standard tests that a CO₂-breakthrough occurs after about 40 minutes if the scrubber is operated in cold water (35 °F/1.6 °C).
However, if the lime itself was pre-cooled to 34 °F/1 °C, a breakthrough occurred almost immediately before the absorption reaction started and the CO₂ in the breathing gas initially dropped again. This illustrates the effectiveness of prebreathing.
... and surprise
But Clarke discovered a surprising dynamic: after a further five minutes, the CO₂-level in the breathing gas rose again, and after seven minutes at the latest, the rebreather failed completely. Although the initial heating of the granules caused by the prebreathing had activated the absorption reaction for a short time, the exothermic reaction was unable to maintain itself.
Clark's computer model confirmed this effect: while the absorption reaction initially started, the cold granules then dissipated the resulting heat. This caused the hot reaction zone to shrink and more and more CO₂ flowed through the scrubber without being absorbed. The system collapsed - the lime failed and a CO₂-breakthrough occurred.
Extensive prebreathing does not guarantee reliable CO2-absorption with cold soaked lime.
Conclusion
This finding means that with a heavily cooled scrubber even thorough prebreathing does not guarantee reliable CO₂-absorption. The initial pseudo-recovery can even provide a deceptive sense of security. Divers in extremely cold environments should therefore avoid allowing the rebreather to cool down completely and store it at temperatures well above freezing point if possible. Long prebreathing alone is not sufficient to ensure reliable function.
Clarke was also able to show that insulating the scrubber can be useful when immersed in very cold water. This means that the border zones of the lime cool down less and remain active for longer. However, this measure is of no use if the lime has completely cooled down.
Answer d) would therefore be the safest solution for the case vignette.
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