Breakthrough - Vignette 1

You are planning a deep dive with your rebreather. What do you pay particular attention to when planning?

a) The running time of the scrubber

b) The maximum breathing gas density

c) The porosity of the chalk

d) a to c

e) Never mind, I have a heating system!

Ice diving in Klöntal. Image: Martin Ammann

Anyone who dives with a rebreather has certainly already dealt with the scrubber - one of the most important components of closed-circuit rebreathers (CCR). Questions such as “How well does it work?”, “When does it fail?” or “How long does it last under real conditions compared to test conditions?” concern many divers. The question also arises as to whether the running time of the canister based on the expected performance differs from the values simulated in tests.

Lack of reliable data in civilian diving research

Unfortunately, there is a lack of data on these topics in civilian diving research. In contrast, the military - for example, the US Navy Experimental Diving Unit (NEDU) - possesses such data to support its divers in the successful execution of missions. However, access to this information is very limited for civilians.

The work of John Clarke

This is where John Clarke comes in. A physiologist by training, he was scientific director of the NEDU for 28 years, with much of his work revolving around rebreathers and hence the scrubber. He compiled his findings from decades of research in his book “Breakthrough: Revealing the Secrets of Rebreather Scrubber Canisters”. This work is considered to be the current state-of-the-art manuscript and was reviewed and approved by the US Department of Defense for classified information prior to publication.

The following blog posts will take a closer look at key findings from this book, which was published in 2023, to offer a deeper insight into how rebreather scrubbers work.

A window into the world of scrubber function

Since even the NEDU cannot conduct an unlimited number of experiments on rebreather diving, Clarke developed a computer model for scrubbers in his spare time. This model is used to simulate a wide variety of situations and takes all the knowledge about scrubbers into account. In addition to basic data such as the amount of CO₂ to be absorbed and the breathing rate, numerous other parameters are taken into account. These include, among others:

• Porosity of the lime: The grain size influences both the CO₂ absorption capacity and the breathing resistance.

• Pressure drop across the canister: A crucial factor for breathing resistance.

• Ambient temperature: It affects the chemical kinetics of CO₂ absorption.

• Chemical kinetics: Clarke does not calculate these deterministically, but stochastically, i.e. he uses probability and statistical laws to achieve realistic results.

How the model works

The model simulates up to 288,000 discrete cells within a CO₂ absorption bed. The heat and CO₂ transfer is modeled in each of these cells. The probability of an absorption reaction increases with temperature, for example, as higher temperatures increase CO₂ diffusivity and more energy is available for chemical reactions. Although this is a simulation that calculates with 5 to 21 million absorbed CO₂ molecules - in comparison, a moderately working diver produces over 160 liters of CO₂ (corresponding to about 43 x 10^23 molecules) during a two-hour dive, which is 10^15 to 10^16 times more molecules than in the simulation - the model enables the investigation of numerous conditions and provides fascinating insights.

Findings for practical use

This brings us to the question raised at the beginning, which was simulated in the model:

• Long dives and canister durations: a deep dive is inevitably accompanied by a long dive time due to the required decompression time. A long canister runtime is therefore essential.

• Porosity of the lime: Due to its larger surface area, fine granular lime offers a higher CO₂ absorption capacity than coarse porous lime and would therefore be preferred.

• Breathing gas density: With increasing diving depth, the breathing gas becomes denser, which increases breathing resistance. A finely granular lime leads to a more densely packed canister and therefore also to a higher breathing resistance.

The simulation shows that a maximum breathing gas density of 6 g/l should not be exceeded and that a lime that is not too finely granular should be selected so that the breathing resistance does not get too high, as in emergency situations, when large breathing volumes are required, the breathing resistance could no longer be overcome. (N.B. A breathing gas density of 6.0 g/dl is exceeded at a pO2 setpoint of 1.3 bar and air as diluent from a depth of 41 m).

A long canister runtime is of no use if you can't breathe!

The lime product and quality control

The simulation was calculated with lime granules of size 8 to 12. This product corresponds to the most commonly used lime, Sofnolime 797, as its granule sizes are similar to the mesh size 8 to 12. However, the question of the exact size distribution of the lime particles in each charge remains crucial. NATO checks the quality of the lime using sieve analysis and creates a granularity distribution curve in order to accurately predict the CO₂ absorption capacity and breathing resistance. If the lime is too coarse, the CO₂ absorption capacity decreases; if it contains too many fine granules, the breathing resistance may be higher than expected. However, we normal consumers do not know the variation range of a lime product.

Plan your dive, dive your plan!

Conclusion

It is therefore crucial to choose the parameters that you can influence - in this case the breathing gas density. A low-density breathing gas is essential for deep dives with rebreathers. Therefore, “the deeper the more helium” applies, which in turn has an effect on the decompression time and the running time of the scrubber.

Solutions a) to c) are therefore correct - because even the best canister runtime is useless if the breathing resistance is too high to be able to breathe.