In February 2023, the “Wetmules” diving team performed the world's first rebreather dive with a hydrogen-based gas mixture, Helihydrox (3% oxygen, 38% hydrogen, 59% helium). A depth of 230 m was reached in a cave in the Pearse Resurgence in New Zealand. The scientific publication is now available. I am happy to summarize it for the Dekoblog audience.
Background
The Wetmules dive team reached a depth of 245 m in the Pearse Resurgence in 2020 using Trimix 4/91 (4% oxygen, 91% helium and 5% nitrogen). Greater depths would have to be reached in order to explore the cave, with two key problems arising in this regard:
With increasing depth, the density of the breathing gas increases due to the rising ambient pressure, which increases breathing resistance and the risk of CO2 retention. At a depth of 245 meters, the gas density was around 7.2 g/l, above the safe upper limit of 6 g/l. This can lead to CO2 intoxication with (sudden) loss of consciousness and an increased risk of cerebral oxygen toxicity, as "CO2 retention almost certainly increases ... cerebral oxygen toxicity".
Another problem is HPNS (High-Pressure Neurologic Syndrome), which occurs at depths of around 150 m and manifests as tremors in the extremities and the trunk and can severely affect divers. It is caused by the high partial pressure of helium and can be alleviated by adding a small amount of nitrogen, which has a slightly narcotic effect. However, nitrogen increases the breathing gas density, which is why it makes even deeper dives impossible.
The solution is a gas that is both light and slightly narcotic, like hydrogen. Hydrogen has been used in various experiments, such as the COMEX Hydra experiments from the late 1960s onwards, to depths of 701 meters. It can be inhaled safely and alleviates HPNS symptoms. However, above a volume fraction of 4.2 - 6% in helium-oxygen mixtures, there is a risk of explosion, especially when electrical equipment is involved like the solenoids in rebreathers. In addition, hydrogen increases the risk of hypothermia due to its high thermal conductivity.
The dive
Trimix 4/91 was used down to a depth of 200 m (PO2 setpoint 0.7 bar). There, the diver initiated the switch to hydrogen by exhaling a small amount of breathing gas into the water and replacing it with hydrogen from a 2-liter carbon fiber cylinder carried for this purpose. In this way, hydrogen was gradually introduced into the rebreather's breathing circuit, resulting in a mixture containing approx. 38% hydrogen at a depth of 230 meters.
After the changeover, the diver did not notice any adverse effects or changes in temperature sensation. Above all, he did not feel any narcosis. The symptoms of HPNS also vanished completely after the change to hydrogen.
During the ascent from dive minute 25, hydrogen was gradually flushed out of the system from a depth of 200 m (dive minute 27) and replaced with the original Trimix while increasing the oxygen setpoint to 1.3 bar.
Above a depth of 150 m, only trimix was used for decompression calculations. Hydrogen was not included due to the lack of established protocols and the short exposure time (11 minutes below 200 m, another 8 minutes until complete washout from the loop). The dive was completed after 13.5 h, including decompression stops in 3 habitats at 27 m, 16 m and 7 m.
Discussion
During the ascent from dive minute 25, hydrogen was gradually flushed out of the system from a depth of 200 m (dive minute 27) and replaced with the original Trimix while increasing the oxygen setpoint to 1.3 bar.
Above a depth of 150 m, only trimix was used for decompression calculations. Hydrogen was not included due to the lack of established protocols and the short exposure time (11 minutes below 200 m, another 8 minutes until complete washout from the loop). The dive was completed after 13.5 h, including decompression stops in 3 habitats at 27 m, 16 m and 7 m.
There were no obvious negative physiological effects of hydrogen breathing during or after the dive. The diver felt no narcotic effect or discomfort due to cooling, which is consistent with the aforementioned earlier experiments. There was also no decompression sickness. However, the authors acknowledge that the hydrogen exposure was relatively short and that a longer exposure could pose other challenges, particularly in terms of decompression stress and hypothermia.
Conclusion
The publication concludes with cautious optimism regarding the use of hydrogen in deep rebreather diving. While the dive is promising in terms of HPNS attenuation and gas density management, cautious progress and further research are essential in the future. The handling of hydrogen in oxygen mixtures under high pressure remains critical due to the risk of explosion.
Meanwhile...
... the same diver reached new depths in a 2nd and 3rd dive with rebreather hydrogen breathing. In the notorious South African Boesmansgat he reached 284 m accompanied by a buddy who also breathed hydrogen. This means that 4 rebreather dives with hydrogen have been carried out to date.
The fact that both divers developed severe decompression sickness during the last dive shows that the venture is not without risk. This brings the morbidity index to 50% (2 decompression incidents in 4 dives). It is an open question whether the altitude of Boesmansgat (> 1500 m above sea level) contributed to this. The decompression obligation at this altitude corresponds to a dive at a depth of 337 m at sea level.
Things remain exciting - and dangerous.
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