It’s hard to tell if technology moves fast or slow in the world of diving. It’s easy to see incremental improvements to equipment and the growing adoption of technologies like rebreathers or the use of trimix.
Many of the challenges that underpin sports and technical diving have been largely solved problems for over 70 years. It has just taken time for them to permeate from commercial and military diving into mainstream diving practices. Fundamental changes are nowadays pretty rare.
So it piqued my interest hearing that a group of divers had successfully tested a gas that had long been shelved for being too dangerous: hydrogen.
Why would any diver be tempted to add a gas best known for zeppelin explosions into their breathing mixes? So first, it’s important to understand what problems the current combinations of breathing are trying to fix and what challenges remain as we descend deeper.
In the Beginning
As early as 1835 it was observed that breathing air at increased pressures resulted in the feeling of being intoxicated. Nitrogen narcosis. It’s drilled into every trainee diver but many will never experience its real impact diving on air below 50m (thankfully it’s gone out of fashion). The effect quickly moves from a light feeling of inebriation (the classic example of a couple of beers) to significant sensory impairment as you progress deeper.
Removing some or all the nitrogen out of air was necessary but what to replace it with? Helium was the solution. It has practically no narcotic potential when breathed at increased pressures making it ideally suited to replace the nitrogen in air. It also has the added benefits of being inert and very light.
The density of a gas becomes important at extreme depths. The higher the density of the breathing gas the harder the body has to work to breath. It can reach a point where the body is effectively producing more carbon dioxide than it can ventilate out – causing hypercapnia. The result can be fatal.
The US Navy were the first to pioneer the use of helium in the mid-1930s and demonstrated its benefits successfully salvaging the submarine USS Squalus from 75m.
Its use grew in commercial and finally into mainstream technical diving in the late 1980s. Commercial and military divers predominantly use heliox (helium-oxygen) a mix that totally removes nitrogen and any narcotic potential. Technical divers, on the other hand, favour the more common trimix (oxygen-helium-nitrogen) as it actually reduces decompression times and is more cost effective (more on this later).
So why hydrogen?
The earliest recorded instance of using hydrogen in a diving mix was by Swede Arne Zetterstrom in the 1940s. This innovation was originally driven by necessity – helium was not available outside the USA.
Helium, at the scale of the universe, is abundant but ironically on earth it’s pretty hard to find. It cannot be synthesised and the only viable source is produced as a byproduct of the radioactive decay of Uranium or Thorium. If this decay happens in the right place, like into a gas well, it can be tapped off and refined.
The US was the only source of helium at this time and due to its scarcity and important applications (airships and barrage balloons) it was not allowed to be exported. Zetterstrom had mixed results with his trials, dying on his final test dive (though hydrogen was believed to have been a factor in the accident). It looked like a developmental dead end.
As further reserves were found and restrictions lifted, helium’s use as a diving diluent grew. Advances in saturation diving techniques pushed dives ever deeper exposing a novel physiological limitation of helium. Divers breathing helium mixes below 150m were experiencing debilitating tremors, dizziness and nausea. Scientist termed the condition High Pressure Nervous Syndrome (HPNS).
Research into the condition suggested that there are two methods to ameliorate its onset: reducing the rate of compression (i.e. rate of descent) or add another diluent (such as nitrogen to spike the mixture).
Proof of concept
French undersea engineering company COMEX pioneered hydrogen research from the late 1960s onwards. Testing it as both a sole diluent (hydrox) or addition to a heliox mix (hydreliox). Hydrogen has a number of benefits to the Uber-deep diver: it’s abundant, relatively inexpensive and even lighter than helium. It obviously has one major explosive drawback but it also has a similar drawback to nitrogen: it’s narcotic. In fact, It’s calculated to have twice the narcotic potential as nitrogen.
The world has a finite reserve of helium. You can’t make it and you’d have to wait geological timescales for any more to produced naturally. This restricted supply and an ever growing demand in wider industry has led to repeated stories declaring the world is running out of helium. Technical divers feel this as the price of helium rises and supply struggles to keep up.
Helium is expensive, far more so than hydrogen. However, this isn’t a particularly accurate description as the demise of helium reserves are greatly exaggerated (the Earth’s supply seems pretty robust). In reality rising prices stimulates the supply side making it more attractive to capture and refine it from existing sites.
COMEX’s experience proved hydrogen’s theoretical potential with the company conducting open water dives to 534m and, in 1992, a simulated chamber dive to 701m. This was the high-water mark (or deep water?) for research into hydrogen as the need for deep commercial diving reduced as the increased use of remotely operated vehicles (ROVs) stymied any future research.
Present Day
Today it’s technical divers and not companies or militaries pushing the limits of human physiology. In 2023 the Wetmules, a team of technical divers, successfully tested hydrogen as a diluent on a rebreather down to 230m. Dr Richard Harris conducted the test, best known as the doctor that assisted with the Thai cave rescue. The motivation was to try and prove a workable solution to the dual threat of the increased work of breathing and HPNS at extreme depths.
Hydrogen was seen as a potential tool to solve a particular issue: how to explore deeper into the cave system of The Pearse Resurgence, New Zealand.
The explosive risk of hydrogen had to be carefully mitigated both preparing and during the dive itself. If the oxygen content of a mix were to rise above 5% even static could be an ignition source. This risk is even harder to manage when using a rebreather: given the changing nature of breathing mixes on ascent and descent.
The dive was a success and a significant milestone in the technical diving community. However, as Harris himself noted a single successful ultra-deep dive does not prove the overall safety of hydrogen as a breathing gas. Though It is certainly a first step.
Final Thoughts
We’re not going to be seeing cylinders of hydrogen on dive boats any time soon. It’s potential application is niche, aiming to solve particular problems at extreme depths and only useful to 0.0001% of divers. For them its a potential tool in the locker.
The reason it fascinates me is it shows there are still areas of genuine technological and scientific advancement in the world of diving.
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