Hydrogen Diving: The Good, The Bad, the Ugly

In the preceding blog post, I reminded the reader that the Earth’s supply of helium is limited. It is not a renewable resource.

Being a diving professional, I am not concerned about the consequence of a helium shortage on party balloons. But I am thinking about the potential consequences on diving.

So, knowing that hydrogen has both good and bad traits, it would be prudent to begin thinking about whether or not there is a way to safely substitute hydrogen for helium in technical, scientific, commercial and military diving.

Perhaps the word “bad” is too much of an understatement. Perhaps “horrible” would be a better descriptor for something like the Hindenburg disaster.

With that sobering reminder of what can happen, we now cautiously move on to the science.

First, we begin with the explosion hazard of hydrogen in binary mixtures of hydrogen and oxygen.

For diving in the 10 to 20 bar range, 326 to 653 fsw range, the upper explosion limit is 94.2 molar percent. So that means that if a binary gas mixture contains 96% hydrogen and 4% oxygen, it should not explode when ignited.

Those underlined words are important. An explosive mixture of hydrogen and oxygen will not explode without an ignition source. Proof of that is exhibited in many college introductory chemistry lectures, and documented in the following YouTube video.

Arne Zetterström

As a forecast of our potential future, during World War II, Sweden was deprived of a ready source of helium coming from the U.S. and elsewhere. So, the clever and industrious Arne Zetterström conducted a series of experimental deep, hard hat dives from 1943 to 1945 using a mixture of 96% hydrogen and 4% oxygen on dives ranging from 12 to 17 bar.

Once at depth, Zetterström switched from a non-hydrox gas mixture to the “hydrox” gas mixture. His initial test dive was to 111 msw (362 fsw, 12 bar), progressing through six dives to a maximum depth of 160 msw (522 fsw, 17 bar).

That dive series was successful. Unfortunately, on the last dive on 7 August 1945, Zetterström died tragically when his dive tenders mistakenly pulled him directly to the surface from the bottom depth of 522 fsw. He died from fulminant decompression sickness.

From the above table we see that modern measurements confirm that Zetterström chose his gas mixes wisely. At a 96 mol% of hydrogen, he was above the upper explosion limit. If there had been an unexpected ignition event, his breathing gas mixture would not have exploded.

I have confirmed the oxygen partial pressure for Zetterström’s dives using PTC Mathcad Express 3.1 and will share the process.

First, I show pressure conversions familiar to Navy divers and diving scientists, but not known to most others.

For Zetterström’s 111 msw (362 fsw) dive, the partial pressure of oxygen (PO2) would have been 0.478 atm, at the top end of the target range (0.4 to 0.48) for U.S. Navy chamber oxygen atmosphere during saturation diving. A PO2 of 0.48 is believed to be the highest PO2 tolerated for extended periods. Saturation dives sometimes last over a month.

For Zetterström’s 6th and last dive, to 160 msw (522 fsw), the oxygen partial pressure was 0.7 ata, about half of what it normally is in modern electronic rebreathers with fixed PO2.

A far more detailed story of the Zetterström Hydrox dive series can be found in this book.

Arne Zetterström Memorial Dive

In 2012, the Swedish Historical Diving Society and the Royal Institute of Technology (KTH) Diving Club, Stockholm, conducted an Arne Zetterström Memorial dive to a relatively shallow depth of 40 msw or 131 fsw. The original 96% – 4% ratio of hydrogen and oxygen was maintained, resulting in a gas mixture with a PO2 of 0.20 atm.

As reported in the KTH Dive Club’s Dykloggen (dive log) report of July 2012, the team lead was Ola Lindh, Project Leader and Diver. Åke Larsson, another diver, contributed the following information about that dive.

The Hydrox divers used open circuit scuba, with back mounted air, and for decompression, bottles of hydrox and oxygen.

The Swedish divers did not go deeper than 131 feet because they were just above the mud at that depth in a quarry. Plus, they did not yet have details of Zetterström’s decompression plan for deeper diving.

Today, they do possess the wartime hydrogen decompression plan, so deeper hydrogen dives may be forthcoming.

Three gas mixtures – hydrogen, and air (nitrogen and oxygen)

When you mix an inert gas like nitrogen (or perhaps helium?) with hydrogen and oxygen mixtures, that greatly reduces the explosion hazard. But as this video shows, sooner or later the ratios might change enough to become explosive.

Naval Medical Research Institute

I spent 12 years working as a diving biomedical researcher at the Naval Medical Research Institute (NMRI) in Bethesda, MD.

Main entrance to the Albert R. Behnke Diving Medicine Research Center, at NMRI.

My laboratory was in the Behnke Diving Medicine Research Center building, but the hyperbaric hydrogen facility was situated a safe distance behind the main building. In the unlikely event of an explosion, the main Behnke facility and its hyperbaric chamber complex would be preserved.  

The hyperbaric hydrogen facility was used to test the effects of high-pressure hydrogen and biochemical decompression on pigs, rather than risk human divers. And all of that was done safely, thanks to the professionalism of Navy divers and scientists.

Dr. Susan Kayar checking on the hydrogen diving pigs.

Kayar, a member of the Women Divers Hall of Fame, used at 230 msw (751 fsw) a gas mixture of 88% hydrogen, 2% oxygen, balance helium with a slight amount of nitrogen. That 88% hydrogen mixture put the gas mixture well above the 71.3% upper explosion limit for three gas components at 24 bar pressure. The resulting PO2 was 0.5 ata.

Compagnie Maritime d’Expertises (COMEX)

COMEX and their human-rated hyperbaric chambers are located in Marseilles, France.

When it came to manned hydrogen diving, the effect of hydrogen narcosis forced COMEX to operate below the upper explosion limit during its long series of experimental hydrogen dives.

In 1985, COMEX’s Hydra V was the first manned hydrogen dive to 450 msw. Hydrogen fraction was 54%, helium fraction was 45%, and oxygen fraction 1%. PO2 was a nominal 0.45 atm, the same partial pressure used by the U.S. Navy for saturation dives.

In 1988 during Hydra VIII, the first open water hydrogen dive, the depth was 534 msw, or 1752 fsw. Hydrogen fraction was 49%, helium fraction was 50%, and oxygen fraction 1%. The resulting oxygen partial pressure was 0.54 atmospheres.

The following video documents the record-breaking Hydra VIII dive.

The 534 msw Hydra VIII depth record was broken by Hydra X, a 701 msw, 2300 fsw chamber dive. The gas mixture was the same as in Hydra VIII, hydrogen fraction 49%, helium 50%, and oxygen percentage 1%. Due to the increase in depth, PO2 rose to 0.7 atm, an oxygen partial pressure frequently used in older U.S. Navy rebreathers.

The head of the Diving Medicine Department at NMRI, CAPT Ed Flynn, M.D. (glasses and grey hair sitting on the right side of the console), was performing physiological studies on both Hydra VI and VIII. In essence he was the Patron Saint of the NMRI Hydrogen Research Facility.

Shallow Hydrogen Diving

What have the previous studies taught us? Well, for one thing, the Swedes showed in their Arne Zetterström Memorial dive that you can get away with oxygen concentrations close to normoxia, PO2~0.21 ata. The disadvantage of normal atmospheric partial pressures of oxygen, compared to higher pressures, is related to decompression time. There is a decompression advantage when breathing oxygen pressures of 1.3 to 1.45 ata. Virtually all modern electronic rebreathers use those oxygen pressures for that reason. But as the KTH Dive Club showed, hydrogen decompression can be safely handled at relatively shallow depths.

For recreational divers, there is an economic advantage for reducing helium usage by substituting nitrogen. We don’t yet know what the economic and safety comparison would be when using helium diluted hydrogen versus pure hydrogen.

Hydrogen, helium, and oxygen were the standard gases used by COMEX. But they were likely chosen to lessen hydrogen toxicity. Hydrogen toxicity would not be a problem at shallow depth. And in fact, the KTH Dive Club reported no toxicity problems.


As proud as I have been of the record-breaking COMEX hydrogen research program, and of the highly imaginative U.S. Navy hydrogen research program, it has not been lost on me that the first deep human hydrogen dives were conducted by an undoubtedly low-cost program led by a single Swedish Naval Officer, Arne Zetterström.

Now, I find it remarkable that the people testing hydrogen diving at relatively shallow depths, would also be Swedish. Unlike the COMEX and NMRI projects described above, I suspect the KTH Dive club was not sponsored by multimillion dollar programs.

You have to admire the Swedish chutzpah.

Disclaimer: The author is no longer employed by the Navy or Department of Defense. All opinions are my own, and not those of any government agency. This document is posted purely for historical and educational interest. At risk of violent death, under no circumstances should the reader be tempted to explore the production, storage, or use of hydrogen without thorough and certified safety training.