Tag: NMRI

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.

Retrospection

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.

Knock Yourself Out (Carbon Dioxide – The Diver’s Nemesis)

Most rebreather divers start off their diving career with open-circuit diving; that is, with scuba. And some of them pick up bad habits. I happen to be one of those divers.

With scuba you start the dive with a very limited amount of air in your scuba bottle. New divers are typically anxious, breathe harder than they have to, and blow through their air supply fairly quickly. More experienced divers are relaxed and enjoy the dive without anxiety, and thus their air bottles last longer than they do with novice divers.

So early in a diver’s experience he comes to associate air conservation with a sign of diver experience and maturity. When you are relaxed and physically fit, and your swimming is efficient, your breathing may become extraordinarily slow. Some call it skip breathing — holding your breath between inhalations.

I was once swimming among the ruins of Herod’s Port in Caesarea, and my dive buddy was a Navy SEAL. I started the dive under-weighted, so I picked up a 2000 year old piece of rubble and carried it around with me as ballast. In spite of the very inefficient style of swimming which resulted, my air supply still lasted longer than that of my SEAL buddy.

At first I was annoyed that I had to end the dive prematurely, but then I began to feel somewhat smug. I had used less air than a frogman.

As a physiologist I knew that I may well have been unconsciously skip breathing, which would have raised my arterial carbon dioxide level, potentially to a dangerous level. But all ended well, and I could not help being glad that I was not the one to call the dive.

It is important for rebreather divers to understand that they don’t have to be breathing elevated levels of carbon dioxide to run into physiological problems with carbon dioxide. It’s the carbon dioxide in your arterial blood that matters. It can render you unconscious even when you’re breathing gas with no carbon dioxide at all.

MK 16 rebreather diver

Normally the body automatically ensures that as you work harder, and produce more carbon dioxide in your blood stream, that you breathe more, forcing that CO2 out of your blood, into the lungs, and out through your mouth. It works like an air conditioner thermostat; the hotter it gets in the house, the more heat is pumped outside. In other words, arterial and alveolar CO2 levels are controlled by automatic changes in ventilation (breathing.) In fact you can predict alveolar levels of CO2 by taking the rate at which CO2 is being produced by the body and dividing it by the ventilation rate. This relationship is called the Alveolar Ventilation Equation, or in clinical circles, the PCO2 Equation.

Normally, CO2 production and ventilation is tightly controlled so that normal alveolar and arterial CO2 is about 40 mmHg, mmHg being a unit of so-called partial pressure. 40 mmHg of arterial CO2 is safe. [One standard atmosphere of pressure is 760 mmHg, so ignoring the partial pressure of water vapor and other gases, a partial pressure of 40 mmHg of CO2 is equivalent to exhaling about 5% carbon dioxide.]  

When a diver is working hard while breathing through a breathing resistance like a rebreather, as ventilation increases respiratory discomfort goes up as well. For most people, when the respiratory discomfort gets too high, they quit working and take a”breather”. But there are some divers who hate respiratory discomfort, and don’t mind high levels of arterial CO2. We call these people CO2 retainers.

Navy experimental deep sea divers; photo credit: Frank Stout

As an example, I once had as an experimental subject a physically fit Navy diver at the Naval Medical Research Institute during a study of respiratory loading. The test was conducted in a dry hyperbaric chamber under the same pressure as that at 300 feet of sea water. The experimental setup in the chamber looked somewhat like that in the figure to the right although the diver I’m talking about is not in this photo.

The diver was exercising on the bicycle ergometer while breathing through a controlled respiratory resistance at 300 feet in a helium atmosphere. The diver quickly learned that by double breathing, starting an inspiration, stopping it, then restarting, he could confuse the circuitry controlling the test equipment, thus eliminating  the high respiratory loading.

As he played these breathing pattern games my technician was monitoring a mass spectrometer which was telling us how high his expired CO2 concentration was going. The exhaled CO2 started creeping up, and I warned him that he needed to cut out the tricky breathing or I’d have to abort the run.

The clever but manipulative diver would obey my command for a minute or so, and then go back to his erratic breathing. He joked about how he was tricking the experiment and how he felt fine in spite of the high CO2 readings.

That was a mistake.

When you’re talking, you’re not breathing. Since his breathing was already marginal, his end-tidal CO2, an estimate of alveolar CO2, shot up in a matter of seconds from 60 to 70 and then 90 mmHg, over twice what it should have been. When my technician told me the diver’s exhaled CO2 was at 90 mmHg, I yelled “Abort the run”. But the diver never heard that command. He was already unconscious and falling off the bike on his way to the hard metal decking inside the hyperbaric chamber.

The diver thought he was tricking the experiment, but in fact he was tricking himself. Although he felt comfortable skip breathing, he was rapidly pedaling towards a hard lesson in the toxicity of carbon dioxide.

Keep in mind, this diver was breathing virtually no carbon dioxide. His body was producing it because of his high work level, and he was simply not breathing enough to remove it from his body.

In upcoming posts we’ll look at what happens when inspired CO2 starts to rise, for instance due to the failure of a carbon dioxide scrubber canister in a rebreather. I already gave you one example in the CO2 rebreathing study of my first post in this series. There’s lots more to come.