Rebreather Forum 4, held in 2023 in Malta was yet another Herculean undertaking by Michael Menduno, the executive editor of Global Underwater Explorer’s (aka GUE) InDepth magazine and various international publications (see the above link for details.) Michael was the journalist who coined the term ‘technical diving”, and also created Aquacorp Magazine.
I’ve known and appreciated Michael for decades and gave presentations at his Rebreather Forums 2 and 3. My role on the RF4 Science Panel was to help select the best speakers for the Forum, to herd cats when necessary, and to speak.
The Book
Of course, with a new book (basically a textbook) on one of the most mysterious and technical aspects of rebreather diving, I spoke on the subject of “Demystifying Scrubbers”.
Photo courtesy of Rosemary Lunn
Breakthrough can be found on Amazon in paperback or Kindle, or elsewhere in ePub format.
Photo courtesy of Axel Schoeller (1963-2023)
Presentation Description
Here, I’ve borrowed Menduno’s own words describing my talk.
Plunge into the intricate world of rebreather diving safety and techniques with retired scientific director of the US Navy Experimental Diving Unit (NEDU) and author, Dr. John Clarke, who illuminates the inner workings of rebreather CO2 scrubbers, based on extensive research work by the US Navy and his own modeling efforts. This talk unveils the critical aspects of CCR diving gear selection, especially focusing on the importance of rebreather CO2 scrubbers, soda lime quality, and the proper maintenance of canister duration to ensure diver safety.
From the influence of cold water on CCR diving equipment to the vital role of physiological variation among divers, discover how individual differences significantly impact rebreather canister duration and CO2 absorption rates. Engage with real-life scenarios and experiments that shed light on the potential dangers of compromised soda lime; explore the implications of using indicating soda sorb without proper Navy notification (leading to catastrophic failures in rebreather functionality). Moreover, the discussion covers simulated physical models that offer insights into the dynamics of CO2 absorption in rebreather diving. The models emphasize the importance of understanding your CCR diving equipment and recognizing when changes, no matter how slight, in soda lime granule size or distribution might signal a risk. This video is an essential watch for any rebreather diver seeking to deepen their safety knowledge, highlighting the blend of technical expertise, physiological awareness, and practical vigilance required to navigate the underwater world securely.
All of the RF4 presentations are available at this RF4 link. It is chock full of the best information available, from the best presenters in the field of Rebreather Diving.
Heisenberg’s Uncertainty Principle applied to quantum events avows that there is no certainty until you look. Well, this morning, I looked, and I’m just as confused as ever.
It was a chilly morning in late November. As we warmed up with coffee, I wondered how cold it was outside. So, in the modern style, my wife and I checked the Weather Channel on our phones. One indicated it was 47°F, but the other showed it was 48°F.
That can’t be, I said. So, with identical phones side by side, both tuned into Panama City Beach, Florida weather on the Weather Channel, one phone said it felt like 45°F, and the other said it felt like 43°F.
As Charlie Brown would say, “Good grief.”
Wanting to find some agreement among our devices, I checked a nested set of humidity and temperatures sensors grouped together in our kitchen. Humidity indicators are notoriously inaccurate, yet amazingly, the measured humidity was in reasonable agreement. But inside temperature varied from 70.3°F to 72.8°F.
According to Segal’s Law, “A man with a watch knows what time it is. A man with two watches is never sure.”
This aphorism is falsely attributed to Lee Segall of KIXL, now KGGR in Dallas. Regardless of the source, it is often repeated because it makes such good sense. If you multiply the number of devices three times, as above, the situation is no more precise. (But that’s where statistics comes in, I suppose.)
Giving up on simple things like local environmental parameters, I turned to the latest news on the VAERs update for the vaccines.
I wish I hadn’t. Yes, there is a chance you’ll be fine, but there’s also a small chance you’ll have heart problems and even a small chance you’ll die.
Frankly, my one-time shot at slot machines and the roulette table in Vegas did not end well. So, is there anything we know that can be guaranteed accurate?
Diving
I’ve spent a long Navy career in diving science, so I know there are serious certainties there. If you consume more air than is in your scuba tank, you’ll drown. If you stay down too long and surface too quickly, you’ll get the bends, aka decompression sickness.
But what if I use a decompression computer to plan my dive and follow its guidance to the letter? Unfortunately, there’s still a chance you’ll end up in a treatment chamber. Both people’s health and the water environment change constantly, and no decompression algorithm is perfect, or omniscient.
From an engineer’s perspective, the tensile strength of a bolt is known within strict limits. If the force applied to that bolt exceeds its limits, then bad things might happen. Buildings might fall, or planes might crash. Or your muffler might fall off.
It’s hard to know what the effect of a broken bolt will be unless you understand precisely the function of that bolt. There is uncertainty in the outcome of a bolt breaking.
Uncertainty vexes some engineers to no end. I’ve watched them squirm as I reveal the role of statistics and probability in acceptance decisions about diving equipment. People are not bolts whose tensile and shear strength can be measured. As Heisenberg predicted (out of context), a dive outcome cannot be predicted with certainty.
Equipment Testing
The same thing applies to diving equipment. The Navy Experimental Diving Unit is entrusted with determining the safety and suitability of underwater breathing apparatus. Both physiologists and engineers envision a line in the sand for a given water depth and diver breathing rate.
If a UBA exceeds that line during testing, it should be rejected for military use. Right? After all, a limit is a limit.
Test and Evaluation Laboratory, Navy Experimental Diving Unit, Panama City, Florida. Photo by Stephen Frink.
Well, not exactly. When translating engineering limits into human terms, things get messy. If a published limit is exceeded, just like taking the COVID vaccine, some people will fare well, while others may pass out. In other words, failure is classified as the probability of an untoward event where untoward translates to anything that threatens a diver or a diving mission.
For any given dive, and any given diver, the probability of a dive failure cannot be known precisely. Dive failure, like decompression sickness, is probabilistic.
One type of untoward event: Ice inside a second-stage scuba regulator.
Usually, a UBA evaluated at NEDU is suitable for most diving depths and any foreseeable work/ventilation rate, as shown in Table 1.
The only time that limits were exceeded was at the greatest depth and ventilation rate.
But what if the data had revealed a slightly larger “out of limits” region, as in the next table? What decision regarding safety would then be made?
Hypothetical Test Data
In this hypothetical case, human judgment is required. It is not sufficient to declare the diving equipment unsafe for use. It simply means divers need to pace themselves when working and breathing hard near a depth of 200 feet. Reducing their workload enough to slow their breathing to 62 liters per minute or less (still a high ventilation rate) is a safe way to keep the UBA within limits.
This is nothing new. Every salvage diver knows to occasionally interrupt hard work periods with periods of rest. Catching your breath is kind of important.
Limits are not absolute
As a person with too many watches, or thermometers can attest, you can’t be sure what all the various goal numbers and limit numbers mean. Instead, collectively they should be used as a guide to safe diving.
Hanging on the umbilical in a KM 37.
Whether you’re a sport diver or professional, if an underwater breathing apparatus is functioning normally but doesn’t meet all of the EU (EN250) or U.S. Navy engineering limits under all possible testing conditions, that doesn’t mean it’s not a useful piece of diving gear. You just have to use it judiciously. After all, good human judgment is always required for safely operating life support equipment.
It is a wise diver who remains mindful of their life support system’s limitations and plans their dive to stay within those limitations. That way, the probability of experiencing an untoward event is minimized.
Blood pressure is not the only silent medical killer. Hypoxia is also, and unlike chronically elevated blood pressure, it cripples within minutes, or seconds.
Hypoxia, a condition defined by lower than normal inspired oxygen levels, has killed divers during rebreather malfunctions, and it has killed pilots and passengers, as in the 1999 case of loss of cabin pressure in a Lear Jet that killed professional golfer Payne Stewart and his entourage and aircrew. Based on Air Traffic Control transcripts, that fatal decompression occurred somewhere between an altitude of 23,000 feet and 36,500 ft.
In most aircraft hypoxia incidents, onset is rapid, and no publically releasable record is left behind. The following recording is an exception, an audio recording of an hypoxia emergency during a Kalitta Air cargo flight.
Due to the seriousness of hypoxia in flight, military aircrew have to take recurrent hypoxia recognition training, often in a hypobaric (low pressure) chamber.
As the following video shows, hypoxia has the potential for quickly disabling you in the case of an airliner cabin depressurization.
Aircrew who must repeatedly take hypoxia recognition training are aware that such training comes with some element of risk. Rapid exposure to high altitude can produce painful and potentially dangerous decompression sickness (DCS) due to the formation of bubbles within the body’s blood vessels.
In a seminal Navy Experimental Diving Unit (NEDU) report published in 1991, LCDR Bruce Slobodnik, LCDR Marie Wallick and LCDR Jim Chimiak, M.D. noted that the incidence of decompression sickness in altitude chamber runs from 1986 through 1989 was 0.16%, including both aviation physiology trainees and medical attendants at the Naval Aerospace Medical Institute. Navy-wide the DCS incidence “for all students participating in aviation physiology training for 1988 was 0.15%”. If you were one of the 1 and a half students out of a thousand being treated for painful decompression sickness, you would treasure a way to receive the same hypoxia recognition training without risk of DCS.
With that in mind, and being aware of some preliminary studies (1-3), NEDU researchers performed a double blind study on twelve naïve subjects. A double-blind experimental design, where neither subject nor investigator knows which gas mixture is being provided for the test, is important in medical research to minimize investigator and subject bias. Slobodnik was a designated Naval Aerospace Physiologist, Wallick was a Navy Research Psychologist, and Chimiak was a Research Medical Officer. (Chimiak is currently the Medical Director at Divers Alert Network.)
Three hypoxic gas mixtures were tested (6.2% O2, 7.0% and 7.85% O2) for a planned total of 36 exposures. (Only 35 were completed due to non-test related problems in one subject.) Not surprisingly, average subject performance in a muscle-eye coordination test (two-dimensional compensatory tracking test) declined at the lower oxygen concentrations. [At the time of the testing (1990), the tracking test was a candidate for the Unified Triservice Cognitive Performance Assessment Battery (UTC-PAB)].
As a result of this 1990-1991 testing (4), NEDU proved a way of repeatedly inducing hypoxia without a vacuum chamber, and without the risk of DCS.
The Navy Aerospace Medical Research Laboratory built on that foundational research to build a device that safely produces hypoxia recognition training for aircrew. That device, a Reduced Oxygen Breathing Device is shown in this Navy photo.
Whidbey Island, Wash. (Feb 16, 2007) Lt. Cmdr. James McAllister, from San Diego, Calif. sits in the simulator during a test flight using the Reduced Oxygen Breathing Device (ROBD). The ROBD is a portable device that delivers a mixture of air, nitrogen and oxygen to aircrew, simulating any desired altitude. Combined with a flight simulator the ultimate effect replicates an altitude induced hypoxia event. McAllister is the Director of the Aviation Survival Training Center at Whidbey Island. U.S. Navy photo by Mass Communication Specialist 1st Class Bruce McVicar.
Although NEDU is best known for its pioneering work in deep sea and combat diving, it continues to provide support for the Air Force, Army and Marines in both altitude studies of life-saving equipment, and aircrew life support systems. Remarkably, the deepest diving complex in the world, certified for human occupancy, also has one of the highest altitude capabilities. It was certified to an altitude of 150,000 feet, and gets tested on occasion to altitudes near 100,000 feet. At 100,000 feet, there is only 1% of the oxygen available at sea level. Exposure to that altitude without a pressure suit and helmet would lead to almost instantaneous unconsciousness.
A test run to over 90,000 feet simulated altitude.
Herron DM. Hypobaric training of flight personnel without compromising quality of life. AGARD Conference Proceedings No. 396, p. 47-1-47-7.
Collins WE, Mertens HW. Age, alcohol, and simulated altitude: effects on performance and Breathalyzer scores. Aviat. Space Environ Med, 1988; 59:1026-33.
Baumgardner FW, Ernsting J, Holden R, Storm WF. Responses to hypoxia imposed by two methods. Preprints of the 1980 Annual Scientific Meeting of the Aerospace Medical Association, Anaheim, CA, p: 123.
I once met the Father of the U.S Remote Viewing program, unawares.
A decade ago, at the request of a Navy engineer who ended up being a character in my novel Middle Waters, I invited Dr. Harold E. Puthoff into the Navy Experimental Diving Unit to give a talk on advanced physics. He had attracted a small but highly educated and attentive crowd which, like me, had no idea that the speaker had once led the CIA in the development of its top secret Remote Viewing program.
Puthoff is the Director of the Institute of Advanced Studies at Austin, in Texas, but before that, and more germane to this discussion, Puthoff was a laser physicist at the Stanford Research Institute. It was there that the CIA chose him to lead a newly created Remote Viewing program, designed to enable the U.S. to maintain some degree of competiveness with Russia’s cold war psychic spying program.
6800 feet down in the Desoto Canyon
Psychic spying was purportedly the method used by the two superpowers to visualize things from a distance; not from a satellite, but from what some call the highly developed powers of the mind’s eye. If we believe what we read on the subject, Remote Viewing was eventually dropped from the US psychic arsenal not because it had no successes, but because it was not as reliable as signal intelligence (SIGINT), satellite imagery, and spies on the ground. But, it has been argued, it might be ideal in locations where you can’t put spies on the ground, such as the dark side of the moon, or the deep sea .
Serendipitously, as I started writing this blog post, Newsweek published a review of the Remote Viewing efforts of Puthoff and others in a November 2015 issue. The article seemed fairly inclusive, at least more so than other articles on Remote Viewing I’ve seen, but the Newsweek author was not particularly charitable towards Puthoff. Strangely, the strength and veracity of Puthoff’s science was reportedly criticized by two New Zealand psychologists who, as the Newsweek author quoted, had a “premonition” about Puthoff.
“Psychologists” and “premonitions” are not words commonly heard in the assessment of science conducted by laser physicists, especially those employed by the CIA. The CIA is not stupid, and neither are laser physicists from Stanford.
To the extent that I am able to judge a man by meeting him in person and hearing him talk about physics, I would have to agree with Puthoff’s decision to ignore his ill-trained detractors. Every scientist I know has had detractors, and as often as not those detractors have lesser credentials. Nevertheless, I have the good sense to not debate the efficacy of remote viewing. I don’t know enough about it to hold an informed opinion. However, there seems to be some evidence that it worked occasionally, and for a science fiction writer that is all that is needed.
Nuclear physicist Enrico Fermi.
As my curiosity became piqued by the discovery of the true identity of my guest speaker at NEDU, and as I learned what he had done for the U.S. during the Cold War, I thought of another great physicist, Enrico Fermi, one of the fathers of the atomic bomb. In the midst of a luncheon conversation with Edward Teller, Fermi once famously asked, “Where are they?” The “they” he was referring to, were extraterrestrial aliens.
What became known as Fermi’s Paradox went something like this: with all the billions of stars with planets in our galactic neighborhood, statistically there should be alien civilizations everywhere. But we don’t see them. Why not? “Where are they?”
In most scientists’ opinions, it would be absurdly arrogant for us to believe we are the only intelligent life form in the entire universe. And so ETs must be out there, somewhere. And if there, perhaps here, on our planet, at least occasionally. And that is all the premise you need for a realistic, contemporary science fiction thriller.
But then there is that pesky Fermi Paradox. Why don’t we see them?
Well, they could indeed be here, checking us out by remote viewing, all the while remaining safely hidden from sight. After all, as one highly intelligent Frog once said, humans are a “dangerous species” —fictionally speaking of course.
That “hidden alien” scenario may be improbable, but it’s plausible, if you first suspend a little disbelief. If we can gather intelligence while hiding, then certainly they can, assuming they are more advanced than humans. A technological and mental advantage seems likely if they are space travelers, which they almost have to be within the science fiction genre. Arguably, fictional ETs may have long ago engineered space-time, which could prove mighty convenient for tooling around the galactic neighborhood.
So, if in the development of a fictional story we assume that ETs can remote view, the next question would be, why? Is mankind really that dangerous?
Well, I don’t intend for this post to be a spoiler for Middle Waters, but I will say that the reasons revealed in the novel for why ETs might want to remote view, are not based on fear of humans, but are based on sound science. From that science, combined with a chance meeting with Hal Puthoff, the basic premise of a science fiction thriller was born.
So, to correct what some of my readers have thought, I did not invent the concept of “remote viewing”. It is not fictional; it is real, and was invented and used by far smarter people than myself, or even that clever protagonist, Jason Parker.
The following is reprinted from my article published in ECO Magazine, March 2015. It was published in its current format as an ECO Editorial Focus by TSC Media. Thank-you Mr. Greg Leatherman for making it available for reprinting.
It is the highpoint of your career as an environmentally minded marine biologist. The National Science Foundation has provided a generous grant for your photographic mission to the waters 100 ft below the Ross Ice Shelf, Antarctica. Now you’re on an important mission, searching for biological markers of climate change.
Under Antarctic Ice, photo by Dr. Martin Sayer.
Above you lies nothing but a seemingly endless ceiling of impenetrable ice, 10 ft thick. Having spent the last several minutes concentrating on your photography, you look up and notice you’ve strayed further from safety than you’d wanted. The strobe light marking the hole drilled in the ice where you’ll exit the freezing water is a long swim away. And, unfortunately, your fellow scientist “buddy” diver has slipped off somewhere behind you, intent on her own research needs.
You’re diving SCUBA with two independent SCUBA regulators, but in the frigid cold of the literally icy waters, you know that ice could be accumulating within the regulator in your mouth. At the same time, a small tornado of sub-zero air expands chaotically within the high-pressure regulator attached to the single SCUBA bottle on your back—and that icy torrent is increasingly sucking the safety margins right out of your regulator. You are powerless to realize this danger or to do anything about it.
At any moment, your regulator could suddenly and unexpectedly free flow, tumultuously dumping the precious and highly limited supply of gas contained in the aluminum pressure cylinder on your back. You’re equipped and trained in the emergency procedure of shutting off the offending regulator and switching to your backup regulator, but this could also fail. It’s happened before.
As you try to determine your buddy’s position, you’re feeling very lonely. You realize the high point of your career could rapidly become the low point of your career—and an end to your very being.
The preceding is not merely a writer’s dramatization. It is real, and the situation could prove deadly—as it has in far less interesting and auspicious locations. Regulator free flow and limited gas supplies famously claimed three professional divers’ lives in one location within a span of one month.
There is a risk to diving in extreme environments. However, the U.S. Navy has found that the risk is poorly understood, even by themselves—the professionals. If you check the Internet SCUBA boards, you constantly come across divers asking for opinions about cold-watersafe regulators. Undoubtedly, recent fatalities have made amateur divers a little nervous—and for good reason.
Internet bulletin boards are not the place to get accurate information about life support safety in frigid water. Unfortunately, the Navy found that manufacturers are also an unreliable source. Of course, the manufacturers want to be fully informed and to protect their customers, but the fact remains that manufacturers test to a European cold-water standard, EN 250. By passing those tests, manufacturers receive a “CE” stamp that is pressed into the hard metal of the regulator. That stamp means the regulator has received European approval for coldwater service.
As a number of manufacturers have expensively learned, passing the EN 250 testing standard is not the same as passing the more rigorous U.S. Navy standard, which was recently revised, making it even more rigorous by using higher gas supply pressures and testing in fresh as well as salt water. Freshwater diving in the Navy is rare—but depending on the brand and model of regulator in use, it can prove lethal.
The unadorned truth is that the large majority of manufacturers do not know how to make a consistently good Performing cold-water regulator. Perhaps the reason is because the type of equipment required to test to the U.S. Navy standard is very expensive and has, not to date, been legislated. Simply, it is not a requirement.
Some manufacturers are their own worst enemy; they cannot resist tinkering with even their most successful and rugged products. This writer is speculating here, but the constant manufacturing changes appear to be driven by either market pressures (bringing out something “new” to the trade show floor) or due to manufacturing economy (i.e., cost savings). The situation is so bad that even regulators that once passed U.S. Navy scrutiny are in some cases being changed almost as soon as they reach the “Authorized for Military Use” list. The military is struggling to keep up with the constant flux in the market place, which puts the civilian diver in a very difficult position. How can they—or you—know what gear to take on an environmentally extreme dive?
My advice to my family, almost all of whom are divers, is to watch what the Navy is putting on their authorized for cold-water service list. The regulators that show up on that list (and they are small in number) have passed the most rigorous testing in the world.
Through hundreds of hours of testing, in the most extreme conditions possible, the Navy has learned what all SCUBA divers should know:
• Even the coldest water (28°F; -2°C) is warm compared to the temperature of expanding air coming from a first stage regulator to the diver. There is a law of physics that says when compressed air contained in a SCUBA bottle is expanded by reducing it to a lower pressure, air temperature drops considerably. It’s the thermal consequence of adiabatic (rapid) expansion.
• Gas expansion does not have to be adiabatic. Isothermal (no temperature change) expansion is a process where the expansion is slow enough and heat entry into the gas from an outside source is fast enough that the expanded gas temperature does not drop.
• The best regulators are designed to take advantage of the heat available in ice water. The most critical place for that to happen is in the first stage where the greatest pressure drop occurs (from say 3,000 psi or higher to 135 psi above ambient water pressure (i.e., depth). They do that by maximizing heat transfer into the internals of the regulator.
• First stage regulators fail in two ways. The most common is that the first stage (which controls the largest pressure drop) begins to lose control of the pressure being supplied to the second stage regulator, the part that goes into a diver’s mouth. As that pressure climbs, the second stage eventually can’t hold it back any longer and a free flow ensues.
• The second failure mode is rare, but extremely problematic. Gas flow may stop suddenly and completely, so that backup regulator had better be handy.
• Second stage regulators are the most likely SCUBA components to fail in cold water due to internal ice accumulation.
• Free flows may start with a trickle, slowly accelerating to a torrent, or the regulator may instantly and unexpectedly erupt like a geyser of air. Once the uncontrolled, and often unstoppable free flow starts, it is self-perpetuating and can dump an entire cylinder of air within a few minutes through the second stage regulator.
• A warm-water regulator free flow is typically breathable; getting the air you need to ascend or to correct the problem is not difficult. In a cold-water-induced free flow, the geyser may be so cold as to make you feel like you’re breathing liquid nitrogen and so forceful as to be a safety concern. Staying relaxed under those conditions is difficult, but necessary.
• Water in non-polar regions can easily range between and 34°F to 38°F; at those temperatures, gas entering the second stage regulator can be at sub-freezing temperatures. European standard organizations classify ~10°C (50°F) as the cold/non-cold boundary. The Navy has found in the modern, high-flow regulators tested to date that 42°F is the water temperature where second stage inlet temperature is unlikely to dip below freezing.
• The small heat exchangers most manufacturers place just upstream of the second stage is ineffective In extreme conditions. They quickly ice over, insulating that portion of the regulator from the relative warmth of the surrounding water.
• Regulator “bells and whistles” are an unknown and can be problematic. Second stage regulators with multiple adjustments can do unpredictable things to heat transfer as the diver manipulates his controls. The last thing a cold-water diver should want is to make it easier to get more gas. High gas flows mean higher temperature drops and greater risk of free flow.
• Only manufacturer-certified technicians should touch your regulator if you’re going into risky waters. The technician at your local dive shop may or may not have current and valid technician training on your particular life support system. Don’t bet your life on it— ask to see the paperwork.
• Follow Navy and Smithsonian* guidance on handling and rinsing procedures for regulators in frigid waters. A single breath taken above the surface could freeze a regulator before you get your first breath underwater.
U. S. Navy reports on tested regulators are restricted. However, the list of those regulators passing all phases of Navy testing is available online. If your regulator, in the exact model as tested, is not on that list, do yourself a favor and don’t dive in frigid waters.
The original Editorial Focus article is found in the digital version of the March ECO magazine here, on pages 20-25.
Clarke JR1, Moon RE2, Chimiak JM3, Stinton R4, Van Hoesen KB5, and Lang MA5,6.
1 US Navy Experimental Diving Unit, Panama City, Florida 2 Duke University, Durham, North Carolina 3 Divers Alert Network, Durham, North Carolina 4 Diving Unlimited International, Inc., San Diego, California 5 UC San Diego – Emergency Medicine, San Diego, California 6 OxyHeal Health Group, National City, California
Introduction
The San Diego Center of Excellence in Diving at UC San Diego aims to help divers be effective consumers of scientific information through its “Healthy Divers in Healthy Oceans” mission. In this monograph we explore a research report from the Navy Experimental Diving Unit (NEDU) that is leading some divers to think they should be cold if they want to reduce decompression risk. That is a misinterpretation of the report, and may be causing divers to miss some of the joy of diving. There is no substitute for comfort and safety on a dive.
Background
In 2007 NEDU published their often-cited report “The Influence of Thermal Exposure on Diver Susceptibility to Decompression Sickness” (Gerth et al., 2007). The authors, Drs. Wayne Gerth, Victor Ruterbusch, and Ed Long were questioning the conventional wisdom that cold at depth increases the risk of decompression illness. After conducting a very carefully designed experiment, they were shocked to find that exactly the opposite was true. Some degree of cooling was beneficial, as long as the diver was warm during ascent.
Discussion and Implications
There are some important caveats for the non-Navy diver to consider. First of all, it was anticipated that a diver would have a system for carefully controlling their temperature during the separate phases of bottom time and decompression. Most non-Navy divers do not have that sort of surface support.
Secondly, the “cold” water in the NEDU study was 80 °F (27 °C). For most of us, 80 °F (27 °C) is an ideal swimming pool temperature, not exactly what you are going to find in non-tropical oceans and lakes. The warm water was 97 °F (36 °C), also a temperature not likely to be available to recreational and technical divers.
When testing the effect of anything on decompression results, the Navy uses their extensive mathematical expertise to select the one dive profile that is, in their estimation, the most likely to identify a difference in decompression risk, if that difference in risk exists. In this case the profile selected was a 120 fsw (37 msw) dive with 25 to 70 min bottom time, decompressed on a US Navy Standard Air table for 120 fsw (37 msw) and 70 min bottom time. That table prescribes 91 minutes of decompression divided thusly: 30 fsw/9 min (9msw/9 min), 20 fsw/23 min (6 msw/23 min), 10 fsw/55 min (3 msw/55 min).
A total of 400 carefully controlled dives were conducted yielding 21 diagnosed cases of decompression sickness. Overwhelmingly, the lowest risk of decompression was found when divers were kept warm during decompression. The effects of a 9 °C increase in water temperature during decompression was comparable to the effects of halving bottom time.
That is of course a remarkable result, apparently remarkable enough to cause civilian divers to alter their behavior when performing decompression dives. However, before you decide to chill yourself on the bottom or increase your risk of becoming hypothermic, consider these facts.
Do you have a way of keeping yourself warm, for instance with a hot water suit, during decompression? If not, the study results do not apply to you.
Of the many possible decompression schedules, the Navy tested only one schedule, the one considered to be the best for demonstrating a thermal influence on decompression risk. Although it seems reasonable that this result could be extrapolated to other dive profiles, such extrapolation is always risky. It may simply not hold for the particular dive you plan to make, especially if that dive is deeper and longer than tested.
Most commercial decompression computers do not adhere to the U.S. Navy Air Tables; few recreational dives are square profiles. Furthermore, additional conservatism is usually added to commercial algorithms. NEDU is not able to test the effects of diver skin temperature on all proprietary decompression tables, nor should they. That is not their mission.
The scientific method requires research to be replicated before test results can be proven or generalized. However, due to the labor and expense involved in the NEDU dive series, it seems unlikely that any experiments that would determine the relevance of these results to recreational or technical diving will ever be performed. As such, it may raise as many questions as it answers. For instance, the original question remains; if you become chilled on a dive, how does that affect your overall risk of decompression illness compared to remaining comfortably warm? Unfortunately, that question may never be answered fully.
Thermoneutral temperatures for swim suited divers are reported to be 93 °F to 97 °F (34 to 36 °C) for divers at rest and 90 °F (32 °C) during light to moderate work (Sterba, 1993). So a skin temperature of 80 °F (27 °C) is indeed cold for long duration dives. If your skin temperature is less than 80 °F (27 °C), then you are venturing into the unknown; NEDU’s results may not apply.In summary, beer and some types of wine are best chilled. Arguably, divers are not.
Acknowledgments
Support for the San Diego Center of Excellence in Diving is provided by founding partners UC San Diego Health Sciences, UC San Diego Scripps Institution of Oceanography, OxyHeal Health Group, Divers Alert Network, Diving Unlimited International, Inc. and Scubapro.
Sterba JA. Thermal Problems: Prevention and Treatment, in P.B. Bennett and D.H. Elliot, eds., The Physiology and Medicine of Diving, 4th ed. (London: Saunders, 1993), pp. 301-341.
Compared to decompression computers, digital oxygen control, and fuel cell oxygen sensors, carbon dioxide absorbent is low-tech and not at all sexy. Perhaps because it is low in diver interest, it is poorly understood. In rebreather diving, a lack of knowledge is dangerous.
The U.S. Navy Experimental Diving Unit (NEDU) is intimately familiar with sodalime, the crystalline carbon dioxide absorbent used in a wide variety of self-contained breathing apparatus for both diving and land use. NEDU routinely tests sodalime during accident investigations, during CO2 scrubber canister duration determinations, or during various research and development tasks. They have developed computer models of scrubber canister kinetics and patented and licensed technology for use in determining how long a scrubber will last in diving and land applications.
The types of sodalime in NEDU’s experimental inventory are:
Sofnolime 408 Mesh NI L Grade
Sofnolime 812 Mesh NI D Grade
HP Sodasorb (4/8 Reg HP)
Dragersorb 400
Limepak
Micropore
Absorbent undergoes a battery of quality tests at NEDU, most of them in accordance with NATO standardized testing procedures (STANAG 1411). One test is of the distribution of sodalime granule sizes, and another tests the softness or friability of the granules. One test checks the moisture content of the sample, and another tests the CO2 absorption ability of a small sample of absorbent.
The lead photo is a sample bag of sodalime removed from absorbent buckets, awaiting testing.
From time to time, absorbent lot samples fail one or more of these tests. One failure of granule size distribution was caused by changes in production procedures. “Worms” of absorbent rather than granules of absorbent started showing up in sodalime pails. In another case, absorbent was found to have substandard absorption activity, and in yet another, the material was too soft. Too soft or friable material can allow granules to break down, turning into dust.
This would not be a major problem, except that a diver or miner has to breathe through his granular absorbent bed, and dust clogs that bed, making breathing difficult. In the extreme, labored breathing from unusually high dust loading can result in unconsciousness.
What does the above have to do with this post’s title?
Supposedly, the maxim “Trust in God, but keep your powder dry” was uttered by Oliver Cromwell, but first appeared in 1834 in the poem “Oliver’s Advice” by William Blacker with the words “Put your trust in God, my boys, and keep your powder dry!” If indeed Cromwell did say it, then it dates from the 1600s.
A much more modern interpretation, appropriate for rebreather divers, is as follows: buckets of sodalime with a larger than usual layer of dust at the bottom (due to the mechanical breakdown of absorbent granules during shipment), should be kept dry. In other words, don’t dive it!
Micropore rolled carbon dioxide absorbent on the right, granular absorbent on the left.
Presumably, this is not an issue with Micropore ExtendAir CO2 absorbent since it’s basically sodalime powder suspended on a plastic medium. The diver breathes through fixed channels in the ExtendAir cartridge, not through the powder.
Considering the relatively high cost of granular sodalime, a diver might be very reluctant to discard an entire bucket of absorbent with a non-quantifiable amount of dusting. They certainly will not be performing sieve tests for granule size distributions like NEDU, however, one simple solution to a suspected dusting problem might be to sieve the material before diving it. The only requirement would be that only the dust should be discarded, not whole granules. In other words, your sieve must have a fine mesh.
In NEDU’s experience, quality control issues are not necessarily a problem with manufacturing. Where and how sodalime is stored can apparently have an appreciable effect on sodalime hardness. The same lot of sodalime stored in two different but close proximity locations has been found to differ markedly in its friability. Exactly why that should be, is presently unknown.
Regardless of whether the subject is sexy or not, a wise rebreather diver will seek all the knowledge available for his “sorb”, as it’s sometimes called. After all, the coolest decompression computer in the world will do you no good at all if you’re unconscious on the bottom because you tried to outlast your CO2 absorbent.
The amount of carbon dioxide (CO2) that can be safely inhaled by rebreather divers is a continuing point of conjecture, and vigorous argument. Unfortunately, the U.S. Navy Experimental Diving Unit has confused that issue, until recently.
A non-diver might wonder why a diver should inhale any CO2. After all, the air we breathe contains only a small fraction of CO2 (0.039%). A rebreather is best known for emitting no bubbles, or at most very few bubbles depending on the type of rebreather. It does that by recirculating the diver’s breath, adding oxygen to make up for oxygen consumed by the diver, and absorbing the carbon dioxide produced by the diver. The CO2 scrubber canister is vital to keeping the diver alive. As pointed out in the first post in this series, carbon dioxide is toxic; it can kill.
A CO2 scrubber keeps the recirculating CO2 levels low by chemically absorbing exhaled CO2. However, the scrubber has a finite lifetime – it can only absorb so much CO2. Once its capacity has been exceeded, CO2 passing through the canister accumulates exponentially as the diver continues to produce CO2 from his respiration.
The question rebreather divers want answered is, “How much of that bypassed CO2 can I tolerate?” As we’ve discussed in previous posts, 30% CO2 can incapacitate you within a few breaths. I can personally verify that if you’re exercising you may not notice the effect 7% CO2 has on you, until you try to do something requiring coordination. I’d equate it to the effect of drinking too many beers. There is little controversy about CO2 levels of 5-7% being bad for a diver.
For levels below 5-7% CO2, the U.S. Navy has not been real clear. For instance, 2% CO2 is the maximum CO2 allowed in diving helmets. If CO2 were to climb higher the diver would most likely feel a need to ventilate the helmet by briefly turning up the fresh gas supply to clear CO2.
Since at least 1981, NEDU has defined the scrubber canister breakthrough point in rebreathers as 0.5% CO2. That means that at some point, which varies with CO2 injection rate, ventilation rate, water temperature, and grain size of CO2 absorbent, CO2 begins leaking past the canister, not being fully absorbed during its passage through the canister. Once that leakage starts, the amount of CO2 entering the diver’s inspired breath rises at an ever increasing rate unless work rate or other variables change. By the time the CO2 leaving the canister has reached 0.5%, the canister has unequivocally “broken through”.
I pointed out in my last post that even 0% inspired CO2 may be too much for some divers when they are facing resistance to breathing. And all rebreathers are more difficult to breathe than other types of underwater breathing apparatus because the diver has to force his breath through the rig’s scrubber canister and associated hoses. The deeper the dive the denser the breathing gas and the worse breathing resistance becomes.
In free-flow diving helmets like the old MK 5, and the short-lived MK 12, the diver did not breathe through hoses and scrubber canisters. But those helmets had a high dead space and to keep helmet CO2 at tolerable levels a fresh gas flow of 6 actual cubic feet per minute (acfm; 170 liters per minute) was required. The U.S. Navy allowed up to 2% CO2 in the helmet because 1) the helmets did not have a high work of breathing and 2) due to simple physics the helmet CO2 couldn’t be kept very low.
For rebreathers, none of the above apply. A high breathing resistance is inevitable, at least compared to free-flow helmets, and once CO2 starts rising there is nothing you can do to decrease it again, short of stopping work.
In 2000, NEDU’s M. Knafelc published a literature review espousing that the same limit for inspired CO2 which applies in helmets could be used in rebreathers. Nevertheless, in 2010 NEDU’s D. Warkander and B. Shykoff clearly demonstrated that in the face of rising inspired CO2 concentrations work performance is reduced, and blood levels of CO2 rise, in some cases to dangerous levels. More recent work by the Warkander and Shykoff duo have extended those studies into submersion, however those reports are not yet publicly available.
As a result of both physiological theory and confirmatory data in young, physically-fit experimental divers, NEDU has not relaxed the existing definitions of scrubber canister breakthrough, 0.5% PCO2. Furthermore NEDU will adhere to the current practice of using statistical prediction methods to define published canister durations, methods which are designed to keep the odds of a diver’s rebreather canister “breaking through” to no more than 2.5%, comparable to the odds of decompression sickness following Navy multi-level dive tables. Details of this procedure will be explained in later postings.
Children of the Middle Waters (working title) is a science fiction/thriller that has been completed and is being submitted today for consideration by Tom Doherty Associates, New York. My friend and mentor, the writer Max McCoy, has provided literary criticism and encouragement for the manuscript. Max, who works primarily in the Western genre, wrote a diving-related thriller called The Moon Pool, which happens to involve in its closing chapter the Navy Experimental Diving Unit, and someone a lot like me.
Below is a blurb briefly describing Children of the Middle Waters.
In the deep-sea canyons and trenches of the Earth lie thousands of alien spacecraft and millions of their inhabitants who have to leave soon or risk being stranded forever, or being destroyed. Due to their physiology they have been unable to directly contact humans, but they are adroit at mental contact and remote viewing, when it suits them.
They need the help of two humans to assure their safe escape, an experienced Navy scientist and a beguiling graduate student. But introductions through mental means are slow and suspect, as you might imagine.
The U.S. government is well aware of this deep sea civilization, and is desirous of the weapons the visitors possess, which puts the two unsuspecting scientists in the middle of a conflict between powerful
military forces and powerful intergalactic forces. Things could get messy.
Even worse, jealous friends turn on the unlikely duo and put their lives at risk.
Children combines two separate Native American beliefs and legends with current events. It is a complex thriller with science fact and science fiction mixed in with military action and government intrigue. Also revealed are romantic possibilities that far exceed the capabilities of the mundane, everyday world.
Early American Indian beliefs create an ending for this story that no one could anticipate. It is an ending that causes the protagonist to realize everything he has held dear is wrong, in one way or another. At the same time he discovers a reality that is the greatest blessing that man can receive.
If you’re diving a rebreather (closed-circuit breathing apparatus to be exact), then you know the scrubber removes carbon dioxide from your recirculated breath. Without the scrubber working, you’d go unconscious from carbon dioxide intoxication within a very few minutes of starting the dive.
But do you really know what’s going on inside that scrubber canister?
A stochastic computer simulation developed by the author gives as realistic a glimpse inside as we can get.
Loose granular and rolled sodalime. Click to enlarge.
Carbon dioxide scrubber canisters usually contain a chemical mixture called sodalime that chemically reacts with carbon dioxide in a diver’s expired breath. That material may be in granular form, or in a preformed roll. Sodalime is a mixture of calcium hydroxide and sodium hydroxide, which when it reacts by absorbing carbon dioxide is converted into calcium carbonate (CaCO3, calcite), a major constituent of limestone.
The overall chemical reaction can be simplified to:
CO2 + Ca(OH)2 → CaCO3 + H2O + heat
In the following sequence of images we see a rectangular prism shaped scrubber canister arranged axially such that the diver’s expired breath enters the section from the left, passing completely through the canister section before exiting to the right. A portion of the canister was cut away digitally after the simulation was run to allow visualization of temperatures within the canister interior.
Beginning of the simulation. Click to enlarge.
Initially, the canister is at room temperature, and then is immersed in cold water as the diver begins his dive. Temperature is color coded: the coldest temperature is black, and increasing warmth is portrayed in an intuitive fashion from purple to red to yellow, and finally white, being the highest temperature.
In the first image, CO2 has just started reacting with the sodalime at the entrance to the canister section, with a slight heating resulting. Thermal conduction is cooling the exterior surface of the canister, but most of the inside still remains at room temperature.
In the second image, the reaction front has clearly formed, and the hottest portion of the canister has begun moving downstream. Convection carries heat rapidly downstream to heat the diver’s inspired breath, and is seen to offset canister cooling due to conduction from the surrounding cold water.
Click to enlarge
In the image to the left, the heating front is fully developed, and residual heat has spread almost completely throughout the downstream portion of the canister.
In the next image, to the right, the front is beginning to weaken in intensity.
Finally (lower left figure), the thermal heating in the reaction front, indicative of CO2 absorption effectiveness, is fading out, and the cooling of the canister from the surrounding cold water is beginning to win the tug of war between heat generation and conductive cooling.
At that point in time, the canister is spent, and essentially all of the exhaled CO2 is passing right through the canister without being absorbed. If the diver had not ended his dive before his canister reached this point, he would be at great risk of passing out due to CO2 accumulation.
The last figure (lower right) shows temperature readings at various locations, and at various times (reps) throughout the simulation run. The orange and brown traces marked “temp” are measured temperatures from locations near the entrance to the canister. They rise abruptly as the absorption reactions start, and fall quickly as the reaction front moves past them, downstream.
Click to enlarge
The curves that remain elevated longer represent the average exhaled gas temperature, and the average temperature within the absorbent bed. After reaching a peak, the average bed temperature steadily drops as cold gas from the inlet (exhaled) gas chills the portion of the bed behind the reaction front. Exhaled gas temperature, on the other hand, climbs more slowly, but remains more stable until the bed becomes depleted of absorbent activity.
The monitoring of absorbent canister temperature changes is what makes the rebreather scrubber canister monitors used in the Inspiration and Sentinel rebreathers possible. The Sentinel technology is licensed from the U.S. Navy Experimental Diving Unit.
In the next posting, we’ll see the surprising way that cold canisters fill up with calcium carbonate.
The following is a high definition video of the computer simulation of heat generation and loss in a short cylindrical canister. For best effect go to full screen and 1080p mode.
Further details about the computer simulation involved in the production of these images and video can be found in the paper “Computer Modeling of the Kinetics of CO2 Absorption in Rebreather Scrubber Canisters”, in MTS/IEEE OCEANS 2001 Conference Proceedings, published by the Marine Technology Society; Institute of Electrical and Electronics Engineers; Oceanic Engineering Society (U.S.); IEEE Xplore (Online service).
