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.
Helium is a low density, non-narcotic gas often added to the breathing gas mixture of divers who have to dive deep. Nitrogen, the primary component of air is both dense, making it hard to breathe when diving deep, and narcotic at depths below one hundred feet. That is why nitrogen leads to the so-called “rapture of the deep.” Narcotic divers make bad decisions.
If it weren’t for helium, some of the deepest and most sensitive diving for national security would never have happened. So, it’s really important. Commercial saturation diving in the oil fields of the North Atlantic and the Gulf of Mexico is wholly dependent on the easy to breathe and non-narcotic properties of helium.
Both civilian and government science divers, technical divers, and underwater cave explorers have been able to extend their diving range and safety because of helium in their breathing gas.
Helium graphic from the cover of a 2016 report on helium supply shortages by the American Physical Society, the American Chemical Society, and the Materials Research Society. — Image credit – Ashley Mumford
For those not familiar with the second lightest gas in the periodic table, I’ve included a Fast Fact from the Bureau of Land Management (BLM) at the end of this post.
There are two drawbacks to helium. A source of breathable helium is sometimes hard to locate, and the gas is expensive. Because of that expense and growing scarcity, it is forecast to become increasingly difficult to find, especially in remote locations.
From a Science Direct article, “Ongoing ascent to the helium production plateau – Insights from System Dynamics” in Resources Policy, Vol. 35, Issue 2, June 2010, pgs 77-89.
The primary source of helium, a non-renewable resource, is from gas wells. As shown in the BLM summary at the bottom of this post, the demand for helium is high in scientific, medical, military, and commercial applications.
Cryomodules for an x-ray light source LCLS-II are under construction at Fermilab in Illinois. The cryomodules will house superconducting cavities that are cooled with liquid helium. Credit: Reidar Hahn.
Not on the list, and the least likely to be considered during allocation of an increasingly scarce resource, is civilian diving, and perhaps even military diving.
The above graphical projection made in 2010 does not consider the damping effect of current government policies which make drilling oil and gas wells, and fossil fuels in general, undesirable. While Qatar and Russia have significant helium reserves, helium transported from distant countries will come with a much higher price tag than forecast in 2010. Unfortunately, no one has so far calculated the net cost of reducing the recovery of gas from the ground, and the recovery of the helium contained in that natural gas.
Why might the next century bring a lowering of helium prices as predicted in the graph above? As I’ve explained in Atmosphere, Book 3 of the Jason Parker Trilogy, fusion reactors should hopefully be common place by then, and helium is a byproduct of those fusion reactions. Of course, the above graph reflects a great deal of uncertainty about the next century, even without the uncertainty introduced by government policies. But our immediate concern is this century, not the next.
One approach to helium conservation is by using rebreathers to conserve gas rather than exhaust it into the water column, as is done in open circuit diving like that pictured in the first underwater photo with two Navy divers. In rebreathers, the only helium wasted is that used to keep breathing bags inflated on descent. Unfortunately, that gas is “burped-off” as gas expands on ascent. But the amount of inert gas wasted during rebreather operations is still far less than in open-circuit diving.
A Navy EX-19 rebreather being tested in Morrison Springs, FL. Navy photo by Bernie Campoli.
Another option for holding down helium cost, is to use helium in “Trimix”, a mixture of oxygen, nitrogen and helium. Such mixes become popular for use at depths of 200 feet sea water (fsw) and deeper. It minimizes the cost of helium while simultaneously reducing the effect of nitrogen narcosis.
A common trimix is called 21/35, which has 21 percent oxygen, 35 percent helium and 44 percent nitrogen. Another common mixture is 18/45, with 18 percent oxygen and 45 percent helium. Those helium percentages are considerably reduced from that found in a typical military heliox mixture containing no nitrogen.
But even then, using helium for recreational deep diving may become far too expensive for any but the richest recreational divers. Already, it’s reported that scientific and medical instruments like superconducting magnets and MRI machines have been affected by helium shortages.
When it comes to the DoD prioritization of military saturation diving missions compared to other military options, the availability and cost of helium will inevitability factor into the high-level decision tree.
So, is there an alternative to helium use in diving? Well, yes and no. I’ve written in both this blog and in my novels about the use of hydrogen in diving, as has a biomedical researcher friend of mine, Susan Kayar, Ph.D. in her novel, Operation Second Starfish.
Hydrogen is even lighter than helium, but at great depth it is narcotic. One strange thing about hydrogen narcosis is that at great depth it can result in psychotic manifestations in some individuals. Also, at shallow depth, hydrogen can form an explosive mixture with oxygen, an issue I’ll discuss in my next post. So, it has to be used with great care and attention to details.
Interestingly, the math says that at 200 fsw, the depth where trimix is typically used, hydrogen can be safely substituted for helium. However, only experimentation can prove if that prediction is valid or not. But as helium gets scarcer and more expensive, using hydrogen in place of helium is something worth considering.
[DO NOT CONDUCT YOUR OWN EXPERIMENTS WITH HYDROGEN. THERE IS ALWAYS A CHANCE OF INJURY OR DEATH WITH HYDROGEN. THINK OF THE HINDENBURG!]
Below are links to other hydrogen and forward-looking diving posts in this blog.
Helium is an odorless, colorless, and tasteless gas. Helium, more than 99.9 percent pure, is also used in liquid form at -452 degrees Fahrenheit.
Where does helium come from?
Helium occurs with other gasses in pockets beneath the Earth’s surface. The most economical source of helium is natural gas, all of which contains some helium. Natural gas in the States of Texas, Kansas, Colorado, Utah, and Wyoming is richer in helium than what has been recovered from other States.
How is helium produced?
When a gas pocket containing economically recoverable amounts of helium is found, a well is drilled to release the gas. It travels by pipeline to a processing plant where the helium is separated from the other gasses. One method of separation is a cryogenic process, which uses cold temperature differences to split the components. Another process, membrane filtration, uses molecular size difference to split components.
What is helium used for?
Today, helium plays a prominent role in medical imaging (magnetic resonance imaging), fiber optics/semiconductor manufacturing, laser welding, leak detection, superconductivity development, aerospace, defense, and energy programs.
Is helium renewable (does it naturally replenish itself after humans use it)?
No, helium is a non-renewable resource. That is why the Federal Government stored 44 billion cubic feet of helium in a natural gas reservoir at Cliffside, just outside of Amarillo, Texas. Helium was injected into porous rock 3,000 feet below the Earth’s surface during the 1960s. This rock holds gas like a sponge holds water. Two layers of calcium anhydrite cover the rock, acting as a lid. The sides are surrounded by water.
As evidenced by Under the Pole diving expeditions, rebreathers are being used in some of the most isolated and frigid places in the world. Some of those dive missions are surprisingly deep (111 meters, 330 feet) and long, about 2 hours.
That gives me cause for pause.
I suspect most divers are aware of the 1/3 rule for gas consumption on an open circuit (scuba) cave dive. You should use no more than 1/3 of your air supply on the way in, leaving you with 1/3 for the trip out, and 1/3 of your gas supply available in reserve. Sadly, even that amount of reserve has not saved all cave divers.
Now that cave divers are using rebreathers, the rules, at least for some, have changed. Some savvy rebreather cave divers use the rule of doubles: Always have twice as much oxygen, twice as much diluent, and twice as much canister as you think you’ll need. That plus an open-circuit or semi-closed circuit bailout should keep you safe — in theory.
Gas supply is easy to measure throughout a dive; there is a pressure gauge for all gases. But what about canister duration? Most divers assume they will have more canister duration available than gas supply; which means they don’t need to worry about canister duration. That would be a good thing, if it were true. After all, how many manufacturers provide expected canister durations for various work rates and water temperatures? Maybe, none? Or certainly very few.
I would be very surprised if manufacturers could say with certainty that during a two hour dive in -2°C (28°F) water, at depths to 111 meters that the scrubber can provide double the duration needed. That would be four hours in -2°C water, at all potential diver work rates.
Some of you may say, “Under-the-ice-diving is not like cave diving, so the doubles rule is too conservative.” I invite you to think again. Under polar ice, is there ready access to the surface? Not unless you’re diving directly under the through-ice bore hole the entire time.
In the U.S. Navy experience, obtaining useful data on canister durations from manufacturers is difficult. Duration data as a function of temperature is practically nonexistent. Therefore I will share the following information gleamed from scrubber canister testing in extreme environments by the Navy. While this blogger cannot reveal canister durations for military rebreathers, the information on the coefficient of varation (COV) is not protected. (There is no way to figure out what a canister duration is based solely on the COV.)
The following 4-minute video gives a good introduction to the coefficient of variation.
All rebreather divers should know that canister performance declines in an accelerating manner as water temperature drops between 50°F and 28°F. But what your rebreather manufacturer may not know is that the innate variability of canister durations also increases as water temperature drops. The Navy has found that trend in all types of rebreathers.
So, while canister durations drop considerably in cold water, you’re also less certain about what your canister’s endurance is going to be, because of the increase in duration variability. When canister duration drops and variability increases, a diver’s margin of safety becomes a gamble. Personally, I don’t like to gamble under water.
Coefficient of variation (COV, mean duration divided by the standard deviation) of a typical rebreather. Each data point is the mean of five canisters (n=5).
In the U.S. Navy, published canister durations take into account mean canister performance, and variability. That is accomplished through the use of 95% prediction intervals. The greater the variability in canister duration, the lower the published duration.
This method of determining safe canister durations has been in use by the U.S. Navy since 1999. However, I do not know if manufacturers use similar statistically-based methods for publishing canister durations. If they or you do not take duration variability into account as you dive cold, you may be in for a shock. Due to the nature of statistics, you may have 9 deep, cold dives with no CO2 problems, but find yourself in bad shape on the 10th dive.
If you did have a CO2 problem, it wouldn’t necessarily be anyone’s fault: it could just be a result of canister variability in action.
So, diver beware. Give yourself plenty of leeway in planning rebreather dives in frigid waters. After all, you do not want to become a statistic, caused ironically by statistics.
If you have an interest in understanding the derivation of the prediction interval equation and its application, two videos of lectures by Dr. Simcha Pollack from St. John’s University may be helpful. Part I is found here, and Part 2 is found here.
Thanks to Gene Hobbs and the Rubicon Foundation, NEDU’s original report on the use of prediction limits to establish published canister durations is found here.
Photo taken from the author’s aircraft one stormy Florida Panhandle morning. (click to enlarge)
I was recently flying a private aircraft down the Florida Peninsula to Ft. Lauderdale to give a presentation on diving safety. As I continually checked the cockpit instruments, radios and navigation devices, it occurred to me that the redundancy that I insist upon in my aircraft could benefit divers as well.
In technical and saturation diving, making a free ascent to the surface is just as dangerous as making a free descent to the ground in an airplane, at night, in the clouds. In both aviation and diving, adequate redundancy in equipment and procedures just might make life-threatening emergencies a thing of the past.
Aviation
As I took inventory of the redundancy in my simple single engine, retractable gear Piper, I found the following power plant redundancies: dual ignitions systems, including dual magnetos each feeding their own set of spark plug wires and redundant spark plugs (two per cylinder). There are two sources of air for the fuel-injected 200 hp engine.
There are two ways to lower the landing gear, and both alarms and automatic systems for minimizing the odds of pilot error — landing with wheels up instead of down. (I’ve already posted about how concerning that prospect can be.)
I also counted three independent sources of weather information, including lightning detection, and two powerful communication radios and one handheld backup radio. For navigation there is a compass and four electronic navigation devices: one instrument approach (in the clouds) approved panel mount GPS with separate panel-mounted indicator, an independent panel mounted approach certified navigation radio, plus two portable GPS with moving map displays and superimposed weather. Even the portable radio has the ability to perform simple navigation.
There’s two of just about everything in this Arrow panel.
The primary aircraft control gyro, the artificial horizon or attitude indicator, also has a fully independent backup. One gyro operates off the engine-powered vacuum pump, and the second gyro horizon is electrically driven. Although by no means ideal, the portable GPS devices also provide attitude indicators based upon GPS signals. In a pinch in the clouds, it’s far better than nothing. Of course, even if all else fails, the plane can still be flown by primary instruments like rate of climb, altimeter, and compass.
There is only one sensitive altimeter, but two GPS devices also provide approximate altitude based on GPS satellite information.
Diving
But what about divers? How are we set for redundancy?
Starting with scuba (self-contained underwater breathing apparatus), gas supplies are like the fuel tanks in an aircraft. I typically dive with one gas bottle, but diving with two or more bottles is common, especially in technical diving. In a similar fashion, most small general aviation aircraft have at least two independent fuel tanks, one in each wing.
The scuba’s engine is the first stage regulator, the machine that converts high pressure air into lower pressure air. Most scuba operations depend on one of those “engines”, but in extreme diving, such as low temperature diving, redundant engines can be a life saver. While most divers carry dual second stage regulators attached to a single first stage, for better redundancy polar divers carry two independent first stages and second stages. Two first stage regulators can be placed on a single tank.
An H-valve for a single scuba bottle. Two independent regulators can be attached.
A Y-valve for Antarctic diving with two independent scuba regulators attached.
Even then, I’ve witnessed dual regulator failures under thick Antarctic ice. The only thing saving that very experienced diver was a nearby buddy diver with his own redundant system.
There is a lot to be gained by protecting the face in cold water by using a full face mask. But should the primary first or second stage regulator freeze or free flow, the diver would normally have to remove the full face mask to place the second regulator in his mouth.
Two regulators, one full face mask. Photo courtesy of Michael Lang and Scuba Pro.
Reportedly, sudden exposure of the face to cold water can cause abnormal heart rhythms, an exceedingly rare but potentially dangerous event in diving. If the backup or bail out regulator could be incorporated into the full face mask, that problem would be eliminated. The photo on the right shows one such implementation of that idea.
Nikki Smith, rebreather diver with open circuit bailout in her right hand. Photo courtesy of Rosemary E Lunn (Roz), The Underwater Marketing Company.
Rebreathers are a different matter. Most rebreather divers carry a bailout system in case their primary rebreather fails or floods. For most technical divers, that redundancy is an open circuit regulator and bailout bottle. However, there are options for the bail-out to be an independent, and perhaps small rebreather. (One option for a bail-out semiclosed rebreather is found here.) Such a bail-out plan should provide greater duration than open-circuit bailout, especially if the divers are deep when they go “off the loop”.
U.S. Navy photo by Bernie Campoli.
For some military rebreather divers, there is at least one complete closed-circuit rebreather available where a diver can reach it in case of a rebreather flood-out.
A commercial saturation diver with semi-closed rebreather backpack as emergency bail-out gas.
For deep sea helmet diving, the bail-out rebreather is on their back and a simple valve twist will remove the diver from umbilical-supplied helmet gas to fresh rebreather gas.
The most common worry for electronically controlled rebreather divers is failure of the rig’s oxygen sensors. For that reason it is common for rebreathers to carry three oxygen sensors. Unfortunately, as the Navy and others have noted, triple redundancy really isn’t. Electronic rebreathers are largely computer controlled, and computer algorithms can allow the oxygen controller to become confused, resulting in oxygen control using bad sensors, and ignoring a correctly functioning oxygen sensor.
The U.S. Navy has performed more than one diving accident investigation where that occurred. Safety in this case can be improved by adding an independent, redundant sensor, by improving sensor voting algorithms, by better maintenance, or by methods for testing all oxygen sensors throughout a dive.
In summary, safe divers and safe pilots are always asking themselves, “What would I do if something bad happens right now?” Unfortunately, private pilots and divers quickly discover that redundancy is not cheap. However, long ago I decided that if something unexpected happened during a flight or a dive, I wouldn’t want my last thoughts to be, “If only I’d spent a little more money on redundant systems, I wouldn’t be running out of time.”
Time, like fuel and breathing air, is a commodity you can only buy before you run out of it.
Disclaimer: This blog post is not an endorsement of any diving product. Diving products shown or mentioned merely serve as examples of redundancy, and are mentioned only to further diver safety. A search of the internet by interested readers will reveal a panoply of alternative and equally capable products to enhance diver safety.
Exoplanet Gliese 581d, orbiting the red-dwarf star Gliese 581, only 20 light-years away. (The existence of this planet is currently in dispute.)
In space, there is a so-called Goldilocks zone for exoplanet habitability. Too close to a star, and the planet is too hot for life. Too far from its star, and the planet is too cold for life, at least as we understand biological life, life dependent on water remaining in a liquid state. Earth is clearly in the Goldilocks zone, and so is a purported planet Gleise 581d, from another solar system.
Carbon dioxide absorbing “scrubber” canisters in rebreathers have similar requirements for sustaining their absorption reactions. If it’s too hot, the water necessary for the absorption reaction is driven off. Too cold and the water cannot fully participate in the absorption reactions.
Those with some knowledge of chemistry recognize that cold retards chemical reactions and heat accelerates them. But that does not necessarily apply to reactions where a critical amount of water is required. Water thus becomes the critical link to the reaction process, and so maintaining scrubber temperature within a relatively narrow “Goldilocks” zone is important, just as it is for life on distant planets.
Temperature within a scrubber canister is a balance of competing factors. Heat is produced by the absorption of CO2 and it’s conversion from gas to solid phase, specifically calcium carbonate. A canister is roughly 20°C or more warmer than the surrounding inlet gas temperature due to the heat-generating (exothermic) chemical reactions occurring within it.
Heat is lost from a warm canister through two heat transfer processes; conduction and convection. Conduction is the flow of heat through materials, from hot to cold. Hot sodalime granules have their heat conducted to adjacent cooler granules, and when encountering the warm walls of the canister, heat passes through the canister walls, and on to the surrounding cold water.
You can think of this conduction as water flowing downhill, down a gravity gradient. But in this case, the downhill is a temperature gradient, from hot to cold. If the outside of the canister was hotter than the inside, heat would flow in the opposite direction, into the canister.
Copper is a better conductor of heat than iron (it has a higher thermal conductivity), explaining why copper skillets are popular for cooking on stoves. Air is a poor conductor of heat, explaining why neoprene rubber wet suits, filled with air bubbles, are good insulators. Air-filled dry suits are an even better insulator.
Chemical absorption reactions heat an otherwise cold canister (yellow is hot, red is warm, black is cold.) (Copyright John R. Clarke, 2014).
Convection is the transfer of heat to a flowing medium, in this case gas. You experience convective cooling when you’re working hard, generating body heat, and a cool dry breeze passes over your skin. Convective cooling can, under those circumstances, be delightful.
When you walk outside on a cold, windy day, convective cooling can be your worst enemy. Meteorologists call it wind chill.
There is wind chill within a canister, caused by the flow of a diver’s exhaled breath through the canister. In cold water the diver’s exhaled breath leaves the body quite warm, but is chilled to water temperature by the time it reaches the canister. Heat is lost through uninsulated breathing hoses exposed to the surrounding water.
As you might expect, if the canister is hot, that convective wind chill can help cool it. If the canister is cold, then the so-called wind chill will chill it even more.
Copyright John R. Clarke, 2014.
The amount of heat transferred from a solid object to gas is determined by three primary variables; the flow rate of the gas, the density of the gas, and the gas’s heat capacity. Heat capacity is a measure of the amount of heat required to raise the temperature of a set mass of gas by 1° Celsius.
Both the heat capacity and density of the gas circulating through a rebreather changes not only with depth (gas density), but with the gas mixture (oxygen plus an inert diluent such as nitrogen or helium). The heat capacity of nitrogen, helium and oxygen differ, and the ratio of oxygen and inert gas varies with depth to prevent oxygen toxicity. Nitrogen and helium concentrations vary as well, as the diver attempts to avoid nitrogen narcosis.
Q is heat transferred by convection, and the terms on the right are, in sequence, diver ventilation rate, gas density, heat capacity of the inspired gas mixture at constant pressure, and the difference in temperature between the absorbent and environmental temperature.
The interaction of all these variables can be complex, but I’ve worked a few examples relevant to rebreather diving. The assumptions are a low work rate: ventilation is 22 liters per minute, water temperature is 50°F (10°C), oxygen partial pressure is 1.3 atmospheres, and dive depths of 100, 200 and 300 feet sea water. The average canister temperature is assumed to be 20°C (68°F) above water temperature, a realistic value found in tests of scrubber canister temperatures by the U.S. Navy.
The heat capacities for mixtures of diving gases come from mixture equations, and for the conditions we’re examining are given in the U.S. Navy Diving Gas Manual. (This seems to be a hard document to obtain.)
At 100 fsw, the heat transfer (Q) for a nitrogen-oxygen (nitrox) gas mixture is 34.2 Watts (W). For a helium-oxygen mixture (heliox), Q is 27.4 W. At 200 fsw, Q for nitrox is 59.9 W, and for heliox Q is 50.3 W. At 300 fsw, Q for nitrox gas mixture is 85.5 W, and for heliox, is 59.9 W.
Interestingly, the heat transferred from the absorbent bed to the circulating gas is the same at 300 fsw with heliox as it is at 200 fsw with nitrox.
Photo courtesy of David L. Conlin, Ph.D., Chief – National Parks Service Submerged Resources Center. Photo by Brett Seymour, NPS.
Dr. Jolie Bookspan briefly mentioned the fact that helium removes less heat from a diver’s airways than does air in her short article on “The 36 Most Common Myths of Diving Physiology” (see myth no. 20). Conveniently, heat exchange equations apply just as well to inanimate objects like scrubber canisters as they do to the human respiratory system.
From these types of heat transfer calculations it is easy to see that for a given depth, work rate and oxygen set point, it is better to use a heliox mixture than a nitrox mixture if you’re in cold water. That may sound counterintuitive considering helium’s high thermal conductivity, but the simple fact is, the helium background gas with its low density carries away less heat from the canister, and thereby keeps the canister warmer, than a nitrox mixture does. The result is that canister durations are longer in cold water if less heat is carried away.
In warm water, the opposite would be true. Enhanced canister cooling with nitrox would benefit the canister.
An earlier post on the effect of depth on canister durations raised the question of whether depth impedes canister performance. The notion that increased numbers of inert gas molecules block CO2 from reaching granule absorption sites has little chemical kinetic credence. However, changing thermal effects on canisters with depth or changing gas mixtures does indeed affect canister durations.
I’ve just given you yet another reason why helium is a good gas for rebreather diving, at least in cold water. Unfortunately, these general principles have to be reconciled with the specific cooling properties of all the rebreather canisters in current use. In other words, your canister mileage may vary. But it does look like the current simple notions of depth effects are a bit too simplistic.
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.
Sirens Cove (contributed by Spanish Conqueror to Mythical Mania Wiki)
In Greek mythology irresistibly seductive female creatures were believed to use enchanted singing to beckon sailors to a watery grave.
Why this myth endured through the centuries is difficult to say. However, my theory is that it helped explain to grieving widows and mothers why ships sometimes inexplicably disappeared, taking their crew with them, never to be seen again. By the reasoning of the time, there must have been some sort of feminine magic involved.
The oxygen sensors in closed-circuit, electronically or computer-controlled rebreathers are a magic device of sorts. They enable a diver to stay underwater for hours, consuming the bare minimum of oxygen required. The only thing better than a rebreather using oxygen sensors would be gills. And in case you wondered, gills for humans are quite impractical, at least for the foreseeable future.
I have written, or helped write three diving accident reports where the final causal event in a rebreather accident chain proved to be faulty oxygen sensors. So for me, the Siren call of this almost magical sensor can, and has, lured divers to their seemingly blissful and quite unexpected death.
Those who use oxygen sensors know that if the sensor fails leading to a hypoxic (low oxygen) state, loss of consciousness comes without warning. If sensor failure results in a hyperoxic state (too high oxygen), seizures can occur, again leading to loss of consciousness, usually without warning. Unless a diver is using a full facemask, loss of consciousness for either reason quickly leads to drowning.
EX 19 rebreather (U.S. Navy photo)
Due to the life-critical nature of oxygen control with sensors, three sensors are typically used, and various “voting” algorithms are used to determine if all the sensors are reliable, or not. Unfortunately, this voting approach is not fail-proof, and the presence of three sensors does not guarantee “triple” redundancy.
In one rebreather accident occurring during the dawn of computer-controlled rebreathers, a Navy developed rebreather cut off the oxygen supply to a diver at the Navy Experimental Diving Unit, and all rebreather alarms failed. The diver went into full cardiopulmonary arrest caused by hypoxia. Fortunately, the NEDU medical staff saved the diver’s life, aided in part by the fact that he was in only 15 feet of water, in a pool.
In two more recent accidents the rebreathers kept feeding oxygen to the diver without his knowledge. One case was fatal, and the other should have been but was not. Why it did not prove fatal can only be explained by the Grace of God.
The two cases were quite different. In one the diver broke a number of safety rules and began a dive with known defective equipment. He chose to assume that his oxygen sensors were in better shape than the rest of his rebreather. If he had been honest with himself, he would have realized they weren’t. If he had been honest with himself, he would still be alive.
The other dive was being run by an organization with a reputation for being extremely safety conscious. Nevertheless, errors of omission were made regarding oxygen sensors which almost cost the experienced diver his life.
In the well-documented Navy case, water from condensation formed over the oxygen sensors, causing them to malfunction. The water barrier shielded the sensors from oxygen in the breathing loop, and as the trapped oxygen on the sensor face was consumed electrochemically the sensor would indicate a declining oxygen level in the rig, regardless of what was actually happening. Depending on how the sensor voting logic operated, and the number of sensors failing, various bad things could happen.
During its accident investigation, when NEDU used a computer simulation to analyze the alarm and sensor logic, it found that if two of the three sensors were to be blocked (locked) by condensed water, the rig could lose oxygen control in either a hypoxic or hyperoxic condition. Based on a random (Monte Carlo) sensor failure simulation, low diver work loads were more often associated with hypoxia than higher work rates, even with one sensor working normally.
We deduce from this result that “triple redundancy” really isn’t.
The white circles at the top left of this scrubber canister housing are the three oxygen sensors used in an experimental U.S. Navy rebreather.
When the accident rig was tested in the prone (swimming) position at shallow depth, after 2 to 3 hours sensors started locking out, and the rig began adding oxygen continuously. The computer simulation showed that the odds of an alarm being signaled to the diver was only 50%. The diver therefore could not count on being alerted to a sensor problem.
Unfortunately in this near fatal case the rig stopped adding oxygen, the diver became hypoxic and the diver received no alarms at all.
After NEDU’s investigation, the alarm logic was rewritten with a vast improvement in reliability. The orientation of the sensors was also changed to minimize problems with condensation.
Today what is being seen are divers who extend the use of their sensors beyond the recommended replacement date. Like batteries, oxygen sensors have a shelf-life, but they also have a life dependent on use. Heavily used sensors may well be expended long before their shelf-life has expired.
The Siren, by John Williams Waterhouse.
Presumably, the birthing pains of the relatively new underwater technology based on oxygen sensors have now passed. Nevertheless, those who use rebreathers should be intimately familiar with the many ways sensors, and their electronic circuitry, can lead divers ever so gently to their grave.
Like sailors of old, there are ways for divers to resist being lulled to their death by oxygen sensors. First among them is suspicion. When you expect to have a great day of diving, you should be suspicious that your rebreather may have different plans for you. Your responsibility to yourself, your dive buddies and your family is to make sure that the rebreather, like a Siren, does not succeed in ruining your day.
The best way to ward off sensor trouble is through education. To that end, Internet sites like the following are useful. Check with your rebreather manufacturer or instructor for additional reading material.
I admit it, my early training in physics has made me irritatingly sensitive to the principle of parsimony.
Parsimony, pronounced similarly to “alimony”, can be summed up by the following: the simplest approach to understanding nature should be considered before contemplating a more complicated line of reasoning. In a famous example, it is more probable that planets, including the Earth, orbit around the sun than the visible planets and the sun orbit around the Earth. Of course, in a different time that probability was not obvious to the common man. But then they hadn’t been thinking about parsimony.
Thank-goodness someone (Nicolaus Copernicus) did.
In the search for habitable exoplanets (planets outside of our solar system), the following statement was recently made by astronomer Steve Vogt in response to a storm of skepticism about a potentially habitable planet. “I do believe that the all-circular-orbits solution is the most defensible and credible,” he said. “For all the reasons I explain in detail … it wins on account of dynamic stability, goodness-of-fit, and the principle of parsimony (Occam’s Razor; in Latin, lex parsimoniae).”
William of Occam (also Ockham) was an English theologian of the 14th century. He did not invent the premise behind his razor, but he famously used it to slice through the complicated philosophies of the day and rebut them by an unfaltering demand for simplicity over complexity.
Photo credit: damianskinner.com
Medical students are taught essentially the same principle, albeit using different words: “When you hear hoof-beats, don’t think of zebras.” Wise physicians know that occasionally zebras do show themselves, but they should not be the first thought when a patient presents with unusual symptoms.
If simplicity is to be generally preferred over complexity, then an example in the diving literature comes to mind. This example annoys me to no end, but I’m slowly coming to terms with it. It is the growing popularity of referring to the respiratory effort required to breathe through a scuba regulator or a closed-circuit underwater breathing apparatus (a rebreather) as work (in joules, J) per tidal volume in liters, L.
When work in joules (J) is divided by volume (L), dimensionally the result is pressure (kiloPascals, kPa). To be exact, what is often called work of breathing in diving is actually the average pressure exerted by a person over the entire volume of a breath. The principal of parsimony says that if it is a pressure, if it has units of pressure, then we should call it a pressure (kPa) and not something more complicated, such as Work of Breathing specified with units of J/L.
The light grey ellipsoidal area within this pressure-volume loop is equal to the work (J) of breathing for that breath.
(Examples in the regulatory diving literature correctly using Work of Breathing with units of joules can be found in early editions of NATO STANAG 1410. EN250:2000 is an example using the units of J/L for work.)
I find in my dealings with non-respiratory physiologists, that the concept of work of breathing is difficult to grasp since mathematically it involves a definite integral of pressure over a change in volume. I have made various attempts to simplify the concept, but I still find knowledgeable medical professionals misunderstanding it. In fact, mathematical integrals seem to be as frightening to most physicians as poorly dissected cadavers would be to laymen. Even engineers who certainly should grasp the intricacies of work and power end up confused.
I’m sure it adds to the confusion when some diving physiologists speak in quotients. For example, since a cubit is a length of 48 cm, and a hectare is 2.47105 acres, you could describe a person’s height as 165,400 cubic cubits/hectare. Dimensionally, that would be correct for a six foot (1.8 m) tall individual. However, most people would prefer the units of feet or meters rather than cubic cubits per hectare. Certainly, the simpler description is far more parsimonious than the former.
The shaded area within this triangle is equal to the “Work” inside the previous P-V loop. By dividing by tidal volume, you obtain the average mouth pressure on the vertical axis.
For the same reason, it makes more sense to speak of a descriptor with units of pressure as simply pressure (kPa) rather than a quotient of work per liter (Joules/L).
If describing a simple parameter like pressure as a quotient is not defensible scientifically, is it defensible psychologically?
Maybe. The U.S. Navy has used terms like “resistive effort” to convey the impression that a volume-averaged pressure is something that can be sensed by a diver. To breathe, divers have to generate a pressure in their chest, and that pressure generation requires effort.
“Effort” is admittedly not a hard-science term: it doesn’t even pretend to be. However, the use of “Work of Breathing” connotes hard science; the concept of work is pure physics. But as I have shown, the way it is increasingly used in diving is not pure physics at all. So its use is misleading in the eyes of a purist, and undoubtedly confusing to a young engineer or physicist.
But to a diver, does it matter? Does it somehow make sense? Do divers care about parsimony?
Well, I have yet to find anyone who does not intuitively understand the notion of the work involved in breathing. If they have asthma, or have tried breathing through a too long snorkel, they sense the work of breathing. So I imagine that the inexactitude of J/L is of no import to divers.
However, I also believe that the over-complication of an arguably simple concept should be just as unappealing to designers of underwater breathing apparatus as it was to William of Occam or, for that matter, the designer of the Cosmos.
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.
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.
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