By Reilly Fogarty
Header photo: rEVO dual radial scrubber, courtesy of MARES
“Chemistry can be a good and bad thing. Chemistry is good when you make love with it (Ed.—or go diving!). Chemistry is bad when you make crack with it.” —Adam Sadler
Rebreathers are fickle things, and the dangers posed by their use are in no small part because of the carbon dioxide absorbent they use. Soda lime, also called “sorb” or referred to by the commercial names Sofnolime, Intersorb, or Sodasorb is a granular compound used to remove carbon dioxide from a closed breathing environment. It’s the latest iteration in carbon dioxide scrubbing solutions for diving equipment, and it’s here to stay for the foreseeable future.
Because of its sensitive and operationally critical applications, soda lime must be stable, inert to a wide range of gases, react predictable, and be cost effective to produce. The long list of requirements and challenging applications make it an interesting case study in chemical innovation, but it comes with some serious tradeoffs.
The chemical makeup of soda lime varies slightly by manufacturer, but it consists of approximately 75% calcium dihydroxide (sometimes called slaked lime), 20% water, 3% sodium hydroxide, and (in the case of many manufacturers) 1% potassium hydroxide.The result is a compound that is relatively inert as a dry powder but that can effectively and reliably react with gaseous carbon dioxide. This then begins a series of chemical reactions resulting in the neutralization of carbon dioxide with water, calcium carbonate, and heat as relevant byproducts. (Freeman, 2014). When used appropriately soda lime is extremely efficient and requires little monitoring, but it poses significant toxic and corrosive hazards if misused. Here’s everything you need to know about your sorb.
Modern soda lime is the result of centuries of attempts to safely recirculate exhaled gas to extend gas supplies and exploration potential. As early as 360 BCE, Aristotle describes divers using upturned pots as “diving bells” to capture their exhaled breath, allowing them to dive deeper and stay longer in order to collect food. There were no scrubbers to speak of at that point, and the effects of carbon dioxide on the divers became a topic of research unto itself.
The first device that really began to resemble the modern rebreather was Giovanni Borelli’s 1680 invention, which used copper tubing cooled by seawater to condense impurities in exhaled gas. Stephen Hale is then credited with one of the first chemical carbon dioxide absorbers, using sea salt and tartar (potassium bitartrate, not the sauce) inside a diving helmet to remove impurities in 1726.
A number of other chemical solutions evolved over the course of time, from barium hydroxide to potassium/sodium hydroxide scrubbers, but the first inklings of modern soda lime can be traced back to 1777. Carl Wilhelm Scheele, a Swedish chemist who is credited with independent discoveries of oxygen, chlorine and manganese, recorded an experiment in which he kept bees alive in a glass jar by absorbing the carbon dioxide they produced with lime water. [For a fascinating history of CO2 absorption see: SODASORB® Manual Of CO2 Absorption]. This experiment was duplicated by Henri Regnault and Jules Reiset in 1847 with dogs, and a granular form of the compound was finally patented in 1930 by Charles A. Carey and the Dewey & Almy Chemical Company, Cambridge, Massachusetts, which was later acquired by chemical giant W.R. Grace in 1954.
Several manufacturers have now developed methods to produce soda lime, and it can be found in a number of similar formulations and granule sizes. While we’re familiar with the technology from its use in diving, soda lime is also used in a number of military and aerospace applications, and sees its widest use in the medical field. Anesthesia machines, some ventilators, and a number of more specialized medical applications all involve breathing circuits similar to those used in rebreathers.
As mentioned, soda lime is a granular compound consisting of approximately 75% calcium dihydroxide (Ca(OH)2, 20% water (H2O), 3% sodium hydroxide (NaOH), and 1% potassium hydroxide (KOH) that is designed to neutralize gaseous carbon dioxide in the presence of heat and water (Freeman, 2014). It’s produced in a variety of forms with minor variations. Some products add an indicating agent, most often a purple dye, that illustrates approximately where the reaction front is in the soda lime scrubber and how much unused soda lime remains. Silica is also added to many products to make the granules harder to reduce the formation of alkaline powders which can cause bronchospasm and mechanical complications, and in medical applications additives are included to reduce the potential for reaction with volatile anesthetic gases.
The ability of the powder to absorb carbon dioxide relies specifically on the sodium hydroxide content and can be best understood in a series of three reactions:
(1) CO2 + H2O ⇌ H2CO3
(2) H2CO3 + 2NaOH (or KOH) ⇌ Na2CO3 (or K2CO3) + 2H2O + Heat
(3) Na2CO3 (or K2CO3) + Ca(OH)2 ⇌ CaCO3 + 2NaOH (or KOH)
The first step involves the combination of gaseous carbon dioxide with liquid water and the formation of aqueous H2CO3 or carbonic acid.
(1) CO2 + H2O ⇌ H2CO3
The evolution of the H2CO3 results in a strongly acidic solution with a pH of approximately 3.49, which can be more easily neutralized by the basic sodium hydroxide in the next step of the reaction:
(2) H2CO3 + 2NaOH (or KOH) ⇌ Na2CO3 (or K2CO3) + 2H2O + Heat
In this step, the carbonic acid from the first step is neutralized by either sodium hydroxide or potassium hydroxide, both of which are used as activators to catalyze the formation of sodium and potassium carbonates. These strong bases are ideal for this reaction because they can completely dissociate in water and react with the weak carbonic acid from the first step.
This neutralization reaction results in the evolution of either sodium carbonate and/or potassium carbonate (depending on the original soda lime composition), water and heat. At this point, the gas has entered the soda lime scrubber and passed the reaction front—the area where fresh absorbent meets carbon dioxide in the gas—and is exiting the scrubber. The reaction front will move as soda lime is consumed by the reaction, and the speed and efficiency of the reaction will be affected by factors like temperature, remaining soda lime, heat and humidity, and the concentration of carbon dioxide in the gas.
The final step of the process involves the reaction of calcium hydroxide with the sodium carbonate and potassium carbonate of the products to form calcium carbonate (CaCO3), a stable and insoluble precipitate notably used as a dietary supplement—in toothpaste and in agriculture.
3) Na2CO3 (or K2CO3) + Ca(OH)2 ⇌ CaCO3 + 2NaOH (or KOH)
This step results in the formation of additional hydroxides which are then used to catalyze further reactions with carbonic acid. In this way the hydroxide catalysts are reused, while the calcium dihydroxide is consumed as the scrubber is used. It’s important to note that while the reaction front of the scrubber is typically easy to identify, it is not the only location for carbon dioxide neutralization, which occurs throughout the scrubber. It is just the area in which the greatest level of activity occurs due to the combination of exhaled gas and fresh soda lime.
Unfortunately, soda lime use has not proven to be without danger. Issues with soda lime in diving and space exploration have primarily fallen into one of two categories; difficulties in monitoring during use, and hazards posed by common mechanical or systemic failures. The first category primarily comes from the difficulty that divers and astronauts have in tracking the reaction time and remaining reaction potential of the soda lime. These users rely on the ability of soda lime to neutralize carbon dioxide in their exhaled gas in inhospitable environments where immediate return to the surface or an atmosphere conducive to life is not always possible.
The resulting hypercapnia, spurred by high end-tidal carbon dioxide, can result in unconsciousness or death in the environments that these divers and astronauts work in, and warning signs may be nonexistent or masked by mental and physical impairments caused by other symptoms of high carbon dioxide levels. The hypercapnia that results from high end-tidal carbon dioxide is one of the most dangerous threats that rebreather divers face, and various methods have been developed to track reaction speed and remaining reaction potential.
In medical applications, a dye that is activated by the carbon dioxide neutralization reaction indicates the usage of a scrubber, but this has proven unreliable in rebreathers. Because the dye relies partially on temperature, it can revert in the time it takes for a diver to get out of the water and inspect their scrubber, and it has proven to be an unreliable indicator of scrubber usage in the high-gas density and moist environment of a rebreather. Additionally, the U.S. Navy implicated indicating absorbent as a possible cause of an ammonia-like odor reported during a dive in 1992, and its use was discontinued. Follow-up work showed that ammonia, ethyl and diethyl amines, and aliphatic hydrocarbons were found in both Sodasorb and Sofnolime scrubbers, possibly as a result of a breakdown of the indicating dye, but the work was not able to be reproduced in similar environments, leaving some question as to the source of the contamination.
More modern approaches have used temperature probes, also known as temperature sticks, or “temp stiks,” which were developed independently during the last decade by both the U.S.Navy Experimental Diving Unit (NEDU) and AP Diving, as well as diver physiological metrics to estimate scrubber usage. According to a 2019 paper, temp stiks have been shown effective in providing a timely warning of significant CO2 breakthrough. However, the majority of divers still estimate their scrubber duration via fairly crude calculations based on known reaction potentials, with enormous conservatism factors applied to those calculations.
Approximately 100g of soda lime is known to absorb 26L of carbon dioxide (Freeman, 2014), and some variation of this estimation is used by most training agencies and manufacturers as a basis for their scrubber duration. Some manufacturers do complete more involved laboratory tests to confirm scrubber performance under known conditions, such as the Conformité Européene (CE) EN14143 test, which measures scrubber duration at a depth of 40 m/131 ft, water temperature 4ºC/39.2ºF, 40 liter/minute breathing rate, and a CO2 production rate of 1.6 liter per minute. However, the extent of these tests varies by rebreather manufacturer.
The difficulty in estimating scrubber performance, even with a known reaction potential, lies in the huge variability in absorbent performance based on temperature, gas density, how the scrubber was packed, and the design of the scrubber.
Diver physiological metrics can also be used to estimate scrubber usage. For example, Global Underwater Explorers (GUE) has developed a relatively recent approach to safely estimating scrubber duration, called Absorbent Canister Endurance (ACE) using theoretical soda lime absorption performance. These calculations are only as accurate as their premise, so it’s important to understand that the ACE approach relies on all of the same variables as the CE endurance test, except for the metric for carbon dioxide production. This metric is hugely variable between divers, but can be estimated based on RMV and oxygen consumption with a volume of carbon dioxide produced (VCO2) and volume of oxygen consumed (VO2) assumed to be equal.
Caustic Cocktails Anyone?
Because soda lime relies on extremely caustic sodium and potassium hydroxides to catalyze the carbon dioxide neutralization reaction, the combination of the granular powder with more water than required for the reaction can result in significant injuries. This presents additional challenges, because the scrubbers require some moisture to function, but uncontrolled liquid in a scrubber can dissolve some unreacted soda lime and create a caustic slurry. Ingestion or inhalation of this slurry, which has an estimated pH of 14, can cause burns to the mouth, throat and airway, and cause general respiratory distress.
In at least one case documented at the Department of Emergency Medicine at the University of California, San Diego, small amounts of water endered a Drager LAR V closed circuit oxygen rebreather resulting in an aqueous soda lime solution entering the patient’s lungs and causing an overwhelming burning sensation in his oropharynx and chest, resulting in an emergency ascent that could have caused further injuries. The good news is that this type of “caustic cocktail” injury is often caught quickly by divers, and most often results in minor irritation to the mouth and throat. In the event of a cocktail, the diver should bailout and if possible immediately flush out the caustic fluid from the mouth and oral cavities while underwater. If the soda slurry does get into the stomach it’s not a serious problem; the concern is more the pharynx and esophagus. Drinking water at the surface is encouraged. Note that Divers Alert Network (DAN) is currently conducting its Rebreather Survey 2020 to collect information on caustic cocktails.
Interestingly, in medical applications, soda lime poses an additional hazard. The mechanism is yet-undetermined but desiccated soda lime and high flow application of volatile organic gases, like those used for anesthesia, have been implicated in the production of substantial amounts of carbon monoxide. This phenomenon does not occur in soda lime used at the correct humidity, nor does it occur with diving gases, but it illustrates the importance of maintaining soda lime moisture at appropriate levels.
Storing Your Scrubber
It’s easy to fall into the trap of extrapolating safety data into absurdity, and scrubber storage is one area where message board communities run rampant with safety policing. While it’s true that erring on the side of caution is important, particularly for rebreather divers, the data just doesn’t support the idea that a used scrubber should be discarded immediately. As long as soda lime is kept at the moisture content required by the manufacturer (typically 16-20%) and away from light, heat and contamination, it stands to reason that a used scrubber can be stored and reused as long as usage is carefully recorded and duration estimated conservatively.
Researchers at Laval University and the University of Auckland recently put scrubber storage to the test, storing used scrubbers with known carbon dioxide exposures open, sealed in an airtight plastic bag, and open overnight and then sealed in a plastic bag. These scrubbers were stored in this manner for 28 days, then then put back into a laboratory simulation of a working rebreather and tested until failure. While the scrubber stored in room air lasted an additional 188 minutes, the vacuum sealed scrubber lasted 241 minutes and the scrubber left open overnight and then sealed lasted 239 minutes. In no case did the scrubbers fail spectacularly, and while the sample size is relatively small, it appears that storage in a vacuum sealed bag, with or without leaving the scrubber in room air overnight to dry from use, is a safe way to store and then reuse packed soda lime.
- Freeman, B. S. & Berger, J. S. (2014) Anesthesiology Core Review: Part One Basic Exam. New York, McGraw-Hill Education Medical.
From the blog of John Clarke, retired scientifici director of NEDU:
Shearwater Research: The CO2 Scrubber In A Diver’s Rebreather: How Long Does It Work And How Long Does It Actually Last? by Dan Warkander
Reilly Fogarty is an expert in diving safety, hyperbaric research and risk management. Recent work has included research at the Duke Center for Hyperbaric Medicine and Environmental Physiology, risk management program creation at Divers Alert Network, and emergency simulation training for Harvard Medical School. A USCG licensed captain, he can most often be found running technical charters and teaching rebreather diving in Gloucester, Massachusetts.
Risk-Takers, Thrill-Seekers, Sensation-Seekers, and … You?
It’s likely that many in our community no longer think of tech diving as a risky activity, or perhaps even appreciate how important taking risks may be to one’s personal health—let alone that of our species. Fortunately, InDEPTH’s copy editing manager Pat Jablonski dived deep into the origins, meaning, and benefits of regularly taking risks, and even offers a thrill-seeking quiz for your edgy edification. What have you got to lose?
by Pat Jablonski. Title photo courtesy of Katelyn Compton Escott.
“Life without risk is not worth living.” – Charles Lindbergh
What defines a risk? What is involved in taking a risk?
Difficult questions to answer, because something that feels risky to one person might be yawn-worthy to another. Risk taking, unscientifically, is something you do that gets your blood up, raises your heartbeat, awakens your senses, and makes you hyper-aware of your surroundings.
Surely we can agree that the Covid pandemic has added an unexpected level of risk to everyday life. Add poor drivers, mass shootings, contentious politics, global climate change, and many are left believing that meeting each day is risky enough. But that’s not true for people who identify as risk-takers or thrill-seekers.
“Everyone has a ‘risk muscle’. You keep in shape by trying new things. If you don’t, it atrophies. Make a point of using it once a day.” – Roger Von Tech
There are many activities that go to the trouble of defining the level of risk involved with a specific activity, and while that’s not the purpose of this article, you should know that scuba diving ranks fairly high on the risky behavior scale–higher than skydiving and rappelling. And, cave/wreck diving or freediving isn’t on any risk scale we could locate. We can assume it’s up there—near or at the top.
Divers are a fairly small niche group for many reasons. One of them could involve the degree of danger associated with the sport. Answer this: Do dry land people ever ask you why you would want to take such a chance with your life in order to go where you weren’t meant to go?
It’s a reasonable question, albeit a hard one to answer.
“A life without risk is a life unlived, my friend.” – Big Time Rush
Kevin Costner’s Waterworld aside, humans have (yet) to be born with gills or webbed toes. Still, there you are. You’ve spent unmentionable amounts of money. You’ve carved out a whole day, or maybe weeks, away from your to-do list. You’re suited up and look like an alien. You’re on a quest to explore the aquatic world where you’re able to breathe only with a cumbersome apparatus. You’re planning to explore inner space! You’re going to delve into that amazing realm that’s off limits to most people.
You may look all matter-of-fact, cool as a cucumber, another day at the office, but it’s a thrill, isn’t it? Inside, you’re a kid with butterflies in your tummy who’s getting away with something big and exciting. Okay, it’s true–you and your team are highly trained, your equipment is top-notch, every box is checked off, and you are behaving responsibly. However, you’d have to be in a coma to not realize that what you’re about to do is taking a risk. Who doesn’t know that people have died doing what you’re doing? Answer honestly: How much more exhilarating is the experience when you know it’s not a walk in the park? Our own Michael Menduno admitted that “the feeling of being more alive lasted for days” after a dive.
So, you’re a diver. Does that mean you’re a risk taker? A thrill seeker? A sensation seeker?
Let’s dive into that subject, first by taking a little quiz, shall we?
From A Death Wish to Life Is Precious
In the past, too many mental health professionals treated risky behavior like a disease in need of a cure, focusing on the negative side of risk, even using government funding to address risky behavior and stamp it out.
Before that, Sigmund Freud might have even believed that thrill seekers had a death wish; in fact, it’s what was believed for many years.
Modern-day science doesn’t support either theory.
“Only those who will risk going too far can possibly find out how far one can go.” – TS Elliot
For our purposes, we’re focusing on the positive aspects of taking chances, pushing boundaries, and seeking experiences that make life feel . . . more alive. Richer. Fuller. We want to examine what goes into the psyche of a person (like you?) who is enthusiastically willing to engage in an activity already identified as dangerous, possibly even by the people who are engaging in it, and hear what some experts on the subject have to say about such people.
The University of Michigan’s Daniel Kruger proposes that taking chances is a fundamental part of human nature going all the way back to our ancient ancestors—prehistoric humans who had to constantly put their safety on the line in their fight for survival. Think fighting off a wooly mammoth with a stick. Kruger believes we have consequently retained many of those same instincts today, and he believes that it’s a good thing.
This writer, who is related to a major risk-taker, has always believed that heart-quickening experiences are essential for a well-lived life. I’m convinced and have long proposed that those pulse-pounding moments are often accompanied by a deepened understanding of and appreciation for one’s life—perhaps all life. And I’m happy to report that current science confirms that belief.
“If you are not willing to risk the unusual, you will have to settle for the ordinary.” – Jim Rohn
Dr. Kruger is one of the scientists who proposes that taking risks means “seeking that moment when life feels most precious.”
This should not be news for you diving adventurers out there.
Nature vs. Nurture: Born That Way or Learned To Love Adventure
Another scientist, Marvin Zuckerberg, affirms the theory that risk taking is in our DNA. “Certain people have high sensation-seeking personalities that demand challenges and seek out environments that most people’s brains are geared to avoid.” I’ll go out on a limb and say that underwater caves or shipwrecks would qualify as environments most would avoid.
Dr. Cynthia Thompson, the researcher behind a 2014 study from the University of British Columbia, was early to look at the genetic factors that might make a person predisposed to participating in extreme sports, ones that are typically defined as activities where death is a real possibility. The results of her study revealed that risk-takers shared a similar genetic constitution, a genetic variant that influences how powerful feelings are during intense situations.
Most scientists agree that personality is a complicated mix of genetic and environmental influences. The “nature vs. nurture” dilemma is alive and well. Dr. Thompson concluded that people who engaged in so-called high-risk sports were not impulsive at all, not reckless either. Instead, “they’re highly skilled masters of their discipline who take a very thoughtful approach to their sports.”
A study conducted in 2019 examined human boundaries, people who pushed them to their limits and beyond, and what made those people tick. Zuckerman labeled such people “sensation seekers” and defined them as “people who chase novel, complex, and intense sensations, who love experience for its own sake, and who may take risks to pursue those experiences.” Is that you?
“History is full of risk-takers. In fact, you could say that risk-takers are the ones who get to make history.” – Daniel Kruger
Other experts posit an alternate theory—one proposing that modern society in the age of seatbelts, guardrails, child-proof caps, safety precautions, laws, rules, and regulations has dulled the sense of survival. In other words, life has flattened out and no longer feels exciting, or risky. So, is one of the reasons we seek excitement because of boredom?
Maslow’s Theory of Self-actualization
I don’t honestly know who was the first proponent of risk-taking being a positive thing, but the work of Abraham Maslow, the founder of humanistic psychology, was one of the first. Maslow became one of the most influential psychologists of the twentieth century, and he developed a theory of human motivation that advocates for “peak experiences.” Peak experiences are not attained without risk.
“One can choose to go back toward safety or forward toward growth. Growth must be chosen again and again; fear must be overcome again and again” – A Maslow
He proposed that, in addition to meeting basic needs, all humans from birth seek fulfillment in terms of what he called self-actualization—finding their purpose/being authentic. Self-actualization involves peak experiences—those life-altering moments that take us outside ourselves, make us feel one with nature, and allow us to experience a sense of wonder and awe. Maslow also believed that those who were able to have such peak experiences tended to seek them out rather than waiting for the next random occurrence. Hence the anticipation of the next dive?
“Do one thing every day that scares you.” – Anonymous
Out of Your Comfort Zone Into A World of Wonder
Psychologist Eric Brymer from Queenstown University of Technology in Brisbane, Australia, has spent years studying extreme athletes and has this to say: “They’re actually extremely well-prepared, careful, intelligent, and thoughtful athletes with high levels of self-awareness and a deep knowledge of the environment and of the activity.”
Recent research backs up what some extreme sports athletes have been saying for years, even if only to themselves.
“What participants get from extreme sports is deeply transformational, a sense of connecting with a deep sense of self and being authentic, a powerful relationship with the natural world, a sense of freedom,” says Brymer. “They get a strong sense of living life to its fullest as if touching their full potential.”
Brymer’s comments mirror what Maslow, the founder of humanistic psychology, said back in the 1940s.
We’re not advocating for taking stupid chances (such as diving without proper training, or necessary precautions) and we don’t believe anyone reading this article does that. We simply intended to focus on the scientific evidence that supports adventurers—people who get a thrill from an activity that offers—as a bonus; a chance to feel awakened from the mundane and thrust into a world of wonder.
Risk-takers and sensation- or thrill-seekers chase unique experiences. Often, those experiences bring awareness of important issues or increase essential knowledge about the planet we share. Many people overanalyze and dither when faced with an unfamiliar situation; they shy away from unsettling circumstances. Risk-takers face the unknown and trust themselves to prevail. Learning to scuba dive, for example, pushes people out of their comfort zone, takes them into a realm foreign and mysterious. Diving forces divers to pay complete attention to a task, to focus with laser-like precision in order to conquer misgivings, and to attain a skill that few others have. Confidence comes with accomplishment. Leadership emerges. Fear is overcome.
Sensation-seekers see potential stressors as challenges to be met rather than threats that might defeat them. With action, resilience develops. High sensation-seekers report lower perceived stress, more positive emotions, and greater life satisfaction. Engaging in extreme activities brings them peace.
What does it bring you?
Bandolier: Risk of dying and sporting activities
National Geographic: What Makes Risk Takers Tempt Fate? Recent research suggests that genetic, environmental, and personal factors can make people take on risky—even potentially fatal—challenges.
Healthday: Taking Risks By Chris Woolston HealthDay Reporter
Pat Jablonski heads up the copy edit team for InDEPTH. She is a blogger, a writer of stories, a retired tutor, English writing teacher, and therapist. She’s a friend, a wife, a proud mother and grandmother. She is also a native of Florida, having spent most of her life in Palm Beach County. She has a B.A. in English from FAU in Boca Raton and an M.S.W. from Barry University in Miami. She learned to swim in the ocean, a place she thinks of as home, but she doesn’t dive.
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