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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.
Learning from Others’ Mistakes: The Power of Context-Rich “Second” Stories
Proper storytelling is a key to learning from the mistakes of others. Human Factors consultant and educator Gareth Lock explains the power of context-rich stories to inform and help us to develop the non-technical skills needed to make better decisions, communicate more clearly, and lead/teach more effectively.
by Gareth Lock
Header image courtesy of Gareth Lock. Divers from Red Sea Explorers’ examining a magnificent gorgonian coral.
Diving can be a fun, sociable, and peaceful activity; it can be challenging and technically difficult; and it can be a way of escaping the hustle and bustle of modern life. Sometimes new wrecks are discovered, caves have new line laid in them, new encounters with wildlife are experienced, and in many cases, courses are completed where both instructors and students have learned something new.
However, it can also be scary, harrowing and frightening if things don’t go to plan or if the plan was flawed in the first place.
Fortunately, the majority of dives which take place are the former and we consider the outcomes to be positive. If we think about it, the goal for every dive should be to surface, having had an enjoyable time, with gas reserves intact and no-one feeling physically or emotionally injured. But how do we achieve this goal considering the inherent risks we face while diving?
The easy answer would be to have effective training, to have the correct equipment, and to have and apply the right mindset. These three things together then lead to safe diving practices. You could say that the majority of safe diving practices and safely designed and configured equipment comes from feedback following accidents, incidents, and near misses. You only have to look at the work which the late, famed cave explorer Sheck Exley did in terms of cave diving fatalities and his “Blueprint for Survival” to see how procedures and equipment have evolved.
What do we learn?
There are accident and incident reports available to us. What do we learn from them? Bearing in mind that the majority of reports which divers see are either in social media or summarised in reports like the Divers Alert Network Annual Incident Report or the BS-AC Annual Incident Report.
For example, the following incident reports are written in a style similar to those you would find on social media or in an organization’s incident report.
An inexperienced diver entered the water to provide support for a guided dive to 24m. They got separated from their buddy, made a rapid ascent to the surface after nearly running out of gas. They were recovered on the boat without any symptoms of DCS being present.
A diver on the final dive of a rebreather training course entered the water from a dive boat. The diver swam to the side of the boat to receive their bailout cylinder to clip on. While sorting their gear out alongside the boat, they appeared to go unconscious and descend below the surface. The diver was recovered from 38 m/124 ft and despite CPR and first aid being applied, they were pronounced dead on arrival at the hospital ER. On inspection, the oxygen cylinder on their rebreather was found to be turned off and the controller logs showed that the pO2 had dropped to 0.05 while they were on the surface.
How much learning do you get from these reports? What emotions did you feel while reading them? What did you think was the primary cause of each of these events? If you were to choose two or three words to describe the causes, what would they be?
Human error? Complacency? Inexperience? Rushing? Not paying attention? Overconfidence? Naivety? Arrogance? Stupidity? Who was it? Where was the instructor? Were they certified? Which agency? Were they qualified?
All of these are normal responses, and they make up the first story.
The First Story
The first story is the narrative we hear, and we start to make immediate judgments on. We can’t help making judgments, even when we try not to. We make judgments because we compare the stories we’ve just read or heard to our own previous experiences. We match patterns to what we ‘know’ and then fill in the gaps with what we think happened, all the time thinking about whether it was the ‘right thing’ to do based on our own experiences.
This ‘filling in gaps’ is normal human behavior. Because our brains are constantly trying to make sense of the situation when we don’t have enough information about a scene or a situation, we reflect on what we’ve seen, read, and heard in the past and then make a best guess or closest fit. During this process, we will be subject to a number of biases, and one of the strongest at this stage is called confirmation bias. This is where we think we know the answer to the question, then as we read or hear something in the story that aligns with our reasoning, we stop looking any further because we have confirmed our suspicions.
In many cases, we carry on and don’t think anything of the learning opportunities presented because we know what happened, we know that ‘we wouldn’t do that’ because we would have spotted the issue before it became critical. We often make use of counterfactuals (could have, should have, and would have) to describe how the incident could have been prevented.
Unfortunately, this means that often we don’t learn. There is a difference between a lesson identified and a lesson learned—a lesson learned is where we make a conscious decision to accept how we do things based on the conditions and outcomes, or we actually put something in place which is different than what was there before and see how effective it is to resolve the problem encountered.
If we are to make improvements, we need to look at the errors, mistakes, and deviations that were made. However, we must recognize that errors are outcomes, not causes of adverse events. If we want to stop an adverse event from occurring, we need to look closer at the conditions which led to the error occurring i.e., the error-producing conditions.
The easiest way to look for error-producing conditions in an event that has already happened is to get those involved to tell context-rich stories. This becomes the second story.
The Second Story
Second stories look much deeper than what we first hear. They look at the context, the local rationality, the conditions, especially those conditions which might lead to errors. Ultimately, they expose the inherent weakness and gaps in any system, where the system includes people, paperwork, equipment, relationships, the environment and their interactions.
Second stories also highlight how divers and instructors are constantly adapting and changing their behaviors/actions to deal with the dynamic nature of diving. They describe ‘normal work’. This adaptation could be moving dive sites, increasing or reducing the time for a course, the order in which skills are taught or the amount of gas used/planned for a dive. Second stories describe the difference between ‘Work as Imagined’, which is what is written down, what is expected to happen, and against which compliance is assessed, and ‘Work as Done’ which is what actually happens in the real world and takes into account the pressures, drivers, and constraints which are faced by those on the dive or the course.
The easiest way to see what a second story looks like is to tell it, and the following account is the same recreational event as above but told as a second story.
An Advanced Open Water (AOW) diver with around 50 dives was acting as an ‘assistant’ to the instructor and dive-centre owner on a guided dive with five Open Water (OW) divers and recent graduates from the school they themselves had learned at. The AOW diver felt a social obligation to help the Open Water Scuba Instructor (OWSI) who was leading the dive, because the OWSI had done so much to help her conquer her fear of mask-clearing during her own training. However, she was also wary that, over time, her role had moved from being a diver on the trip to being almost the divemaster by helping other divers out, which she wasn’t trained to do. In addition, the instructor regularly asked her, at the last minute, to help out and change teams to ensure the ‘experience’ dives happened.
On this particular occasion, the AOW diver was buddied with a low-skilled OW diver who acted arrogantly and did not communicate well. In fact, she didn’t believe that three of the five on this trip should have received their OW certificates, given their poor in-water skills. As they approached the dive site, the visibility could be seen to be poor from the boat and the surface conditions weren’t great. The instructor said to the AOW diver, “Don’t lose the divers. I want you at the back shepherding them.”
They entered the water and descended to 24 m/78 ft and made their way in the poor visibility. On two occasions, the OW buddy had to be brought back down by the AOW diver as they ascended out of control. At one point, the OW diver turned around quickly and accidently knocked the AOW diver into the reef. Unfortunately, the AOW diver became entangled in some line there, and the OW diver swam off oblivious to the entanglement. When the five divers and instructor reached the shot-line ready to ascend, the instructor realized the AOW diver was missing. The instructor couldn’t trust the five divers to ascend on their own and didn’t have enough time to wait at the bottom and conduct a search, so the six ascended. On the surface, the buddied OW diver said that the AOW diver had swum off looking at fish in a certain area.
In the meantime, the AOW diver had managed to free herself; but in her panic, while stuck on the bottom, she breathed her gas down to almost zero and had to do a rapid ascent. She surfaced, feeling very scared and sick with panic, just as the instructor was speaking to the other six on the surface. On seeing the AOW diver break the surface, the instructor swam to her but turned and shouted at the other divers, admonishing them for abandoning their buddy on the bottom. The AOW diver felt very alone and wanted to give up diving as she was not given the opportunity to tell her side of the story.
Observations on potential contributory factors and error-producing conditions:
- Deviation of standards on the part of the instructor/dive-center owner taking OW divers to 24 m/78 ft, maybe driven because of the need to generate revenue and offer something unique.
- Authority gradient between the instructor and AOW diver meant that the AOW diver felt they couldn’t end the dive before they even got in the water or once in the water.
- Inferred peer pressure to help out when they weren’t qualified or experienced enough to act in a supervisory role.
- Poor technical skills on the part of the OW divers and the AOW limited their situation awareness to be aware of hazards and risks.
- Limited awareness on the part of the instructor regarding the location of all the divers during the dive.
- Positive note – good decision on the part of the instructor to ascend with the five OW divers in poor conditions and not keep them on the bottom or get them to ascend on their own.
A full account of the second event can be found here where you can also download a guide which contains more detail than the video covers and also gives you details on how to run a learning event at your dive center or in your own classes.
We can see that the learning opportunities have increased in the second stories. They allow certain issues to be identified like time pressures, financial pressures, peer-pressure, authority gradient, teamwork, leadership, decision-making and situation awareness. These aspects are rarely captured or recounted in the narratives we see online or in incident reports. There are a number of reasons:
- They are often considered ‘common sense’,
- Our brains are constantly looking for simple answers to complicated or complex problems, and one of the easiest ways to do this is to find an individual or piece of equipment to ‘blame’ rather than look wider.
- Those involved don’t consider these factors to be important so they don’t write them down.
- Those involved don’t know about these error-producing conditions or human factors so they don’t know to include them.
- There is no formalised and structured investigation process for diving incidents by diving organisations to facilitate the capture, analysis and sharing of second stories.
Telling second stories isn’t enough to create learning though. We have to work out how to change our own behaviors, and that is where the free materials and courses which The Human Diver provides come in. They help develop these non-technical skills in divers, instructors, instructor trainers, and dive center managers/owners to help them make better decisions, communicate more clearly and lead/teach more effectively. Ultimately, it is about having more fun on the dive, and ending each dive with the goal described at the start of this article intact and creating learning in the process.
Since 2011, Gareth has been on a mission to take the human factors and crew resource management lessons learned from his 25 year military aviation career and apply it to diving. In 2016, he formed The Human Diver with the goal to bring human factors, non-technical skills and a Just Culture to the diving industry via a number of different online and face-to-face programmes. Since then, he has trained more than 350 divers from across the globe in face-to-face programmes and nearly 1500 people are subscribed to his online micro-class. In March 2019, he published ‘Under Pressure: Diving Deeper with Human Factors’ which has sold more than 4000 copies and on 20 May 2020, the documentary ‘If Only…’ was released which tells the story of a tragic diving accident through the lens of human factors and a Just Culture. He has presented around the globe at dive shows and conferences to share his passion and knowledge. He has also acted as a subject matter expert on a number of military diving incidents and accidents focusing on the role of human factors.
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