Counterlung Configurations and their Impact on Diver Safety & Performance

by Paul Haynes. Lead image: RB80 diver by Sean Romanowski

While there has been considerable recent interest in frontmounted and sidemount rebreathers, backmounted counter lungs (BMC) are the prevalent choice in the tech community. However, many tekkies don’t fully appreciate the functional differences between counterlung designs and how these can impact performance, and in particular, diver safety. Fortunately, military and tech CCR veteran Paul Haynes takes us for a deep dive into the workings of various counterlung designs, and discusses some of the hazards of BMC and possible remedies. 

Backmounted counterlungs (BMC) are now prevalent among the sport/technical rebreather community, and this configuration is the basis for a number of popular units. The increasing popularity of BMCs is quite understandable. Positioned behind the diver, the counterlung(s) are out of the way and leave a relatively clean front side that many consider easier to don and doff. Plus, they provide a larger field of view.

During a recent rebreather configuration workshop, a mixed gas trainee mentioned that their original instructor had advised them that the choice between frontmounted and backmounted counterlungs is simply a matter of personal preference—that there is no functional difference. Following this advice, the trainee opted for a BMC configuration. This choice was understandable as there has been a trend towards BMC rebreathers over the last decade. 

However, to suggest that there are no functional differences between counterlungs positioned either on the front or on the back is fundamentally incorrect. Indeed, due to notable performance and safety implications, all rebreather divers and rebreather instructors should be aware of and fully understand the differences. My aim therefore is to discuss the functional differences between various counterlung designs / configurations and explore how these differences relate to rebreather performance, practical diving, and safety.

Having worked in rebreather development for over twenty-five years, I have yet to encounter the perfect counterlung solution. Indeed, such an aspiration is possibly unachievable: each rebreather design compromises as it strives to balance functional and ergonomic requirements and performance.

In my main line of work, these competing requirements are driven primarily by military operational requirements instead of market trends.

Figure 2: The position of the counterlung(s) in relation to the diver’s lungs can have profound implications to rebreather functional performance and safety. Left: frontmounted counterlung; Centre: backmount counterlung; Right: over-the-shoulder mounted counterlungs. Illustration by Bori Bennett

Why Do We Need a Counterlung?

Let’s start with a fundamental question: Why do we need a counterlung? To facilitate ventilation and the re-circulation of breathing gas around the breathing circuit (or ‘loop’), divers need a compliant volume somewhere in the breathing circuit—a flexible volume that freely collapses when we inhale and expands when we exhale, hence the term counter-lung. If you imagine trying to breathe in and out of a rigid breathing loop, inhalation would require divers to suck a negative pressure, preventing full inhalation; ventilation would thus not be possible. 

However, those familiar with rebreathers will recognise that all rebreather loops incorporate compliant (flexible) and non-compliant (non-flexible) volumes. For example, carbon dioxide (CO₂) absorbent canisters are non-compliant, as are counterlung manifolds (T pieces), breathing hoses, and dive surface valves (DSV)/bailout valves (BOVs). Critically, the compliant volume has to match or to exceed the human tidal lung volume; this varies with work-rate and between individuals. As a consequence, the European rebreather standard EN14143 requires a minimum compliant volume of 4.5 litres to account for an extreme work-rate and divers with a large maximum tidal volume.

The compliant volume of some rebreathers takes the form of a single counterlung. However, these days, rebreather designs that partition the compliant volume into an inhale and an exhale counterlung are more common. Why, then, complicate the design with separate counterlungs? While in many cases a single counterlung might be the easiest design solution, a disadvantage of such a configuration is that breathing gas exhaled directly into the CO₂ absorbent canister can be driven quickly through the CO₂ absorbent bed. This reduces the opportunity for CO₂ to be absorbed at high work/breathing rates. This is particularly important toward the end of the canister’s life, where CO₂ can pass through the absorbent bed and be re-inhaled. 

Separating the compliant volume into an inhale and exhale counterlung reduces breathing gas peak velocity, which extends the time exhaled gas is able to linger or dwell in the absorbent bed. Prolonging canister dwell time increases the CO₂ absorption capacity potential before CO₂ canister break through finally occurs. In summary, inhale and exhale counterlungs smooth out gas flow and increase the CO₂ absorption potential of a canister, helping facilitate longer canister durations for a given mass of CO₂ absorbent.

 Another advantage is that, if appropriately designed, the exhale counterlung acts as a natural water trap, preventing or delaying moisture from entering the CO₂ absorbent bed which, if excessive, can increase breathing resistance and potentially reduce canister duration. Such a feature is particularly important in the event of a partial loop flood on the exhale side of the breathing circuit where a well-designed exhale counterlung can limit the likelihood of an alkaline chemical burn injury (caustic cocktail) that might arise if alkaline solution is aspirated/ingested.

Hydrostatic Imbalance

It is important that all rebreather divers are taught the principle of hydrostatic imbalance as it applies to counterlungs. However, this subject is sometimes brushed over during training, particularly where rebreather instructors are aligned with and financially incentivised to promote a particular manufacturer’s rebreather. Often referred to as static lung load (SLL), hydrostatic imbalance is the pressure difference between the diver’s lungs and the actual counterlung(s). 

Why does hydrostatic imbalance matter? As readers likely know, water is a dense (and thus relatively heavy) medium. As a consequence, the hydrostatic pressure exerted over a given area at a depth of 10 m/33 ft is approximately equal to the pressure exerted by 1 ATM over the same area. If we assume that 1 ATM of pressure is equal to 1,000 mbar, then 1 m of water depth is equal to 0.1 ATM, or 100 mbar. Breaking this down further, every 1 cm of water column is thus equal to 1 mbar of pressure. Using cm and mbar metrics for this discussion, we now have a relatable human scale connection between vertical distance in the water column and pressure. 

When testing underwater breathing apparatus (UBA), manufacturers frequently use a common fixed anatomical reference point called lung centroid—the centre of the average human lung (Figure 3).

Note: Some UBA test laboratories might use the sternal notch (Figure 3) as the hydrostatic imbalance / SLL reference point.

During unmanned rebreather laboratory testing for hydrostatic imbalance, the laboratory breathing simulator is typically set to a Respiratory Minute Volume (RMV) breathing rate of 62.5 L/min and pressure at the mouth is measured at the end of exhalation where, for a brief moment, gas velocity is zero: hence the term static lung load. The UBA is then repeatedly tested in different diver role and pitch orientations in order to build a full hydrostatic imbalance picture for a specific rebreather design (Figure 4). 

As the distance between lung centroid and the counterlung varies with different rebreather architectures, so does hydrostatic imbalance. The closer the counterlung is positioned to lung centroid, the less hydrostatic imbalance will occur: highly desirable when striving to optimise breathing performance.

Due to human anatomy, over-the-shoulder (OTS) counterlungs that extend onto the chest generally have the least hydrostatic imbalance compared to counterlungs positioned on the back; the latter, regardless of the rebreather architecture, stand further away from lung centroid due to their relative position to the spinal column and shoulder blades. 

Frontmounted counterlungs (FMCs) that extend down the chest onto the upper abdomen can provide poor hydrostatic imbalance performance when the diver is in a vertical, head-up orientation because breathing gas has to be forced down the water column (and into a higher-pressure zone) during exhalation. With gas naturally wanting to rise to the highest point in the water column, this can result in an uncomfortable positive pressure at the mouth and the sensation of needing to vent excess loop gas while head up (Figure 5). The opposite may be the case if the diver is in a vertical, head-down orientation. However, the hydrostatic imbalance of an FMC is reduced when the diver is in a prone position which, of course, is divers’ typical orientation for the majority of the dive; the slight positive pressure benefits ventilation (Figure 2, Centre). The subject of hydrostatic imbalance is, therefore, very nuanced; truly optimised OTS and FMC designs place as much of the counterlung volume on the upper chest as is ergonomically possible (Figures 11 and 12).

Notes: 

1. 1 kPa is, in practical terms, equal to 10 mbar
2. The data presented in Figure 5 references the suprasternal notch which, without adjustment, favours OTS counterlungs. European rebreather standard EN14143:2013, therefore, includes different maximum permitted positive and negative hydrostatic imbalance figures applicable to differing orientations when referencing the sternal notch (Figure 3).

Depending on the specific design, BMCs housed within a rebreather backpack could potentially position the counterlung approximately 15 to 25 cm/6 to 10 in above lung centroid in a prone (i.e., horizontal swimming) orientation. During inhalation, such a rebreather could generate a negative pressure of between -15 to -25 mbar as the diver strives to suck the gas down the water column into their own lung, which is located in a higher-pressure zone (Figure 2, Left). When on their back (supine), this would then result in a positive pressure of approximately 25 mbar. This would create a sensation of “hamster cheeks” and the need to vent excessive loop pressure. If the diver did this, venting the loop could cause the ADV to free-flow. 

Alternatively, a counterlung mounted on the chest when in a prone orientation (Figure 2, Centre) might, for example, result in approximately 7 mbar of positive pressure. It is worth noting that a positive pressure during inhalation is considered to be physiologically preferential compared to a negative pressure as the respiratory system is able to generate a greater exhale force compared to inhale force; in other words it is easier to blow out than suck in, so a slight positive pressure to assist with inhalation is beneficial to ventilation when using a UBA. This is particularly relevant when breathing high density gas when a potential for hypoventilation exists resulting from effort-independent ventilation, preventing alveoli gas exchangeand in particular the elimination of CO₂. ⁽²⁾ 

So, divers should appreciate that the human respiratory system has not evolved to move gas from one pressure zone to another; it’s unable to effectively manage negative hydrostatic imbalance generated by certain BMC rebreather designs that potentially expose the diver to an increased risk of pulmonary oedema (which is discussed in detail later). Therefore, to minimise BMC generated hydrostatic imbalance, numerous BMC rebreather manufacturers strive to place the counterlung(s) as close to the back of the shoulders and upper rear torso as is ergonomically possible as opposed to placing them within a ‘backpack’. However, anatomical real estate is limited in this area, and partial inflation of a wing Buoyancy Compensator (BC) (which also occupies this space) can restrict the full expansion of the counterlungs of certain BMC rebreathers. This can inhibit ventilation at high work-rates which, of course, is the time when you want ventilation to be as unrestricted as possible.

Alternatively, a well-designed OTS design will minimise hydrostatic pressure imbalance in all orientations, especially  in a prone position: the counterlung(s) is on the approximate same plane or height in the water column as the diver’s lung centroid, with a slight positive pressure (Figure 2, Right). It should also be noted that lung centroid placement can vary between individuals (Figure 3) and so the impact of hydrostatic imbalance can vary between divers⁽¹⁾. This, at least in part, might help explain why some divers report breathing discomfort in a certain orientation while others using the same rebreather do not. It is also worth noting that the agreed position of lung centroid on the laboratory manikin is standardised in order to help provide consistent test results between laboratories. The author is not aware of any female-specific testing allowances; this might explain hydrostatic imbalance experiential differences between individuals, particularly with OTS and FMC. 

Physiological and Safety Implications of Hydrostatic Imbalance

What then are the main physiological and related safety implications of high hydrostatic imbalance? Firstly, any resistance to breathing restricts the ability to ventilate, which is necessary for efficient alveoli gas exchange and, most critically, the elimination of CO₂ from the circulatory system. Human trials have demonstrated that due to gas density, when breathing air, ventilation is reduced by approximately 50% at 30 m/100 ft and reaches only to approximately 45% of surface ventilation capacity at 40 m/130 ft—the maximum recommended depth for an air diluent. This is a significant reduction in ventilation capacity. However, the metabolic consumption of oxygen and resulting generation of CO₂ for a given amount of work, is independent of depth.  As a consequence, CO₂ self-poisoning can occur during high-work rates when breathing gases at elevated densities due to an inability to efficiently eliminate CO₂. This can lead to severe hypercapnia, the early symptoms of which are increasing respiratory and psychological distress. If the hypercapnic feedback cycle is left unchecked, this can quickly result in loss of consciousness (LOC) and drowning if the airway is not protected from water aspiration. 

Considering this and other hazards while rebreather diving (hypoxia and hyperoxia, for instance) in the absence of a Full-Face Mask (FFM), the author recommends the use of a Mouthpiece Retaining Strap (MRS) when rebreather diving. Evidence suggests that MRSs increase the probability of surviving LOC underwater⁽³⁾. 

Any additional factors that impede ventilation, such as a high hydrostatic imbalance, can compound the problem of CO₂ retention and the resulting self-induced hypercapnia. It is important to note that the hypercapnia scenario discussed here must not be confused with hypercapnia arising from inhaled CO₂ as a result of absorbent canister CO₂ breakthrough. 

As previously discussed, some BMC designs can create a notable negative hydrostatic lung load and breathing difficulty in a head-down position. In addition, dependent upon Automatic Diluent demand Valve (ADV) positioning (if it’s not isolated), hydrostatic pressure difference between the ADV and the bottom of the counterlung can cause ADV free-flow—since the counterlung is higher in the water column when the diver adopts a head-down orientation (Figure 4). 

I have experienced this phenomenon on numerous occasions when descending into narrow cave and wreck passages using one very popular rebreather. This unintended diluent flush causes increased loop volume, a lowering of Oxygen Partial Pressure (PO₂), automatic injection of oxygen, and buoyancy control disruption; none of these are conducive to optimising life support system performance or diver safety. 

Some rebreathers are so readily prone to this phenomenon that initiating a diluent flush by lowering the ADV in the water column is a standard operating procedure. As a consequence, certain rebreathers require a hydrostatic imbalance test concession requiring the ADV to be isolated in certain head-down orientations in order to pass the hydrostatic imbalance requirements of European rebreather standard 14143 (Figure 4) (more about the potential safety implications of this later). 

To limit the detrimental effects of excessive static lung loads (and protect efficient ventilation), the European rebreather standard defines a maximum tolerable hydrostatic imbalance of between -20 to +20 mbar for all orientations with reference to lung centroid (Figure 4). However, having dived and witnessed unmanned laboratory testing of numerous sport diving BMC rebreathers, analysing the results and personal experience brings into question claimed compliance with the EN rebreather standard.

To conclude this section, do BMC configurations result in inferior hydrostatic imbalance performance when compared to OTS and FMC configurations? In general, empirical test data suggest yes, but not in all cases. Such a generalisation should therefore be used with caution. For example, a particular rebreather in the defence sector uses a rearmounted bellows as the counterlung attached to a counterweight of an appropriate mass to offset the hydrostatic imbalance; from personal experience, this configuration offers excellent breathing performance. 
Perhaps more relevant to the readership, one UK manufacturer has launched an innovative sport/technical diving BMC electronic Closed-Circuit Rebreather (eCCR) that positions the CO₂ absorbent canister on top of the counterlungs when prone. The weight (mass) of the canister pressing down on the counterlungs helps offset the negative hydrostatic imbalance created by positioning the counterlungs on the back. The manufacturer reports a small, but beneficial, positive hydrostatic imbalance in the prone position. Conversely, if a diver is supine, the weight of the CO₂ absorbent canister—which is attached to the counterlungs—hangs down and offsets the hydrostatically-generated positive pressure. The design of this very compact rebreather arguably incorporates the best of all worlds: hydrostatic imbalance is minimised, the diver has the advantages of a clean front, and (during pre-dive assembly and testing) the diver is able to tailor the compliant volume precisely to their personal maximum tidal volume, thus avoiding all the disadvantages of excess loop volume.

Pulmonary Oedema

Over time, research has improved our understanding of pulmonary oedema (fluid in the lungs) resulting from the hydrostatic effects associated with immersion. In hindsight, a number of diving specialists I have engaged with now have a strong suspicion that Immersion Pulmonary Oedema (IPO) led to numerous diving fatalities that were originally attributed to other causes. If a predisposition to IPO exists in an individual—which can vary day-to-day based on a number of variables such as over hydration, high blood pressure, or high work-rate, then any negative hydrostatic imbalance can facilitate the extraction of fluid from the circulatory system across the alveoli barrier, resulting in IPO.

During military manned diving rebreather trials in a wet chamber under scientific experimental test conditions, as the test diver’s chamber attendant, I witnessed a case of IPO play out in front of me. Convinced he was aspirating water, the rebreather test diver—a maritime Special Forces operator in peak health, became seriously ill and could have died had the medical team not intervened. After this incident, the test diver was permanently retired from diving for the rest of his military career. 

I am also aware of one UK rebreather instructor who required hospitalisation due to IPO, an incident that was strongly suspected by his hyperbaric physician to have resulted from the high negative hydrostatic lung load that arises from the particular BMC rebreather he was diving at that time. The hazard of hydrostatically induced IPO is therefore not academic supposition; it is a very real diving hazard being increasingly reported and documented⁽⁴⁾. 

The growing understanding of this potentially fatal condition, calls  into question the safety of ADV isolation valves and the negative static lung loading that can arise if the ADV is left isolated over an extended period, which many BMC rebreather divers do to prevent ADV free-flow in a head-down orientation and/or to help ensure optimal loop volume.

Breathing Circuit Gas Volume Management

Divers using an eCCR are taught to maintain optimum loop volume to avoid excessive buoyancy variation and to minimise the need for oxygen injection during decompression stops as the electronic control system strives to maintain a constant PO₂. During routine operation, when breathing from OTS, BMC, and FMC configurations, loop volume is readily managed during an ascent by venting excess gas past the lips or via the nose. However, a safety-critical requirement is also the ability to manage the gas volume in the loop during an emergency open circuit bailout ascent. During such a scenario, the diver no longer has a direct oral interface with the loop; so, assuming the DSV has been closed (this happens by default with a BOV when switching to open circuit), the loop is now sealed. 

To help prevent a lung overpressurisation injury—during, for example, an uncontrolled ascent or the rapid injection of gas into the loop—every rebreather should incorporate a pressure relief valve (PRV), also known as an over pressure valve (OPV). The PRV/OPV should be positioned somewhere in the loop to automatically open at a maximum preset pressure (for example, 40 mbar, per the European rebreather standard). 

However, since PRVs are often assigned the function of  a ‘water dump valve,’ they are most often positioned on the bottom of the countelung. As a result, to vent excess gas, the loop has to be full and under positive pressure before the PRV will automatically open, which causes excess positive buoyancy.  This can potentially result in an uncontrolled ascent and decompression illness (DCI) [Ed. note: DCI encompasses two diseases, decompression sickness (DCS) and arterial gas embolism (AGE)].

With FMC and OTS counterlungs, the venting of a sealed loop during an emergency open circuit bailout ascent is relatively easy to manage; the PRV is typically located on the exhale counterlung. With a secondary function as a water dump valve (and being manually adjustable, like a drysuit vent valve, the PRV can be adjusted to relieve at a much lower pressure, like 5 mbar (Figure 11). However, the location of the PRV in the loop or on the exhale counterlung is critical. During a high-stress open circuit bailout scenario, despite the contemporary training emphasis to maintain horizontal trim, it is likely that the diver will instinctively adopt a head-up orientation.

 I have witnessed this reality on numerous occasions and, for those who might contest, I recommend reviewing rebreather accident reports. Divers are also highly likely to adopt a head-up orientation when experiencing hypercapnic respiratory distress—they’ll try to reduce their respiratory load by subconsciously adopting a more comfortable breathing orientation to offset a BMC-induced negative hydrostatic load. 

In some scenarios, such as in a steeply ascending narrow cave passage, the diver has little option but to adopt a head-up orientation. In such scenarios (regardless of the PRV relief pressure), if the PRV is positioned on the bottom of the exhale counterlung (which is typical), and the diver adopts a head-up orientation, the loop has to be full to provide sufficient internal gas pressure to automatically open the PRV.

As discussed, a full loop will result in excess positive buoyancy that can quickly lead to an uncontrolled ascent. To manage this during an emergency open circuit bailout, appropriately designed FMC and OTS counterlungs can be compressed using the forearms to maintain buoyancy control and prevent loop over-inflation (Figure 11). Manually compressing the counterlungs is, of course, not possible with BMC rebreathers; divers can reach neither the PRV nor the counterlungs, and having witnessed and experienced an uncontrolled ascent during an open circuit bailout scenario with a BMC rebreather, not being able to quickly and effectively manage the loop volume presents a notable hazard for multiple BMC rebreather designs.

It is the author’s opinion that, as the trend towards BMC continues, training agencies are not adequately addressing this hazard. This might be due (in part) to an overemphasis on the PRV as a ‘water dump valve’ and not on its primary function: facilitating loop venting while maintaining buoyancy control. Agencies’ current guidance generally suggests closing the oxygen cylinder to avoid oxygen injection during ascent and burping the loop via the DSV/BOV. Such suggestions are procedural BAND-AIDS which try to mitigate what is arguably a deficiency in design functional safety. 

Loop venting restrictions are also particularly relevant in unconscious diver rescue scenarios: both the unconscious diver and the rescuer could easily end up in an uncontrolled ascent situation. A recent review of rebreather rescue procedures highlights a training and information gap in this area; for example, one well-known rebreather manufacturer does not include a written diver rescue procedure in its manual. 

In many instances, what information is available does not adequately cover a rebreather rescue scenario. When I recently discussed this situation with an internationally renowned rebreather instructor trainer examiner (ITE), he described his and a colleague’s attempt to execute a training rebreather rescue from depth in order to develop more detailed procedures. When acting as rescue divers, neither he nor his colleague were always able to maintain control their own or the rescue diver’s buoyancy—because the PRV was located at the bottom of the exhale BMC.

As cave diving instructors, they experimented in a cavern environment in order to help avoid a direct uncontrolled ascent. However, since they were both repeatedly pressed up hard against the cavern ceiling, the instructors surfaced somewhat battered and bruised. Diver rescue techniques often taught by diver training agencies typically place the distressed diver in a vertical, head-up orientation; this controlled exercise highlights the challenges of a rebreather diver rescue where the PRV is located at the bottom of the loop in some BMC designs.

Conclusion

It has been said that, while designing rebreathers is simple in principle, designing a rebreather that performs in extreme conditions is a significant engineering challenge. Having been involved in rebreather design and testing for over twenty-five years, I can attest to the validity of this statement. Despite the simplicity of the principle, rebreather hydrostatic imbalance is a very nuanced subject, and unmanned laboratory testing of the same rebreather model can yield different results based upon how the rebreather is set up in the laboratory 

One set of test results can, therefore, never be relied upon to draw definitive conclusions. Also, the broader subject of rebreather work of breathing (WOB) includes a number of additional respiratory loads not discussed here. These include resistive effort, elastance, inertia, and CO₂ dead space. Hydrostatic imbalance is therefore just one chapter in the rebreather WOB story, and a number of critical performance and safety implications can arise with differing counterlung configurations. 

What, then, are the key takeaways? Hopefully it is now apparent that there can be notable performance differences between counterlung designs that potentially impact diving safety. High negative lung loads can cause hypoventilation leading to hypercapnia and potentially LOC, particularly when breathing gas at elevated densities and at high work rates. In addition, a high negative lung load can induce IPO—a potentially fatal condition if not treated quickly. With consideration to this, an ADV isolation valve should be used with caution as very small descents can generate potentially harmful negative hydrostatic lung loads. 

Changes in hydrostatic imbalance is also of particular concern when using sidemount / bailout rebreathers, which require to be mounted  on or near the same plane as the diver’s lungs to avoid excessive lung loads. 

Counterlung designs that result in a slight positive pressure are potentially beneficial to ventilation, particularly when in a prone orientation at elevated breathing rates. However, an overly high positive hydrostatic imbalance can cause notable discomfort and a sensation for the need to vent excess loop volume—this could potentially cause ADV free-flow and disrupt buoyancy and PO₂ control. 

During an open circuit bailout scenario, where a head-up orientation is an instinctive response, a PRV/OPV positioned on the bottom of a BMC increases the possibility of an uncontrolled ascent. This is a notable hazard—especially when divers need to decompress or safely ascend during a rescue. Rebreather designs that minimise hydrostatic imbalance promote effective ventilation, which help reduce the probability of encountering the diving hazards and resulting maladies described here. Therefore, a rebreather that has been type-tested in accordance with an international standard offers the customer a certain level of confidence in functional performance and safety; however, divers should understand the nuances and implications of each rebreather counterlung design and PRV/OPV placement.

References

1. Broad individual immersion‐scattering of respiratory compliance likely substantiates dissimilar breathing mechanics Olivier Castagna^1,2*, Guillaume Michoud³, Thibaut Prevautel⁴, Antoine Delafargue⁵, Bruno Schmid¹, Thomas Similowski⁶ & Jacques Regnard⁷ 
2. Fatal respiratory failure during a “technical” rebreather dive at extreme pressure. Simon J Mitchell ¹, Frans J Cronjé, W A Jack Meintjes, Hermie C Britz
3. Haynes P. Increasing the probability of surviving loss of consciousness underwater when using a rebreather. Diving and Hyperbaric Medicine. 2016 December;46(4):253-259

• Edema (IPE): A Comparative Study of Military and Recreational Divers Dorian Wolff^1,2†, Olivier Castagna^3,4,5*†, Jean Morin⁵, Henri Lehot⁵, Romain Roffi⁵, Arnaud Druelle⁶ and Jean‐Éric Blatteau⁵

DIVE DEEPER

InDEPTH: Increasing The Probability Of Surviving Loss Of Consciousness Underwater When Using A Rebreather by Paul Haynes

With thirty-plus years of rebreather diving experience, author Paul Haynes is a CCR mixed gas instructor trainer, shipwreck explorer, and former UK Special Forces diving instructor, supervisor, and swimmer delivery vehicle (SDV) Operator. Upon leaving the military, Paul began a second career with Divex, now JFD, the world’s largest manufacturer of professional diving equipment. Now running his own diving/maritime operations consultancy, Paul has remained at the forefront of military underwater life support systems design, testing, and training for over two decades. Incidentally, a long time ago, he trained the editor-in-chief of InDEPTH mag to dive a rebreather 😉