1.2 Breathing Loop
The breathing loop consists of subassemblies for the mouthpiece (Dive Surface Valve (DSV) or Bailout Valve (BOV)), breathing hoses, counterlungs, often t-pieces connecting hoses to the counterlungs, and a scrubber canister. These subassemblies direct the diver’s breathing cycle through the closed system in a manner that allows for scrubbing the CO2 rich exhalations. Here, we’ll explore critical breathing loop components and functions with some opinions on configuration logic.
1.21 DSV/BOV
First, the mouthpiece assembly (dive surface valve or DSV) directs breathing gas flow using directional mushroom/flapper valves. The direction of flow should be well understood by the diver. My opinion is that the most intuitive direction is diver’s RIGHT to diver’s LEFT, drawing on the historical convention that open-circuit gas, or life sustaining inhalations, are supplied from the diver’s right side via a second stage regulator coming over the right shoulder. Further, within closed circuit diving, the RIGHT to LEFT flow is consistent with the rich on right, lean on left technical diving mantra for gas placement, where oxygen enriched gas is being delivered from the right side, and oxygen lean gas is exhaled to the left side. That said, there are variations in the marketplace for reasons I will touch on elsewhere within this content.
The mouthpiece assembly also provides a means to switch from one mode to another mode (i.e. off loop and back on loop). When a bailout valve (BOV) is utilized, this switching provides immediate open-circuit access. When a BOV is not utilized, the switch simply closes the mouthpiece and would force the diver to immediately access to suitable open-circuit bailout ideally positioned in proximity to the diver’s mouth (on a necklace for instance).
1.22 Breathing Hoses
The next components in line are breathing hoses. Breathing hoses should have a large bore to accommodate the passage of higher density gas at high ambient pressure while moved via the lungs. Standard rebreather hoses of 1.5” inside diameter (ID) are a good benchmark. This size is about 50% larger in diameter than a human trachea, thus reducing mechanically imposed breathing restrictions. A sound breathing loop design will make best efforts to eliminate or reduce restrictions at t-piece junctions, through the head section of the rebreather,, and through the scrubber bed itself. The cross-sectional area of any restrictive pathway should always match or exceed this 1.5” breathing hose area. When following this general rule of thumb, resistive work of breathing is kept to a minimum and is frankly of little concern physiologically within routinely encountered depths. Historically, there have been breathing hoses on the consumer market with 1.25” diameter, and even smaller on pendular systems. Their added resistance may unlikely be noticed on shallow dives, though may become noticed on deeper dives (versus a 1.5” hose). Similarly, larger 1.75” diameter hoses have been incorporated into units earmarked for deeper depths and/or to compensate for other restrictions within the breathing loop.
1.23 Counterlungs
Breathing hoses direct gas to flexible counterlungs. Counterlungs (typically one inhale and one exhale) should have a volume to sufficiently accommodate the average adult male diver’s vital capacity of approximately 4 L, plus an additional 20-25% (assume 6 L total flexible bag volume). This vital capacity is the maximum volume of gas that the lungs can be inflated to, though this cannot be sustained during a breathing cycle. During normal breathing, it’s the tidal volume that we are primarily concerned with. When dived correctly at ‘minimum loop volume’, only the diver’s tidal volume of about 0.5 to 1 L will be exchanged per breath. This small volume exchange against such large bag volumes results in not much bag movement, so consequently limits the work of breathing by negating full counterlung inflation against the weight of water or any restrictive components within the rebreather itself.
| from Measuring Metabolic Rate, Clark Cotton 2019. |
While there is great debate on best location for counterlungs, the actual work of breathing is highly variable based on the orientation of the diver in the water – this is true for every rebreather on the market. The technical term used is hydrostatic lung loading, which when coupled with resistive work of breathing is generally considered the cumulative 'work of breathing (WOB)' experienced by the diver. Simply, the breathing loop's distance away from the human lungs can add positive or negative pressure during the breathing cycle. For this reason, counterlungs should be positioned as close to the human lungs as possible, and if awkward body positions are required during the dive (such as a tight confined space, working upside down, etc.), lung position should be considered in the assembly. Any bench or analytical testing related to work of breathing associated with the counterlungs is therefore rather subjective.
There are good reasons for front, back, top of shoulder, and/or along the side counterlung configurations, all pending mission requirements, and there are units on the market that utilize any variety of these positions. The subjectivity also varies diver to diver – diver A may say “the work of breathing is good”, whereas diver B says “the work of breathing is not good” – both on the very same system. The intricacies of fit to personal preference play in considerably, even with units that have “tested” or “approved” work of breathing measurements.
1.23a Counterlung T-pieces
Each counterlung incorporates a threaded port to fit breathing hoses. T-pieces are commonly used, which provide both an inlet and outlet for the counterlung. T-pieces typically incorporate an anti-collapse mechanism, as well as a baffle to direct any water to the counterliung. Positioned properly, the counterlungs act as a water trap, and the t-pieces play an important role in isolating accumulated water from the scrubber or other critical components such as sensors.
1.23b Counterlung Pressure Relief
In the unlikely event that the counterlung volumes were exceeded, such as during periods of extremely strenuous work or during an ascent, an overpressure relief valve (OPV) will vent the excess gas volume. This OPV function ensures that the counterlungs do not burst, and the diver’s lungs are not subject to an overpressure injury (for instance, from a runaway ‘stuck-open’ gas injection).
The OPVs should be strategically located. At low positions on each the inhale and exhale counterlung the OPV doubles as a path for water evacuation. OPVs are frequently a spring-loaded seat. The spring or release pressure should allow the rebreather to vent at pressures lower than what would cause a lung over pressure injury. This is quite low – just 40 cm H2O roughly which equates to just 0.5 psi. This can be challenging to achieve reliably without leaking from a mechanical standpoint – it’s a very delicate seated surface at such a low pressure.
An OPV with this low cracking pressure positioned high on the back will be prone to leak as the hydrostatic pressure within the loop likely exceeds this minimal amount when shifting body positions. For this reason, many OPVs are adjustable and can be tuned to diver preference and unit configuration. Dialing the OPV down, to vent at a higher pressure of 1-2psi serves to protect the breathing loop hardware, though will not help with lung protection. Worth noting is that pressures of 1-2 psi in the loop will be noticeably uncomfortable and trigger the diver to take corrective action via venting from the nose, or manually actuating a relief valve if a positive pressure scenario is encountered.
1.24 Scrubber Water Tolerance & Evacuation
Detailed information on scrubbers will be provided in subsequent sections, though important to note here is the canister itself being an integral component to the breathing loop that must be gas tight and therefore sealed from water ingress. It is also advantageous for the ability to evacuate water from the exhalation side of the breathing loop and/or scrubber exhalation cavity via a manual pump or relief valve located at the bottom of the scrubber can. This location will naturally accumulate condensate from the scrubber’s insulative space, as well as water produced as the byproduct of the scrubber reaction. Using a pump allows for periodic water drainage without having to waste diluent gas during an aggressive positive pressure loop flush.
The breathing loop must be reasonably flood tolerant. Period.
With the breathing loop of a rebreather operating at ambient pressure, a leak of the contained atmosphere will result in water ingress. Too much water means dive over, and, depending on where the diver is within the planned excursion, this could present major problems. Within the realm of sport diving limits, simple open-circuit bailout techniques provide a direct means to escape harm’s way and come back home. Extended range dives, where the physical or physiological surface is no longer an option for immediate retreat, present challenges to open-circuit bailout strategies. In some cases, additional closed circuit bailout scenarios are considered. Assuming we want to move in a direction which maximizes the in-water capability of rebreather technology, we need to mitigate the risk of water ingress and most certainly consider how to manage a catastrophic flood. These types of emergencies are rare, but they do happen.
I can speak from experience in ascending from sub-400 fsw dives along a vertical wall face where the overpressure relief valve (OPV) was aggressively venting to account for the gas volume expansion and then was stuck open due to reef ‘spooge’ falling off the reef face from my exhaust bubbles, dislodging the material and then getting stuck within the seating surface. This resulted in a catastrophic flood – three dive days in a row mind you – and gave me great pause in reviewing OPV design and placement within the rebreather breathing loop. Issues there will be presented later, but, in short, had the overall unit configuration been designed to be more flood tolerant, and possibly even afford the opportunity for flood management if not fully flood recoverable, my botched 3.5 hour dives would not have turned into 5 hour dives with substantially added stress and potentially compromising the overall safety of the dive team. Lesson learned – the rebreather should be reasonably flood tolerant. We’re underwater after all, and despite the longest positive/negative pre-dive checks, water will find its way in one way or another, and this should be manageable by the diver to preserve breathing loop integrity, particularly on very long dives.
Considering this, designs of the breathing loop should provide adequate spaces that serve as a water traps. This may be an exhale side counterlung, a cavity on the exhale side of the scrubber, or even a little ‘snot trap’ in proximity to the exhalation hose. Depending on the mission, this pocket of intruded water – even if only a small volume – could cause problems it makes its way to the scrubber bed or simply create annoying gurgles during a dive. A sound breathing loop design provides for evacuating water from this accumulating area. As mentioned, this could be an OPV, a manual pump, or other mechanism that the diver has confidence in operating. While absorbent pads can be effective to manage condensate and metabolic water, it is not this small volume that concerns me - it's the more substantial volumes from loose lips, a failed seal somewhere, or torn hoses or counterlungs.
Management of water ingress should also be intuitive for the diver. The most probable scenario is that ‘loose lips’ allow for a trickle of water into the loop, which drains towards the exhalation side of the loop and has to be addressed. Therefore, it is important to apply logic in the breathing loop direction that is intuitive for the diver.
1.25 Further Discussion of Loop Direction
On day one of open-circuit scuba training, we learn that the primary second stage comes over the right shoulder and to the diver’s mouth. Air is supplied from the diver’s right. Why confuse this with rebreathers that breathe left to right?
This may be perceived as a personal pet peeve, but let’s explore some additional logic on the matter. Following the trials and tribulations of basic scuba diving, we move onto technical diving. There, we follow a ‘rich on right, lean on left’ mantra where decompression cylinders that are rich[er] in oxygen are carried on the diver’s right and the others on the left. So, organizationally and philosophically, richer life sustenance is coming from the right-hand side.
On mainstream rebreathers, cylinder mounting conventions have followed this ‘rich on right’ mantra, with the oxygen supply cylinder on the diver’s right and the diluent on the diver’s left. When I’m diving, in my head, oxygen replenished gas is being inhaled and oxygen depleted gas is being exhaled. So, the oxygen replenished gas should be coming from the right-hand side (where the oxygen cylinder is located). That would place an inhalation counterlung on the diver’s right-hand side; and therein lies our problem from an engineering and configuration standpoint.
Again, call it opinion, but logic tells me to inject oxygen before the scrubber – that would be the exhalation side of the breathing loop. The reasons for this are to encourage thorough blending of the breathing gas with the newly introduced bolus of oxygen and not flood the oxygen sensors [on the inhalation side of scrubber] with pure oxygen, which would create erratic behavior and possibly falsifying high pO2 alarms. If following my right-to-left breathing logic, then at some point in the gas distribution system, the oxygen injection pathway must then cross from the supply on the right to an injection point on the left [exhale side]. On units that inject directly into over the shoulder counterlungs, this becomes confusing and problematic as an oxygen button would end up on the diver’s left [exhale side] and we don’t want that! So, many of the over the shoulder counterlung units are found with the unit breathing left to right such that the oxygen injection button is on the diver’s right [exhalation] counterlung. This keeps with the rich-on-right configuration mantra but fails to satisfy my ‘breathe from right to left’ philosophy. You will find a mix of right to left and left to right breathing units on the market. It’s confusing and a fundamental roadblock in standardizing the technology.
I have gone to fairly great lengths to satisfy this personal requirement in the systems I have designed and dive, and this remains among the main reasons I am not partial to many of the off the shelf rebreather systems – I want to breathe RIGHT to LEFT and, therefore, more intuitively manage water ingress from my left side. Embracing this right-to-left configuration mantra and philosophy lends itself well to the psychology of manually managing the breathing atmosphere and the physical loop as built upon our established habitual conventions in open-circuit diving.