1.0 Tenets of the Rebreather Envelope and Principles of Design

To leverage rebreathers, broadly, as a platform that may push forward our relationship with this Blue Planet, there are several basic tenets that require further consideration in how the technology becomes embedded into contemporary education, diver training and the end-use regimen.

Each of these tenets may be embraced with more appropriately developed and applied ‘standards’. Standards of practice, or better yet their non-regulatory counterpart "best practices", are very important, though my opinion is that the industry has trended towards standards that are actually prohibitive of short-term success. Further, also prohibitive of exploring the possibility of opening up a true mass market for this technology in time. This is due to standards development being motivated by capturing market share, not by community advancement. These issues need to be readdressed from a productive use or mission-tailored standpoint, aligned with physics and physiology, rather than a purely equipment performance or training standpoint within somewhat arbitrary performance criteria. Below highlights what I believe to the tenets of the rebreather envelope, each requiring improved standardization in how they are embedded within the cultural view of rebreather use.

We must have an expanded understanding of atmospheric management. This includes its conditional and physical changes from at depth to space, traversing the spectrum of what a human may be exposed to, and where physiological limits exist. This forms the base for all diving and other applications of personal life support, though is never well presented outside of training on a specific piece of equipment. Fundamentals of atmospheric gas contents, photosynthesis, carbon sequestration, basic partial pressures, respiratory cycles, gas density, buoyancy, physics of sound through various media, basic oceanographic processes, and so on need to be part of a layperson’s science education, and only then reinforced during a dive training program with specificity to the activity of diving with specific equipment. Think an expanded open water scuba academic curriculum introduced at the middle school science level – where an enthused student may begin aspirations to learn more about and spend more time out on and underwater.

We need to back way up and re-ask, rhetorically, 'what exactly is a rebreather?' There are more than a dozen 'rebreathers' available on the market today, ranging from those with large sales and distribution networks to those producing short runs of 10 or fewer units per year. Aside from the advertised claims of the manufacturer, there is little, if any, hard data available to the consumer to enable an informed purchase decision. Moreover, many of these claims are highly speculative, derived from assumed or experiential performance, rather than from hard analytical data assessing performance. There has yet to be a universally accepted standard for the development or performance of various independent subsystems – there are only full unit standards, which are a disservice to market expansion and are a contrast to what people actually practice [by modifying their units almost immediately out of the box].

      Of critical importance is scrubber design. Too much faith is placed on assumptions in duration. What environmental factors and physical unit characteristics affect scrubber duration? Can these be monitored? In short, there is no publicly available hard evidence to sell the consumer on any margin of safety, or specific performance, in choosing between axial or radial, split or single, cylindrical, oval or otherwise. This information gap needs to be filled, as the scrubber is the heart of any rebreather system. Therefore, it is a disservice to ‘require’ that an end user adopt a full system that meets certain performance criteria. Performance criteria should match the utility requirements of the operation and fall within broadly accepted design criteria that fulfill safety concerns of the given operation. Developer benchmarks as they apply to assembling a system that meets operational risks or hazards needs some attention - though there is no consensus on what that means at this juncture. For example – we have unit performance criteria specifying scrubber durations of X hours at Y depth and Z conditions. Well, what if the mission or operation required twice the stated duration? Or half the stated duration? Obviously then, this specific rebreather may not a good choice – why are there no options for scalability, or even a broadly accepted understanding of how to assemble a unit that will accommodate the 2x duration? This understanding should be no different than understanding that a half full cylinder will yield less dive time than a full cylinder for open circuit diving. The whole as the sum of the parts should be understood by the end user – if you can’t build a rebreather, you shouldn’t be diving one.

      We must adopt training that embeds understanding of basic system level features as applied to atmospheric management. In most cases, unit-specific training is presently required to purchase and dive any given unit. Some claim that this is a money-making scheme of the industry. Perhaps, but consider this; today's units are still like apples and oranges. Conversely, when we train on open-circuit scuba, we can use any brand of regulator, each with its advertised performance advantages, and this is apples and apples. Taking a new course to use brand X instead of brand Y regulator would be ridiculous. Rebreathers should be the same. This is the ONLY way that a mass market will be opened for the mainstream consumer. For this to happen, rebreather manufacturers need to assemble to afford baseline know-how in understanding minimum subsystem performance but also end-user operability. Unit specific assembly and maintenance would then be easily addressed with a workshop or even self-study just reading a manual, rather than full end-user course. Just one prime example – some manual addition valves (MAVs) check closed in the upstream position, and some in the downstream position. This has a significant impact on how one responds to a valve failure, and if/how the unit can continue to be dived while aborting. Understanding ‘why’ this is the case should not be a unit-specific topic of study. It should be part of universally understood rebreather know-how and theory from which an educated consumer can then make an informed purchase or use adoption decision.

     The diver must be capable of identifying and mitigating risks. Rebreathers are predominantly used for recreational/technical diving and private exploration. They are not common in industry due to limited consideration of alternate diving methods. Decision making should prioritize risk mitigation guided by professionals in the diving community. Standardized approaches will help manufacturers create value-added products that mitigate recognized and encountered risks.

By contrast, today, the innovation drive comes from a desire to push oneself further – this is very dangerous and should not be the impetus for industry wide advancement. Rather, we should be marketing what has been and is achievable [to benefit humanity] via advancement of technology. How many new fish have been discovered? How was the wreck documented? How was the tunnel penetrated? Learning about these capabilities is far more important than marketing a specific make or model of rebreather. The risk mitigating steps taken to accomplish these tasks resulted in the selection of rebreathers as the best tool for the job – that must be brought forward, as it promotes creation of market opportunities. Rather the converse – of making the dive for fun and making an accidental discovery; while exciting, this approach does not push capacity building at the scales needed for our human benefit.

It is imperative to emphasize the value of the technology, not solely market the hardware. Showcasing the technology in a utility capacity is a crucial aspect. Numerous intricate details exist within all components and systems, shaped by the developer's experiences. These subtleties must be promoted as enhancing performance and adding value, rather than just hardware features. 

For example, one of the more obvious observations in configuration inconsistencies among and between units is routing of the oxygen and diluent gas distribution systems. Again, there is no data to support any benefit or detriment to injecting oxygen or diluent either pre- or post- scrubber, nor is there any evidence suggesting that specific injection mechanisms are more or less effective; however, analysis of some incidents do shed light on this and warrant discussion. I place considerable effort on theory behind this topic in later sections, though theory is theory. Likewise, the 'rich on right, lean on left' mantra from open-circuit technical diving is still not consistently applied to rebreathers. This can cause problems for an individual crossing between units. It is even more of a concern when working/diving in a team when a team member is not familiar with a specific configuration and attempts to render assistance in an emergency.

The diver must fully understand these nuances and where or if they add value aligning with his or her own risk mitigation strategy and diving philosophy. The hardware is irrelevant, unless it adds value to the dive, and so divers must be trained to recognize risk and determine whether a meaningful change mitigates the risk to consequently add value or is just gimmicky. 

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The principles of design presented throughout consider the many complications posed by these tenets and presents my personal fundamental designs and feature set logic as a basis for adding value to any rebreather system – this is my open-sourced and make/model agnostic design logic. The reader is encouraged to review these principles against their own rebreathers to generate thought, discussion, and future research. My hope is that the reader will begin to ask serious questions of themselves - do I agree with this configuration? Questions should also be asked of the developer/manufacturer - did the design come from logic aligned with my own? These design principles are not specific engineering or component designs, rather an open presentation of why I dive what I dive, to mitigate risks and enhance the value of my time spent underwater, at least as it makes sense to me – not because I was told to do it this way.

The journey begins with the interplay of 'what we dive' and 'how we dive', based on logical deduction and operational needs. The development process focuses on five principal subsystems within rebreather platforms.

These include:

1.      Chassis and Fundamental Configuration

2.      Breathing Loop

3.      Carbon Dioxide Removal

4.      Gas Distribution

5.      Oxygen Monitoring

The design and configuration logic for each subsystem is described in the following sections. In practice, the principles I describe could be applied to any number of makes and models. As such, actual subsystem assembly, its specific use, and maintenance would be subject to additional end-user documentation and checklists and would be the products of a component, subassembly, or full system manufacturer. Considering this perspective, we should also question the role of the said ‘rebreather manufacturer’; whereas a manufacturer of components and assembler or developer of systems maintain distinct responsibilities and confusing the two only mucks up the already gray areas of product liability.

 

For an individual savvy enough to do so – it is entirely possible and reasonable to acquire a mish mash of subassemblies or components, possibly all from different suppliers, and build a very high performing rebreather. So, who is the manufacturer? Well, YOU are. This critical tenet must be embraced moving forward – if you can’t build a rebreather, then you shouldn’t be diving one. It’s not hard to do though it requires a certain type of instruction, and learning, both of which are overlooked within the current paradigm.