4.1 RD1 (Rebreather Day 1)
I was admittedly at a bit of a slump as a working diver in the summer of 2018. The diving seemed to dry up. The phone just wasn’t ringing, the competition had been working far too cheaply, and the local community was swamped with young divers who were out doing the hustle that I did for such a long time.
What to do? Welcome to shellfish harvesting.
In Rhode Island, harvesting quahogs (our hard-shell clam) is sound business and is carried out in two principal ways – through bull raking (done wading or from a boat) or diving. The dives are shallow, typically not more than 20 feet deep, and with some good local knowledge which I had acquired from years of doing commercial work out in Narragansett Bay, it’s reasonable to put in a hard day’s work for a decent day’s pay - so that’s what I did. One way or another diving has always been a crutch in between other diving related ventures, and so off to the murky abyss of quahog beds I went.
For the first few months, I dived multiple (3-4) aluminum 80’s a day, which accounted for say 4-6 hours of bottom time per day. The challenging part was getting out of the water several times to switch cylinders. During that seemingly short break, major setbacks would present themselves. First, I’d lose my spot in the clam bed – the surrounding sediment that I worked hard to clear away would fill in the work area, and I would lose 15-20 minutes of productive harvest time for each new cylinder/dive. Second, once getting out of the water, psychologically, it’s always just that much harder to get back in.
So, my first crack at a solution was employing larger cylinders. I invested in two steel 121’s. This meant longer individual dive times and fewer surface periods to swap cylinders. Cumulative dive times were about the same, but my harvest yield went up about 10%. The ability to stay with the area I was working and prevent sediment backfill made a huge difference, and so my workdays were a little shorter, more productive, and it was psychologically easier to be doing two long dives per day rather than 3-4 shorter ones.
After another couple of months of this, everyday mind you, those big, honking cylinders were destroying my back, so I again rethought the daily evolution. Then it dawned on me: I should be doing this with a rebreather. For two decades, I had been searching for a real-world working dive environment that provided a demonstrated need for rebreather tech (aside from deep scientific diving), and I had finally found one – harvesting.
I looked long and hard at my mixed-gas rebreather unit, which I had custom built in 2015, and it offered little appeal for 20-foot dives. It was just too much. It was too big, too heavy, overly capable, and would frankly be dangerous even with air diluent if used exhaustively in very shallow water and zero visibility when pO2 displays may not be easily viewable. I needed something with a much quicker turnaround, something lighter than a steel 121, and something that would be forgiving with respect to atmospheric management.
So, with a little shop tinkering, a very few custom components, and applying the principles mentioned throughout this content, I arrived at a very simple oxygen rebreather that was then purpose built for the work environment.
Enter RD1.
System Description
I dubbed this little oxygen rebreather ‘RD1’, short for ‘Rebreather Day 1’ because after a few tens of hours diving it, it dawned on me that this was simple enough for a novice diver to dive from ‘day 1’, or at least very early in their dive training perhaps even before entry level nitrox training (presuming some very basic instruction were provided about oxygen partial pressures and limitations on depth).
Through the design/build, the unit had to meet some basic criteria, where:
- it had to be back mounted given my aggressively working with my hands and very close to the bottom.
- It had to provide about 4 hours of life support in 20 feet of depth.
- it had to automatically inject oxygen and maintain a suitable loop volume throughout the dive.
- it had to be reasonably flood tolerant.
- it had to provide enough open-circuit bailout during all parts of the dive to make it to the surface and establish positive buoyancy.
Now of course, oxygen rebreathers have been around forever – even back into the 19th century - but it is rare to see them commercially available. So, this design/build effort was put on the bench, and has since become our core platform for all subsequent design/build solutions. The project has been a real home run.
6.12 Technical Overview & Principles of Design
As presented earlier, every rebreather has 5 principal elements. RD1, despite being an oxygen only rebreather, is no different. 
4.12a Basic Envelope and Fundamental Configuration
The basic envelope is that of a back-mounted system and is assembled to a simple aluminum ‘spine’ that fits a standard diver backplate. The spine is designed such that the unit could be switched over to a conventional plastic diver backpack, or even a commercial diver harness, thus allowing the diver to use any style of harness or backpack that he or she is familiar with.
4.12b Breathing Loop
The breathing loop consists of both inhalation and exhalation counterlungs positioned high on the back and favoring the shoulders. The unit breathes right to left, following the conventions previously described, with flow directed by a DSV. The loop is water tolerant with baffles in each counterlung t-piece, and also a manual evacuation pump positioned low on the exhale side of the scrubber.
4.12c Carbon Dioxide Removal
The most significant development from the RD1 design/build was a much dumbed down ‘universal scrubber’ assembly. Very simply, the scrubber is an axial design with absorbent retention screens, some room for oxygen sensors in the head lid, and several through ports in the lids for cable or hose pass throughs. I took the time to get this universal scrubber right in anticipation of using this as a model plug and play scrubber for future design/build projects.
4.12d Gas Distribution
Gas injection was boiled down to oxygen only, so any and all components for diluent injection were omitted. The oxygen system also became grossly simplified – no needle valve or orifice and no MAV. Instead, I ran oxygen to an ADV (though no ‘d’ for diluent) on the exhale counterlung. In this configuration, oxygen would be automatically added with the gentle descents I was making to 20fsw, and then maintain loop volume on demand as I metabolized the oxygen in the loop. Using a conventional ADV (perhaps A’O’V) in this manner makes oxygen management incredibly mindless but does impose a depth limitation.
In the event of needing to bail out from the rebreather, a small air cylinder is fit to the chassis, and this supplies only a second stage regulator for bailing out. In this position, air can be used for sanity breaths or full open-circuit bailout. I determined that using the oxygen cylinder alone for these purposes was insufficient if the bailout were required at the end of a dive when oxygen supply would be diminished. For the dives pursued, 'up' was the bailout plan, so very little gas was required.
4.12e Oxygen Monitoring
In RD1, one or two oxygen sensors are used and monitored with a simple digital pO2 display board. It can be argued that no oxygen monitoring is required for oxygen only rebreathers; however, after diving this unit for the first 100 cumulative hours, I elected to maintain the monitoring capability since pO2 does fluctuate considerably during an oxygen rebreather dive. At the surface, when going on the loop, air from your lungs is introduced and it is nearly impossible to get a truly 100% oxygen purge. Given this, it is not uncommon to casually enter the water at pO2 0.6-0.7 bar, which equates to pO2 1.0 at about 20 feet of depth. Recall earlier that this 1.0 bar pO2 is one of our magic numbers as it allows for 5 hours of diving without CNS trouble. Interestingly then, for those with discipline, it’s reasonable to make short forays into deeper water, perhaps 40 or even 50 feet, with a little oxygen rebreather. I am not advocating this though it is a topic that warrants further discussion in another venue.
4.13 Opportunities
The RD1 design/build project has been rather enlightening. We arrived at a very simple, inexpensive, but incredibly capable rebreather suitable for diving virtually all day in shallow water and without any of the complexities or additional risks that come with a mixed-gas rebreather. What that does for us is quite remarkable since it simplifies very fundamental rebreather diving mechanics and makes principles in atmospheric management easy to introduce to a novice. In particular, concerns for hypoxia in shallow water are virtually eliminated, while reinforcing loop flushing techniques to easily mitigate these risks when moving to mixed gas units. The novice can just get accustomed to breathing off of a loop and recognize changes in loop volume on ascent and descent, learn how to switch on and off the loop to bail out, and begin to recognize how pO2 fluctuates with depth and metabolism. These skills are easily taught in a short pool session. I ran such an exercise at the US Coast Guard Academy in fall 2018, where I introduced about 40 basic scuba students to RD1 in confined water. In all cases, the unit was dived without issue after a very short tutorial, and all students surfaced with smiles after this new experience.
For a working diver, such as marine harvesting where RD1 was born, this simplicity is essential since there is very, very little to go wrong – perhaps even less so than on an open-circuit scuba. The working diver can focus on work without acute attention paid to the intricacies that come with mixed–gas rebreather diving.
As of this writing, I am newly and absolutely convinced that there is an opportunity to use oxygen rebreathers for very early training, right out of basic scuba, which would shift our rebreather training paradigm significantly. A student can spend time learning about the equipment in shallow water and then demonstrate this proficiency to an instructor to ‘graduate’ to incorporating air diluent for depth extensions. By designing the unit in a modular way, such as the case with RD1, only a simple upgrade on the gas distribution subassembly is required to enhance the unit’s capabilities, thus the unit can grow with the diver, and the diver is then building on a longer experience pathway throughout his or her diving career with just one unit that he or she has become intimately familiar with.
No longer would rebreathers be intimidating, or prohibitively expensive to enter the field, and the market would open up tremendously.
Some interesting food for thought.