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Are Oxygen Rebreathers Useful Tools for Tekkies and Sci-divers?

Mixed gas closed circuit and semi-closed circuit rebreathers have become essential tools for technical and scientific divers. But what about their pure oxygen counterparts? Diving contractor, scientist, and inventor Michael Lombardi believes that the time for O2 rebreathers’ may have come—not just for shallow water (≤10 m/33 ft) diving—shades of Hans Haas—but potentially to introduce motivated bubblers to CCR with a simpler rebreather.



by Michael Lombardi. Images courtesy of M. Lombard

A completely fortuitous venture resulted in spending more time on a rebreather than I had in 20 years of rebreather diving, though it wasn’t in the deep/wreck/cave environments often associated with the technology. It was very, very shallow and with nothing but oxygen, offering insights into what may be an overlooked opportunity: introducing rebreathers to a broader market. 

Enter Rebreather Day 1.®

History & Context

In 2017, for-hire diving projects here in Rhode Island slowed a bit, and a colleague suggested commercially harvesting clams—or ‘quahogs’—to fill in some downtime. So, that’s what I did. It’s big business here in the Ocean State and an amazing fishery. The routine is to dive three to five hours per day to limit out. It’s all very shallow (6 m/20 ft or less) and most harvest divers use open circuit, though some use hookah. I started out diving 3-4 Aluminum 80s (AL80) per day. The first issue surfacing was that every time I switched out bottles, I would lose my place in the bed from it backfilling where I had cleared the mud out. So, I switched to two LP112s per day. That helped, but was destroying my back. Eureka moment—I can do this on a rebreather!

As a working diver, I always viewed rebreathers as a tool for the job. As of 2017, the only rebreather remaining in my dive locker was a 100% custom unit purpose-built for more technical interventions. It was far too much for harvesting, and far, far from the key element of being able to roll out of bed and go—everyday. So I put my developer hat on and took to the bench.


The task at hand called for specific criteria—a rugged system that could take the abuse of day-to-day work, back-mounted to keep my front clear since I was working close to the bottom, consumables for five hours, and enough bailout for the short return to the beach. Oxygen was supplied on-demand via a de-tuned ADV T-piece (an addition valve, rather than auto diluent valve). I designed/built two units that met the criteria and went to work. And I made a huge discovery.

I was able to dive all day, literally every day, on a rebreather—all with very low maintenance and turnover time and, most importantly, no chronic tinkering. Just get up and go diving, all while being more productive and arguably safer than on open circuit (OC) for the nature of the dives.

Figure 1 One of two prototype RD1 units purpose-built for quahog harvesting. It is dived as an O2 unit with independent air bailout.

Since this 2017 inception, I’ve started to rethink my rebreather diving—so much so that I branded the unit “Rebreather Day 1®, or RD1 for short, given my new belief that oxygen-only rebreathers might be an overlooked and alternate pathway to broader/early career market acceptance. 

Spending multiple hundreds of hours with the original RD1 units has given me pause with the mainstream deep/wreck/cave paradigm mindset. These more ‘extreme’ experiences have been the way to lure in new rebreather divers, which is important, but remains volume-limited, given various barriers to entry: cost, training, proficiency, and perhaps most importantly perceived complexity. Mixed-gas rebreathers are still unattainable from a mass market standpoint for these reasons. Oxygen rebreathers arguably make these barriers easier to contend with and may be the early introduction needed to shift diving to a rebreather mindset.

Figure 2 An early pool trial with RD1 illustrating its highly compact configuration.

Operational Techniques

Now of course, oxygen rebreathers aren’t voodoo diving science. Oxygen rebreathers predate even open circuit scuba, let alone modern mixed gas rebreathers. There is an ill-informed assumption that the dives are limited to just 45 minutes due to 1.6 bar pO2 exposure (at 6 msw/20 fsw). This stems from being historically unable to monitor these circuits in the olden days. 

Today, fitting a monitor to an O2 rebreather reveals that it’s quite difficult to reach 1.6 bar given residual nitrogen in the lungs at the surface and, with advanced understanding of pre-dive loop flushing (easy to teach), it’s not terribly difficult to maintain lower PO2s and enjoy lengthy dives, even up to 2 ATA depth.

Figure 3 PO2 fluctuations with depth based on different loop flushing patterns at the surface. Upon descent, only oxygen is added, so the pN2 is essentially fixed less minimal on-gassing. Areas in green are within the desired PO2 range while still allowing for lengthy dives. When the loop is monitored, this full spectrum of depths to 2 ATA can be explored using basic loop flushing techniques.

One operational practice I’ve adopted while diving manual closed circuit rebreathers (mCCR) is riding a PO2 ‘bubble’ +/- 1.0 bar throughout the dive. It keeps on-the-fly math easy and doesn’t extend decompression too significantly versus riding a higher fixed pO2. I’ve always felt that we have no shortage of time, so we may as well use it! 

This is important to understand when maximizing the capabilities that a rebreather affords: A PO2 of 1.0 bar allows for five hours of daily diving on CNS tables, and with no pulmonary penalty for multi-day events. I originally designed RD1 for working to this threshold, where the scrubber offers a conservative five hours (based on 1 ATA testing at NOAA rebreather performance criteria) and carries five hours of oxygen based on an assumed metabolic consumption of 1 lpm (368 liters/AL13). 

For conservatism, I always plan that oxygen is my limiting consumable so, with the likelihood of getting a somewhat short fill in a 3000 psi cylinder, running down my oxygen over three to five hours was the go/no-go gauge. With attention to my loop flushing technique during pre-dive and monitoring the loop, I was able to enjoy and benefit from countless three-plus hour duration dives on my new RD1 units.

After a few seasons of diving an oxygen-only unit in this fashion, I felt that a level of rebreather proficiency had been attained that would have otherwise been unattainable, and that proficiency has made my mixed-gas diving much easier as well—the perceived complexity has been alleviated.

A Training Opportunity

While up to my elbows in mud for hundreds of hours digging clams, I had nothing but time to think through this perceived complexity issue, and I believe that we’re skipping a step within contemporary training regimens that may be part of the reason why broader market acceptance has not been achieved.

An open question is, “Can we train an open-water diver on a rebreather from Day 1?” Well, yes, but does it need to be (or should it be) a mixed-gas unit which introduces the ‘perceived complexity’ (dynamic gas contents) that often drive people away? I’m proposing to not even go there at all. Eliminate the complexity that comes with mixing gas in the loop altogether and put the new or novice diver on any rebreather, with only oxygen, and introduce rebreather fundamentals.

Current standards and training programs are written for mixed-gas rebreathers from Mod 1 through Mod 3 or similar. Oxygen rebreathers are not presently considered within this paradigm, though it should be borne in mind that disabling the diluent on any mix rebreather effectively creates an oxygen rebreather, which consequently simplifies the overall system and reduces certain risks for training purposes. 

I ran an experiment with not-yet-certified, open-water students, introducing them to our RD1 units in a pool. Within minutes, every student took to the system very well. This was an isolated event, though the takeaway was several-fold. First, the students knew enough about breathing off of a second stage that I could emphasize simply ‘when in doubt, bail out.’ 

Second, with a good observable (by me, the instructor) oxygen flush, we eliminated any concern about hypoxia (an important skill to engrain for future gas dives during the first and last 6 m/20 ft of depth). The exercise was able to focus on just breathing, managing a breathing loop, understanding what to do with some gurgles, basic buoyancy, and bailing out. It was easy, unintimidating, and brought smiles to faces, the way diving should.

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Now having worked with numerous new-to-rebreather divers using RD1 and oxygen-only fundamentals through a simplified program that I’ve developed, my observation has been that there is tremendous value starting out in this overlooked gap—arguably “Mod 0,” from which the rest becomes much easier in theory and in practice since the system and methods of rebreather diving in general can be taught, rather than a specific unit being taught (each with nuances related to diluent management). On any rebreather, eliminating the perceived complexity (dynamically diluting the loop) leaves us with an oxygen-only unit to start with the basics.

In very little training time (one to two days), and with only entry-level background know-how, it’s within reach to enable the new, young, enthusiastic diver a five-hour-per-day experience in the shallows. Within this refreshed paradigm, the diver can attain proficiency with loop management and basic maintenance, adopt rebreather theory as part of their baseline knowledge, and have experiences that are only possible with a rebreather. From there, I’ve experienced that students very quickly realize on their own what needs to happen to dive deeper—dilute the breathing gas. The student learns the ‘why’ on their own before the ‘how,’ presenting an organic path to a mix unit.

“We reached out to one training agency who was rather forward, stating, ‘We don’t support oxygen rebreathers,’ which I found confusing — why omit this ultra simplified technique?” 

We’ve successfully instituted this ‘Mod 0’ training on the RD1 unit with the University of Arizona’s Scientific Diving Program, where it’s proven to be a valuable tool and technique for research. In an effort to expand the opportunity, we reached out to one training agency who was rather forward, stating, ‘We don’t support oxygen rebreathers,’ which I found confusing — why omit this ultra-simplified technique? 

Figure 4 Scientific Diver Brandon Carr dives RD1 at Biosphere2 in concert with an ROV while evaluating novel intervention techniques. Photo by Jona Silverstein.

Current Opportunities: Kitting Up!

RD1 units, as conceived, are amazing tools; I and others are using them routinely for a variety of shallow water work. Over the past few years, I’ve organically moved components to a production state with partners—all critical components are produced by ISO9001 manufacturers. For those already expert in the field, we offer a ‘kit’ unit which allows the diver to build out to suit. We have end users staying true to its shallow roots, and also evolving them to mix units. 

Figure 5. RD1 mated with our novel Presenter Helmet for science outreach at the University of Arizona’s Biosphere2 facility.

RD1’s inherent simplicity provides an open architecture or building–block-like approach, allowing the end user to reconfigure and evolve the system for their requirements. Conceptually, an RD1 oxygen-only diver could re-plumb to have a very nice mCCR, and then again incorporate any number of accessories for desired features. From the development standpoint, we intend to continue to develop RD1 accessories and upgrade pathways. Just one accessory we’ve been very happy with is our Mouthpiece Retaining Strap (MRS). 

“My hope and expectation is that both the oxygen-only fundamentals concept and the RD1 unit itself prove somewhat disruptive.” 

Figure 6 An overview of the RD1 MRS. An MRS was gifted to each RF4 participant as a show of support to the MRS Safety Initiative of the RTC.

My hope and expectation is that both the oxygen-only fundamentals concept and the RD1 unit itself prove somewhat disruptive. As a methodology, simplified training at or near ‘Day 1’ will broaden the end user population which will, in turn, create the need for alternate training paradigms that accommodate specific development paths. 

I very much want to see people build off this platform as it becomes recognized that the best rebreather for Rhode Island clams may be different than the one for New Jersey wrecks and Florida caves and Caribbean reefs and elsewhere. However, at its core, a rebreather is a rebreather is a rebreather. We can better institute that mentality if we are instructing oxygen-only fundamentals.

This approach further disrupts current conventions in third party testing. It’s extremely important to understand performance criteria windows, practical limitations of components, and overall system architecture, although the industry is already having to contend with the multitude of aftermarket components being routinely integrated into ‘tested or approved’ units that may negate the initial testing altogether. 

A ‘Day 1’ unit and associated knowledge base should eventually take us down a path similar to using a regulator. That level of utility will create the true rebreather market opportunity that would benefit all of us.

Figure 7 A production RD1 unit in its original oxygen-only configuration. The unit incorporates a 2.5kg scrubber, 2 x 4L BMCLs, an AL13 of oxygen fit to a detuned demand valve, and independent AL13 air bailout. The unit fits to any standard backplate, harness, or stab jacket via the centrally positioned spine piece. Accessories such as pO2 displays, a dewatering pump, and various gas plumbing can be integrated to suit.
Figure 8 RD1 in oxygen only mode showing configuration overview.

To participate in the Rebreather Day 1 discussion, join the Facebook Group: Rebreather Day 1 (RD1)

RD1 Oxygen Rebreather Kit: Kit System (Back Mount)


InDEPTH: Portable Habitats: New Technical Diving Capabilities are Well Within Reach by Mike Lombardi

InDEPTH: Professor achieves an underwater first – ‘camping’ by Mike Lombardi

InDEPTH: Decompression Habitats Are Ascendent by Andy Pitkin

Inner Space Explorers: The ISE Oxygen Rebreather Program

Michael is a Rhode Island USA based diving contractor, scientist, inventor, and entrepreneur. He has amassed more than 5000 working dives in very challenging conditions – ranging from inshore marine construction to deep mesophotic coral reefs. He has been diving rebreathers since 2003. Michael Co-Chairs the Marine Technology Society’s Diving Committee and is widely published in the field.  His company has developed portable habitats, undersea tooling, and custom rebreathers all for use within scientific and industrial projects. More on Lombardi Undersea LLC can be found at Michael’s work in the community can be found at You can reach him at:

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Hyperbaric Chambers Are Turning Away Divers. Will There Be One Nearby When You Need It?

Unfortunately, it’s hard to make a business case for treating divers versus wound and burn care victims. As a result, many hyperbaric chambers no longer treat divers, leaving fewer facilities available for divers in need and increasing their post-dive time to treatment. InDEPTH editor Ashley Stewart reports on this growing crisis in the US and what can be done!




By Ashley Stewart

The hyperbaric chamber at the University of California San Diego. Photo courtesy of Sherri Ferguson

Steven Wells was diving on the scuttled wreck of the USS Oriskany off the coast of Florida in 2016 when a problem with his buoyancy compensator caused a rapid ascent to the surface.

Wells’ dive buddies followed the emergency action plan for the Oriskany listed on the Florida Fish and Wildlife Conservation Commission’s website at the time and brought Wells straight to Naval Air Station Pensacola, the nearest facility with a hyperbaric chamber. The facility turned him away because there was no one there to run it.

Wells was taken 30 minutes away to Baptist Hospital, which also has a chamber capable of treating his injuries, but the hospital had years earlier decided only to use it for wound care. Doctors there decided Wells would be taken by ambulance more than an hour away to Mobile, Alabama, the nearest facility that accepts divers.

By the time Wells arrived at the only chamber that would help him, it was too late.

Steven Wells

“I got a call from the hospital saying, ‘Your husband is on life support. You need to get here now,’” Rachel Wells said of her late-husband of more than 23 years. 

Julio Garcia — the program director of Springhill Medical Center’s wound care and hyperbaric facility where Steven Wells was to be treated — told InDEPTH that while no one can be certain how sooner treatment would have affected the outcome of Wells’ case, it would have given him the best chance for a full recovery.

Each year in the US, there are about 400 serious cases of decompression illness (DCI) — a category including both arterial gas embolism and decompression sickness — in divers, according to one 2020 paper. The Divers Alert Network (DAN) hotline dealt with 587 cases annually over the past five years.

The availability of hyperbaric chambers to treat decompression illness is something many divers take for granted. We try to avoid dive-related injuries through training, but expect treatment to be available when we need it. 

The reality — as Steven and Rachel Wells tragically learned — is that only a minority of divers are close to care for diving-related injuries, according to medical professionals in the field. The estimates vary, but it’s generally believed there are about 1,500 hyperbaric medicine facilities in the US and only 67 are currently treating diving accidents, according to DAN.

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The estimates vary, but it’s generally believed there are about 1,500 hyperbaric medicine facilities in the US and only 67 are currently treating diving accidents, according to DAN.

“The problem is only getting worse, not better,” Garcia, the Springhill Medical Center program director, said. Garcia has been sounding the alarm about this problem for more than a decade. His hospital takes patients from as far away as Florida cave country and treated 20 DCI cases in 2022. Those patients had an average transportation time of 11.5 hours, according to an InDEPTH analysis of Garcia’s records.

Florida stands out because it’s a popular diving destination, DAN Research Director Frauke Tillmans said, but the situation is not much better across the US. Many of the 1,500 hyperbaric medicine facilities, like Pensacola’s Baptist Hospital, have transitioned to treating wound care only for economic reasons. Emergency hyperbaric services are expensive to train and staff, and come with increased liability.

Patient briefing before treatment at the Environmental Medicine and Physiology Unit at Simon Fraser University. 

Time to treatment can be important in DCI cases

Time is of the essence when treating DCI. Divers Alert Network Director of Medical Services Camilo Saraiva told InDEPTH time to treatment is a “pivotal determinant” when it comes to outcomes for DCI patients. “Swift intervention significantly influences the effectiveness of therapeutic recompression,” Saraiva said.

Decompression sickness, for example, results from rapid changes in pressure and can form gas bubbles in body tissues. Initiating recompression therapy minimizes bubble size and number, Saraiva said, enhancing their elimination and reducing the risk of further vascular obstruction and tissue damage.

“The timely provision of hyperbaric oxygen therapy not only aids in bubble resolution but also mitigates the potential for neurological deficits and other severe complications, highlighting the critical role of early treatment in optimizing outcomes for DCI patients,” Saraiva said.

The 2018 paper “In water-recompression” stated delays to recompression in military and experimental diving are typically less than two hours and more than 90% of cases are completely resolved during the first treatment.

Frank K. Butler and Richard E. Moon, hyperbaric medicine experts, wrote in a 2020 letter to the Undersea and Hyperbaric Medicine journal editors suggesting a minority of patients who need life-saving hyperbaric oxygen treatment (HBO2) are close to a major hospital with a 24-hour emergency hyperbaric facility.

Julio Garcia’s log on patient time to treatment at Springhill Medical Center. Click to enlarge

“Despite the urgent need for treatment, most hyperbaric chambers will decline to accept emergent patients at present,” Butler and Moon wrote. “Patients may eventually receive HBO2 but after a significant delay and a transfer of several hundred miles. Many never receive indicated HBO2, often resulting in poorer patient outcomes.”

Patients who are delayed treatment, they wrote, face the possibility in some cases of “death, permanent neurological damage, permanent loss of vision, or loss of an extremity, most of which would have been readily preventable had emergent HBO2 been administered.”

Why fewer chambers treat dive injuries

As recently as two decades ago, according to Butler and Moon, the majority of hyperbaric treatment facilities were available 24/7 to treat emergency patients. The percentage of those facilities now treating emergency patients is unclear, but it’s universally agreed the number has fallen significantly.

The reasons for the loss of emergency HBO2 facilities, Butler and Moon suggest, include “a better economic return when chambers focus on wound care patients as opposed to emergencies; the greater legal liability involved with treating high-acuity emergency patients; and the increased training and staffing requirements that would be required to manage critically ill patients — especially diving injuries and iatrogenic gas embolism patients.”

A letter from an administrator at Baptist Hospital — which sent Steve Wells to Springhill Medical Center — viewed by InDEPTH shows the hospital discontinued hyperbaric emergency services in December 2010, citing lack of staffing for specialty trained hyperbaric physicians who can provide 24-hour patient care. Baptist has yet to respond to InDEPTH’s request for comment.

Julio R. Garcia at Springhill Medical Center Hyperbaric Center

There’s also the issue of pay. Garcia, the Springhill program director, said the current rate of pay for doctors who administer hyperbaric treatments regardless of length is around $150. A typical hyperbaric treatment for other conditions is about two hours. Diving treatments are usually six or seven, he said. “What doctor wants to get paid $150 to be up all night for seven hours, at that point making less than the technician?” Garcia said. “The fix is that healthcare payers need to pay more for the supervision of the treatment for diving injuries. Make it something that’s worth a doctor’s time besides the goodness of their hearts.”

Silence from lawmakers

Medical and diving organizations in 2020 sent a letter to the House and Senate, federal government agencies, governors of Florida and California, and the American Hospital Association expressing concerns about the lack of availability of chambers to treat diving injuries.

“There are approximately three million recreational scuba divers in the US,” the letter stated. “In the unlikely event that they suffer a diving-related injury, they trust that the US medical system will provide state-of-the-art care for their injuries, but the steadily- decreasing number of hyperbaric treatment facilities in the US willing to treat them emergently for decompression sickness or arterial gas embolism often places them at much greater risk than they realize.”

Garcia has on his own contacted lawmakers, reporters, medical systems — even private space companies like SpaceX because his facility is also the only one nearby treating altitude decompression sickness from space and air travel.

Little has changed, Garcia said.

Garcia showed InDEPTH a 2014 letter from a Defense Health Agency director who said, while there are three Undersea and Hyperbaric Medicine Society-accredited clinic hyperbaric medicine facilities and two additional facilities that can treat civilian emergencies, they are not staffed 24/7 and not designed for patients with other medical illnesses. Garcia at the time requested the creation of a federal grant to support the expansion of 24/7 hyperbaric services, but the director said that was outside of the agencies’ purview. 

The hyperbaric chamber at the University of California San Diego. Photo courtesy of Sherri Ferguson

Two years after this exchange, Steven Wells was taken to and turned away from one of these facilities — the NAS Pensacola, listed on the Florida Fish and Wildlife Conservation Commission’s (FWC) emergency action plan at the time. 

The Florida Fish and Wildlife Conservation Commission website now shows a map of the nearly 4,000 artificial reefs across Florida’s 1,350 miles of coastline. Two chambers, one in Mobile, Alabama, and one is Orlando, cover 500 of those miles densely packed with dive locations, according to Garcia.

The FWC website now shows a map of the nearly 4,000 artificial reefs across Florida’s 1,350 miles of coastline. Two chambers, one in Mobile, Alabama, and one is Orlando, cover 500 of those miles densely packed with dive locations, according to Garcia. A report from the University of West Florida estimated the sinking of the Oriskany, scuttled in 2006, generated nearly $4 million for Pensacola and Escambia County in the next year alone.

So many reefs, so few chambers! FWC map screenshot

An FWC spokesperson said the agency provides diver safety reminders and recommended actions on its website “as a courtesy” and is not intended for emergency response. FWC and Visit Florida did not respond to inquiries about how much Florida’s government spends on advertising the artificial reefs and other diving activities, or whether any effort to expand the availability of hyperbaric facilities to treat the divers who show up as a result.

“My question is what is my husband’s life worth compared to your chambers,” Rachel Wells, Steven Wells’ widow said. “Why did he have to die?”


DIVER: A Crisis in Emergency Chamber Availability by Dan Orr (April 2022)

Divenewswire: A Crisis Lurking Below the Surface Emergency Hyperbaric Treatment Availability by Dan Orr (August 2021)

Undersea and Hyperbaric Medicine (2020): Emergency hyperbaric oxygen therapy: A service in need of resuscitation – an open letter by Frank K. Butler, MD, and Richard E. Moon, MD

White paper: Access to emergent hyperbaric oxygen (HBO2) therapy: an urgent problem in health care delivery in the United States (2020)

InDEPTH: A New Look at In-Water Recompression (IWR) (2019) by Reilly Fogarty

Diving and Hyperbaric medicine (2018): In-water Recompression, Doolette DJ and Mitchell SJ 

aquaCORPS (1993): In-Water recompression As An Emergency Field Treatment for Decompression Illness by Richard L. Pyle and David A. Youngblood

InDepth Managing Editor Ashley Stewart is a Seattle-based journalist and tech diver. Ashley started diving with Global Underwater Explorers and writing for InDepth in 2021. She is a GUE Tech 2 and CCR1 diver and on her way to becoming an instructor. In her day job, Ashley is an investigative journalist reporting on technology companies. She can be reached at:

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