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Can Mouthpiece Retaining Straps Improve Rebreather Diving Safety?

In this issue of InDepth, we explore the value and efficacy of using mouthpiece retaining straps aka ‘gag straps’ to improve rebreather diving safety. Though few tech divers use them today, a good case can be made for their use as evidenced by this reasoned case made by DAN’s Reilly Fogarty.



By Reilly Fogarty
Header photo by
Reilly Fogarty

Be sure to check out the following stories:

InDepth: RTC Launches New Rebreather Safety Initiative

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

InDepth: A Mouthpiece Restraining Strap Just Might Save Your Life by Andrew Fock

InDepth: Where do Agencies and Manufactures Stand on Mouthpiece Restraining Straps?

We surveyed CCR divers from around the world. Here are the results.

Rebreathers are excellent tools for extending dives and pushing the limits of human exploration. Unfortunately, discussing how best to set up your rebreather in any public forum can push the limits of human patience. The cost, the training, and the experience required to dive a rebreather can lead to strong opinions, and the debates about the commercially available options are endless. In these debates, consideration of important safety features can be lost. 

Among these features are mouthpiece retainers sometimes referred to as “gag straps.”.The concept is neither new nor revolutionary—most are simple rubber straps designed to hold the circuit in a diver’s mouth in case of loss of consciousness. Some designs feature a half or a full face mask. The purpose of the feature is to keep water out of the airway of a diver who has lost consciousness. Beyond a few configuration options, there are few differences between most of the units available to recreational divers. 

Research has shown that mouthpiece retainers offer a real benefit in terms of safety, so regardless of how you choose to configure your rebreather, a loop-retaining device is crucial. 


Retainer strap connected to the mouthpiece.
Photo by AP Diving.

Understanding what kills rebreather divers is complicated by the small number of participants and the problem of separating anecdotes from statistically relevant data in small incident sample sizes. Estimates of the total number of rebreather divers globally range from 6,000–20,000 divers; neither manufacturers nor training agencies provide their sales or certification numbers to the public. A safe estimate of active rebreather divers is generally considered to be around 14,000. 

The military and to a lesser extent the commercial diving community (commercial Saturation divers use rebreathers for bailout) represent other groups of users. Both of these communities use slightly different equipment and adhere to much stricter operational protocols. Military/commercial use and fatality numbers are largely unknown to the public  as well, but there are some exceptions that have proven enlightening. Based on incident reports, DAN Annual Diving Report analysis, and a series of DAN Rebreather Forum meetings, the single largest killer of rebreather divers is drowning.

Based on incident reports, DAN Annual Diving Report analysis, and a series of DAN Rebreather Forum meetings, the single largest killer of rebreather divers is drowning. 

While this may sound unsurprising or even obvious, it illustrates that what kills rebreather divers is not freak accidents or the increasing number of health issues we see in the larger general population of recreational divers. This shows that rebreather fatalities can largely be attributed to some kind of drowning—generally subsequent to loss of consciousness via hypoxia, hyperoxia or hypercapnia. It’s in addressing this cause of mortality among rebreather divers that diver supply valve (DSV) retainers come into play. These devices aren’t a catch-all designed to save divers from their own mistakes or drown-proof undertrained, would-be explorers. What they do is to provide a measure of safety in loss-of-consciousness events and to measurably decrease fatalities. 

Equipment Options

Photo courtesy of rEVO.

These mouthpiece retainers come primarily in two variations. The first is a retainer strap, like a mask strap, that attaches to a rebreather mouthpiece and is positioned behind the head. This strap attaches to a device that will seal around the lips (such as the lip seal on a Drager Safety Strap). The second option is a full face mask designed for CCR use, like the Draeger Panorama, or a partial mask design like the Kirby Morgan M48. There are now several commercially available options for rebreather divers looking for full or partial mask options; as long as the device is intended for rebreather divers and adequately minimizes dead space, it can  reasonably be considered for use. Excessive dead space in a full face mask can lead to unintended CO2 retention, which poses a greater hazard to rebreather divers than it does to open-circuit divers. 

Both options function similarly, sealing the rebreather circuit to the lips and allowing the diver to continue breathing. It’s important to note that losing control of the circuit while underwater will also result in the loss of loop volume and a commensurate decrease in buoyancy, exacerbating any issues that led to the initial loss of control. This type of compounding incident is common among new rebreather divers and difficult to recover from. 


Given the size of the market and the limited availability of military research, it’s not surprising that statistical studies of mouthpiece retainers are difficult to find. While it’s true that case studies abound, and many provide valuable information, the compounding nature of rebreather accidents makes it difficult to determine a single incident catalyst rather than guess at the most likely contributing factors to a fatality. What we know about mouthpiece retainers comes primarily from two studies. The first reviews 54 loss-of-consciousness events in military rebreather diving (Haynes, 2016), while the second reviews 153 diving injuries among French military rebreather divers (Gempp, 2011). 

The Haynes paper begins by reviewing the range of issues created by the initial adoption of rebreathers by recreational divers. The early modifications, protocol creations, and fatalities illustrated incident statistics surprisingly close to what we see now, with “inappropriate gas” causing more than half of all rebreather fatalities in Haynes’ data analysis. This category (in this analysis) indicated a rebreather-delivered gas causing hypoxia, hypercapnia, or hyperoxia rather than a tank of mislabeled open-circuit gas. This holds true with what we now know about rebreather fatalities and serves as the motivation for Haynes’ review of mouthpiece retainers to minimize the fatalities caused by the resulting loss of consciousness in these incidents. While the Haynes paper goes on to cite military adaptations of mouthpiece retainers, case reviews, and expert testimonial, the most educational data cited is taken from the Gempp paper. 

Photo courtesy of Divesoft.

Descriptive Epidemiology of 153 Diving Injuries with Rebreathers Among French Military Divers from 1979 to 2009 confirms both the Haynes and industry data, with gas toxicity causing 68% of injuries. More importantly for our purposes, it reviews 104 cases of gas toxicity with 54 of those resulting in impairment or loss of consciousness in the water. Of these, the outcome was “always favorable” if the diver could be retrieved to the surface. Among the loss-of-consciousness events, only 3 fatalities were recorded. The paper goes on to state that “gas toxicities are frequently encountered by French military divers using rebreathers” but that the low fatality rate can be attributed to strict safety protocols, specifically the “mouthpiece strap, buddy team with link, and diving instructor with open circuit to lend assistance if necessary during training” (Gempp, 2011).  

Both papers are clear about the use of mouthpiece retainers saving lives among the population studied. As with any research, it’s important to note that this may not apply directly to your diving. It’s true that mouthpiece retainers showed very promising results in case studies as well as incident reviews, but the subjects do not represent most recreational divers. Not only do both studies focus on military divers in better physical condition and with better equipment maintenance, but their subjects adhere more strictly to safety protocols and receive significantly more training than recreational divers. 

The lack of training in particular is what brings issues like bail-out protocols to light. It’s true that bailing out with either a mouthpiece retainer or full face mask becomes somewhat more complicated, and the addition of a bailout-valve adds additional opportunities for user error. With adequate training and emergency protocols it seems unlikely that any of these concerns would notably increase risk, but that’s not something we have the data to confirm just yet. 

For now, consider the research, and consider how you dive: do you have enough experience and training to add a mouthpiece retainer to your configuration? If the data applies to your diving, a mouthpiece retainer might just save your life. 


Haynes, P. (2016, December). Increasing the probability of surviving loss of consciousness underwater when using a rebreather

Gempp, E. (2011). Descriptive Epidemiology of 153 Diving Injuries with Rebreathers Among French Military Divers from 1979 to 2009.

Additional Resources:

Mouthpiece Retaining Straps: Discussion on CCR Explorers
A Survival Guide for Rebreather Diving, by Paul Haynes

When he’s not working with DAN on safety programs, Reilly Fogarty can be found running technical charters and teaching rebreather diving in Gloucester, Mass. Reilly is a USCG licensed captain whose professional background includes surgical and wilderness emergency medicine as well as dive shop management.

Diving Safety

What Happened to Solid State Oxygen Sensors?

The news in 2016 that Poseidon Diving Systems would be incorporating a solid state oxygen sensor in their rebreathers sent a buzz through the rebreather community. Galvanic sensors, along with their legacy-1960s “voting logic” algorithms to boost reliability, had long been considered the weakest link in closed circuit rebreathers. Many heralded Poseidon’s subsequent 2017 roll-out as the dawn of a new era in rebreather safety. Five years later, Poseidon remains one of two companies (the other strictly military) to have adopted optical sensors. Technology reporter and tech diver Ashley Stewart examines some of the reasons why.




by Ashley Stewart.

Header image: Karst Underwater Research (KUR) rebreather divers at Weeki Wachee. Photo by Kirill Egorov

For years, it’s been said there’s a revolution coming for the closed-circuit rebreather— a new, more reliable, safer replacement for the traditional electro-galvanic oxygen sensor, widely considered the weakest component of rebreathers. In March 2017, that revolution looked to be just over the horizon. Poseidon Diving Systems began shipping an offboard solid state sensor to supplement the MKVI’s and SE7EN’s galvanic sensors and offered to license the technology to other manufacturers. Though Poseidon subsequently incorporated the solid state sensor into its SE7EN rebreathers, nearly five years have passed, and not much else has changed.

Poseidon remains the only manufacturer using solid state sensors in recreational rebreathers. No other companies have licensed Poseidon’s technology. Major tech diving manufacturers—including JJ-CCR and Divesoft—say they don’t believe the technology in general is ready for use in rebreathers. Some manufacturers worry that the sensors won’t function accurately in humid environments over a wide range of pressures, and they claim that addressing these challenges will be costly. Meanwhile, divers who tested Poseidon’s sensors offered mixed reviews, and even the inventor who sold the sensor validation technology patent to Poseidon believes they should be used along with traditional sensors. (Poseidon gives divers the option of combining the sensors).

Poseidon’s solid state sensor integrated into the SE7EN rebreather. Photo courtesy of Poseidon.

Oxygen sensors are the enabling technology that made mixed gas rebreathers possible, replacing rebreathers that could only be used with pure oxygen. In 1968, marine scientist Walter Starck introduced the first commercial CCR, called the Electrolung, which used polarographic sensors. The next year, BioMarine Industries launched its CCR-1000, the predecessor of the US Navy’s Mk-15/16. The unit was the first mixed gas rebreather to use galvanic sensors, which do not require a power supply. 

In addition to removing a diver’s exhaled carbon dioxide, a rebreather must measure and maintain a safe and efficient level of oxygen, as measured by the partial pressure of oxygen, or PO2, via oxygen sensors.

Measuring PO2 correctly is critical, and failures can be fatal. Too little oxygen can cause hypoxia and loss of consciousness, and too much can result in central nervous system toxicity and convulsions. Since sport divers began using CCRs over twenty years ago, both conditions have caused numerous drowning fatalities.

With the exception of Poseidon and military manufacturer Avon Underwater Systems, modern close circuit rebreathers have more or less used the same type of sensor since the 1960s. Rebreathers typically use three galvanic sensors, averaging the readings of the two closest sensors and ignoring the third in a protocol called “voting logic,” originally created by Starck in response to the sensors’ noted unreliability.

Even with this voting logic, however, the sensors can be unreliable (See “PO2 Sensor Redundancy” in Additional Resources below). The galvanic sensors are cheap and time-tested, but they need to be recalibrated before every dive and expire after about a year. The new sensors—called “solid state sensors” or optical sensors—are expected to be more precise, reliable, and durable, though significantly more costly.

An illustration of luminescent quenching technology

Galvanic sensors are essentially wet-cell batteries that generate a millivolt current proportional to the PO2 in the loop. Conversely, Poseidon’s solid state sensor uses luminescent quenching, wherein a red LED light excites the underside of a special polymer surface, which is covered with a hydrophobic membrane and exposed to the gas in the breathing loop. A digital color meter then measures the responding change in fluorescence, which is dependent on oxygen pressure, and an algorithm calculates the PO2.

Experts more or less agree that the right solid state sensor could make rebreathers safer, but the market is split on whether the technology is ready for use in rebreathers and just how much better they’d have to be to justify the cost.

Field Test Results

Poseidon advertises its sensor as “factory-calibrated and absolute, delivering unsurpassed operating life, shelf life, and calibration stability.” Richard Pyle, a senior curator of ichthyology at Hawaii’s Bishop Museum who works with Poseidon-affiliated Stone Aerospace, has tested Poseidon’s sensors for years, initially as a passive offboard check against Poseidon’s traditional galvanic sensors. Later, in November 2019, he said he began testing Poseidon’s prototype with the solid state sensor as the primary sensor in the unit. “From my perspective as a rebreather diver, this is the most significant game-changing way to know what you are breathing,” Pyle said. “We will never go back to the old oxygen sensors.”

Poseidon divers at 110 m/359 ft. Photo by John L. Earle

Pyle said he’s yet to fully analyze the data he’s collected to compare the performance of the solid state sensors against the galvanic sensors, but that  he’s had zero failures with the solid state sensors in the time he would have expected to have 50 to 100 failures with the galvanic sensors.

Likewise, Brian Greene, a Bishop Museum researcher who has tested the Poseidon sensors with Pyle, estimated that he’s made hundreds of dives with the solid state sensors without failure. But, not everyone has had this experience.

Sonia Rowley, an assistant researcher at the Department of Earth Sciences in University of Hawai’i at Mānoa, told InDepth that she experienced a variety of repeated failures when testing Poseidon’s system alongside Pyle beginning in 2016 and 2017, and Rowley dictated to InDepth specific dive logs detailing many of the failures. She wrote about her experience in the book “Close Calls.

Poseidon CEO Jonas Brandt said the company has tested the sensors since 2017 at different depths and temperatures, and that it has only seen one possible failure.

Arne Sieber is a sensor technology researcher who said he developed the O2 sensor validation technology used in the Poseidon rebreather and sold the patent to Poseidon. Sieber is now researching uses for the solid state sensor including in the medical market. He told InDepth he believes the best way to incorporate the solid state sensors into rebreathers would not be to substitute one for the other, but to combine sensor types and design a rebreather that incorporates both. 

Traditional galvanic sensors have advantages over the solid state sensors, Sieber said—they’re cheap, simply designed, low-voltage, and time-tested. Also, while solid state sensors are very accurate at measuring low PO2, they become less sensitive at about 1.6 bar, and are more prone to incorrect readings of higher PO2 levels than galvanic sensors. As for whether the sensors can function in humid environments, Sieber said the sensors can work well in liquids, such as when used for blood analysis (though the sensors are used to measure much lower partial pressures of oxygen) and for measuring oxygen content in the sea. Liquid can delay the amount of time it takes a sensor to read a partial pressure, but it does not falsify the results, Sieber said. Of Poseidon’s system, Sieber said, “It’s a good start. It’s very important that someone starts. Someone always has to be the first one.”

Brandt said divers have the option of combining sensors in the company’s SE7EN rebreather, using either two galvanic sensors, two solid state sensors, or one of each, and said it could be argued that using one of each sensor is the most reliable.

Meanwhile, a catalyst may be coming to encourage the development and adoption of solid state sensors in Europe, Sieber said. European Union rules restrict the use of hazardous substances in electrical and electronic equipment, but galvanic sensors (which have an anode made of lead) have been granted an exemption in medical products because there is not a suitable alternative. The exemption is set to expire.

Poseidon’s solid state sensor sells to end users for as much as around $1,500USD, and its SE7EN rebreather units use a maximum of two onboard sensors. [Note: Poseidon sells the sensor for 6800SEK plus VAT from its website, which equates to 944 USD, some outlets in the states sell them for much higher]. Galvanic sensors, meanwhile, cost around $100USD, last one year and divers use three at a time. And, that’s just the cost of the sensors themselves: Manufacturers have to make significant investments in, and upgrades to, electronics systems to accommodate solid state sensors. 

Fathom rebreather lid showing its galvanic sensors. Photo courtesy of Fathom Dive Systems, LLC.

As for how long the sensors actually last, even the manufacturers don’t yet know. Poseidon has some from 2014, and they still work but have to be factory calibrated every two years. Galvanic sensors need to be replaced annually, while solid state sensors are expected to last much longer.

Brandt chalks the debate about its sensors up to competitiveness in the market. “I don’t think anyone likes that somebody cracked the nut,” Brandt told InDepth. Poseidon is ready to share the technology with other dive companies and manufacturers, Brandt said, but there have been no deals to date. “We wanted to raise the bar in technology and safety with the rebreathers, and to be honest, we haven’t said to anyone in this business that this technology is exclusive or proprietary.”

Market Interest

When the company first debuted its sensor, Brandt reported that companies like Hollis and Shearwater Research expressed interest in licensing the technology, but nothing has come so far of those discussions. Brandt did say one manufacturer reached out right before the pandemic. He declined to say which, but shared that it was a European company. Hollis brand manager Nick Hollis said his team recalls a conversation with Poseidon, but that it was back in 2014 or even earlier.

Shearwater director of sales and marketing Gabriel Pineda said the company is still interested in solid state sensors, but they see an issue with the price. “If you make the economic case of traditional galvanic sensors versus solid state or optical sensors, you have to dive a lot, and it takes a long time for these to make economic sense for a diver.”

Of course, Shearwater is not a CCR manufacturer, but the company is interested in seeing whether the sensors would be viable for use with its electronic control system that is used by a majority of rebreathers on the market. Shearwater currently has no immediate plans to license the technology from any manufacturer but Pineda said the interest remains. 

Meanwhile, Poseidon’s solid state sensor CCR is still making headway, Brandt said. The current biggest buyer of the Poseidon units is the military (Brandt said three European Union countries’ forces are actively using the sensors). The sales have continued throughout the pandemic, and, over the past six months, Poseidon has started an upgrading program, allowing divers to add the new sensors to their old units. Poseidon is looking into a program Brandt compares to Apple Care, where customers can pay a fee for maintenance throughout the life of the sensor.

Solid state sensors replace numerous galvanic sensors which have a one-year life. Photo courtesy of Richard Pyle.

Meanwhile, Avon Underwater Systems is using three solid state sensors in its MCM100 military rebreather. Kevin Gurr, a rebreather designer and engineer who sold his company, VR Technology Ltd., to Avon, said the company uses the sensors “because of the increased safety and the decreased user burden as far as daily calibration.”

Gurr, who designed and produced the Ouroboris and Sentinel closed circuit rebreathers at his prior company VR Technologies Ltd., believes it’s the cost that has discouraged other manufacturers. “It shouldn’t be about cost at the end of the day,” Gurr said. “The digital interface is so much safer.”

Martin Parker, managing director of rebreather manufacturer AP Diving, said his company follows solid state sensor development but has yet to come across a sensor that meets its accuracy requirements. One such sensor using luminescence quenching can achieve good accuracy through a replacement disk the user must apply to the sensor surface after each use.

“Having been in the diving business for 50 years, we don’t believe it is on any diver’s wish list to have to re-apply every diving day a new component, as simple as that is to do,” Parker told InDepth. “With no easy external measure of accuracy prior to the dive, it is easy to foresee that many divers would ‘push their luck’ and use the discs for multiple days, then when they get away with it, they would encourage other divers to do the same… with the inherent risk of DCS or O2 toxicity.”

Parker said that he’s aware of two additional sensors under development, but neither has shown a working product yet. He declined to identify any of the manufacturers, citing commercial sensitivity. “Hopefully, we will get these to evaluate in the next 12 months,” Parker said.

Divesoft co-founder Aleš Procháska said he believes Poseidon’s approach to the sensor could “lead to success.” Speaking generally about solid state sensors rather than about Poseidon’s specifically, Procháska said his company isn’t yet utilizing solid state sensors because he believes the sensors are unable to function in humid environments with extreme water condensation and not applicable over a  wide range of pressures. To be able to use one of these sensors in a rebreather, Divesoft wants it to be durable in high humidity, consume less energy, and have a good price-to-lifetime ratio. 

“It is possible to build a CCR with the currently available O2 solid state sensor but not without sacrificing important properties of the breathing apparatus,” he said, such as size and energy. “Overall, the reasons why no one currently sells this technology on the market seems to be quite simple. It’s extremely difficult to come up with a suitable and functional principle that would lead to a cheap, small, and low energy consuming solid-state sensor. Despite this, I do believe that it’s only a matter of time until someone solves this one.” Asked via email about the status of DiveSoft’s own work on the technology, Procháska replied, “Well, as I said earlier, it’s just a matter of time,” adding the text, “Aleš smiles.”

JJ-CCR lid showing its three galvanic sensors. Photo by Kees Beemster Leverenz.

Halcyon COO Mark Messersmith said that divers are slow to embrace new technologies in general, and the current sensors just aren’t deficient enough to merit widespread adoption or the investment from manufacturers. “It’s not unlike many other technologies,” Messersmith told InDepth. “People are often slow to embrace a new technology if the existing technology is functional. The existing tech needs to be vastly deficient, and existing oxygen sensors are still largely functional.” 

The bottom line: Solid state sensors might very well be safer, but there isn’t enough incentive for the market to make them a reality. 

David Thompson, designer of the JJ-CCR, told InDepth they don’t use the sensors because he doesn’t believe the technology is ready yet and research in that area is extremely expensive and difficult for what he believes is essentially a small market. “Analog cells have a long history, and in the right hands are very reliable, easily available, and have a long history of working in a rebreather environment which is very hostile,” Thompson said, adding that high humidity and temperature in a rebreather is a challenge for any sensor. “I am sure it will be in the future, but that future won’t be here yet.”

Additional Resources:

InDepth: Where Have All the Sensors Gone? Assessing the Global Oxygen Sensor Shortage

Rebreather Forum 3 Proceedings: PO2 Sensor Redundancy by Nigel A. Jones p. 193-202

Alert Diver: Oxygen Sensing in Rebreather Diving by Michael Menduno

Wikipedia: Electro-galvanic oxygen sensor

Photo by Daniel McMath 

Ashley Stewart is a Seattle-based technology journalist and GUE Tech 1 diver. Reach her via email:, Twitter: @ashannstew, or send a secure message via Signal: +1-425-344-8242.

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