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Diving Safety

The Flexibility of Standard Operating Procedures

Instructor trainer Guy Shockey discusses the purpose, value, and yes, flexibility of standard operating procedures, or SOPs, in diving. Sound like an oxymoron? Shockey explains how SOPs can help offload some of our internal processing and situational awareness, so we can focus on the important part of the dive—having FUN!

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By Guy Shockey

Header Image by Derk Remmers

At first glance, the title reads like a bit of an oxymoron. How can a standard operating procedure (SOP)—which implies a ‘one size fits all’ solution to problem solving—also be flexible? How can flexible also be firm?

One of the things that initially attracted me to Global Underwater Explorers (GUE) was the presence of SOPs. For anyone with a military background, SOPs were our bread and butter. You can create a good SOP while you have the time to think and plan. You can put them into practice, refine them over time, and keep them in place until new or better information comes along to change them. 

For example, airline pilots have a binder full of SOPs for various contingencies. When something comes up, they turn to the correct page and find a list of actions to follow. Pilots understand that these SOPs represent the collective knowledge of many aviators and engineers that have come before them. Many of them have also been revised multiple times, codified, and then even revised after that. Some SOPs require commitment to memory because there may not be a lot of time, and pulling out a three-ring binder or flipping through your iPad to the correct page isn’t the appropriate action. In that case, then those same pilots practice these situations regularly in simulator training. 

One of the primary values of an SOP is that it frees up a lot of situational awareness information processing. You are able to match up “mental models” to the current situation and, rather than processing your information in small bite-sized pieces, you are able to process “chunks” of information that match patterns of something that you know or are familiar with. 

Let me create an analogy that may help make this clearer. If I were to give you a bowl of tomato sauce, some slices of pepperoni, some mushrooms, some cheese, and a piece of baked dough, you could eat them all one at a time and try to figure out what it was you were eating. Or, I can put all those ingredients on that same piece of dough, bake it, and you would instantly know that you were eating pizza. You don’t have to process all the ingredients one at a time. You already have an existing mental model that says “pizza.” We do this when we solve problems. We pattern-match and identify existing mental models all the time, and it’s actually the only way we can actually think as fast as we do. Many problems are actually solved with multiple mental models being applied together. 

Photo by Derk Remmers.

Having an SOP gives you the ability to solve problems more efficiently and effectively because you have a ready-made mental model or solution to a recognized problem. Think of every first aid course you have ever taken and the “ABCs” of first aid. SOPs are incredibly valuable in nearly every environment that includes potential risks. 

If an SOP is shared, it also allows diverse groups to work together. It is no surprise that SOPs from various militaries of the world are often similar, even if they are written in different languages. From personal experience, NATO countries can coordinate and execute complex military operations because they share common SOPs that, if not identical, are very similar and don’t require much adjusting to mesh together. Common expectations and goals can be shared toward a common purpose. 

When in time-sensitive environments, many of these SOPs and the corresponding mental models they help develop can be lifesaving. This doesn’t just apply to the military, but also to law enforcement, paramedics, firefighters, pilots, and any other profession that is often faced with time pressures in making critical decisions. 



Do you share a common operational picture?

There is an interesting term often used in military circles called the “common operational picture” (COP). This is exactly what it sounds like, and is sometimes referred to as “a single source of truth.” Everyone involved in a decision-making cycle needs to be privy to the information that affects their decision. Sharing that information allows us to make informed decisions that often include SOPs. You could argue that we are creating a mental model that lets us apply another mental model!  

Alright, so how exactly does all this apply to diving and GUE diving in particular? I’m pretty sure that many of you have already connected many of the dots. 

In the GUE world, our divers create a COP at the beginning of the dive. We help reiterate this COP with our GUE EDGE pre-dive checklist, which is a great example of an SOP! We review the goals, team roles, our equipment, and the operational parameters of the dive, all in a standardized format that efficiently accommodates teammates from multiple different languages and cultures. I have performed GUE EDGEs in about 10 different languages and I only speak two!  The fact that we were doing this in a standardized fashion meant I could follow along and knew what they were talking about. 

As the dive plan complexity increases, so too does the COP become more complex. Some of our more ambitious exploration projects require even more time spent in planning than actual execution. But because there is a COP, coupled with SOPs (I know that’s a lot of acronyms), these projects usually go off without a hitch. 

Photo by Derk Remmers.

During the dive, there are multiple times that we have team-expected actions that are based on SOPs, and this contributes to and reinforces our COP. It is almost as if we are filling in a PDF form as we go along and confirming the various pieces of information that we need to complete the entire “form” or plan. 

In the case of emergencies, we have ready-to-implement SOPs for just about any equipment malfunction from valve failures to losing your mask. We practice these SOPs so that, in real time, we can employ them in a timely fashion and resolve the problem. These SOPs are just like the ones I mentioned at the beginning of this article and were developed over time and refined with successive reviews and after-action analyses. Finally, they have been codified, and you can now find them in our GUE SOP manual! You will also notice that this manual is of a particular “version,” which tells you that the SOP is constantly being fine-tuned in a dynamic process.

How Can An SOP Be Flexible?

In reality, it isn’t the actual SOP that is flexible, but it is the degree of flexibility it provides to the dive plan itself that is of value. Let me give you an example from the technical diving world. 

Imagine the team is diving on a wreck and experiences a delay on the bottom for whatever reason. It could be that it was done on purpose (discovery of pirate gold!) or maybe it was imposed upon the team as a result of any number of problems, like dealing with an equipment problem or an entanglement, for example. The dive is longer, the decompression obligation is now going to be longer, and there are some decisions to be made. 

Having an SOP here can help provide a solution to the problem with no mess and no fuss. The divers dig into the bag of tricks they learned in GUE technical training, and because of their common operational picture and team-expected actions, they apply the SOP they practice regularly and modify their decompression schedule to suit the new bottom time. What could have been an exciting moment for many divers turns into just another discussion point for their debrief after the dive!

Photo by Alexandra Graziano.

So, while SOPs are usually not flexible in and of themselves, they allow for a great deal of flexibility while diving by freeing up mental processing power and providing ready-made and practiced solutions to potential problems. 

GUE SOPs presuppose the presence of personal diving skills at a high level, and assume that factors such as good buoyancy and trim are second nature. In fact, many of the SOPs state the first step in resolving a problem as “stabilize” or “stop” in all three dimensions. GUE divers see that, as the diving gets more complex, the SOPs also get more complex. For a new GUE Fundamentals diver, demonstrating some of the SOPs required to pass muster as a Tech 2 or CCR 2 diver look more akin to channeling “the force” than anything else. However, like most things, perfect practice produces perfect performance, and so it’s just a matter of putting in the repetitions. 

For me, diving has never been the end but the means to the end. Anything I can do to make those means take up less mental and physical horsepower means that I can devote more of the same to the end goal. And at the end of the day, I am really all about that pirate gold!

Additional Resources:

Note that GUE members or divers taking a GUE course receive access to GUE’s 30-page manual, Standard Operating Procedures.


Guy Shockey is a GUE instructor and trainer who is actively involved in mentoring the next generation of GUE divers. He started diving in 1982 in a cold mountain lake in Alberta, Canada. Since then, he has logged somewhere close to 8,000 dives in most of the oceans of the world. He is a passionate technical diver with a particular interest in deeper ocean wreck diving. He is a former military officer and professional hunter with both bachelor’s and master’s degrees in political science. He is also an entrepreneur with several successful startup companies to his credit.

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.

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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: ashannstew@gmail.com, Twitter: @ashannstew, or send a secure message via Signal: +1-425-344-8242.

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