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Calibrating Rebreathers Equipped With A Shearwater HUD At Altitude

Rebreather instructors Michael and Josh Thornton describe how to calibrate your Shearwater-equipped rebreather to dive at altitude; it’s something they do frequently. Are you ready to get high, and then get down? Good! Do these steps first.

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By Michael Thornton and Josh Thornton

Header Image: courtesy of Josh Thornton

Diving rebreathers at altitude requires additional steps for calibration to ensure the pO2 display is accurate. At 4500 ft/1,372 m elevation, the atmospheric pressure is an average of 859 mBar (0.85 ATA) as opposed to an average of 1013 mBar (1 ATA) at sea level. This means when the rebreather loop is flushed with 100% oxygen, the pO2 display should be reading 0.85 and NOT 1.00 (Actually 0.98 — Shearwater’s default value). 

You may ask: who dives rebreathers at altitude? Many people do. 

The altitude of Salt Lake City, Utah is approximately 4500 ft/1,372 m. Dive shops in this area frequently teach open water courses at 6,000 ft/1,829 m and frequently dive and teach at lakes such as Fish Lake, which is at altitude just shy of 10,000 ft/3,000 m. Similarly, Lake Tahoe, which straddles the borders of California and Nevada, is at an altitude of 6,220 ft/1,897 m. There are many popular dive sites at altitude. 

Photo by Josh Thornton.

If a diver decides to dive a rebreather at altitude, it is important that they know how to properly calibrate the machine for that altitude. When diving at Fish Lake, for example, the pO2 reads .15 instead of .21 (when the sensors are exposed to air), and should read .70 instead of 1.0 (when the sensors are exposed to 100% oxygen). 

Older Shearwater firmware for the primary handset allowed the diver to choose between a “sea level” setting and an “auto” setting in the altitude settings for the controller. Depending on the firmware, the diver may need to select “Auto” in order to calibrate at altitude. The specifics of this are covered in the user manual of the rebreather. 

Newer Shearwater firmware defaults to an altitude setting where the controller uses the pressure sensor to determine atmospheric pressure, and allows for an accurate calibration at higher altitudes where the atmospheric pressure is less than that at sea level.  For example at 4500 ft/1,372 m flushing with 100% oxygen and calibrating the handset in altitude mode will display the correct pO2 of 0.85. 

Courtesy of Sherwater.

However, since the Shearwater Heads Up Displays (HUDs) do not have a pressure sensor, if calibrated at 4,500 ft/1,352 m using the button interface, the HUD will incorrectly display a pO2 of 1.0 instead of 0.85. Unfortunately, if the HUD pO2 reading differs by 15% from the controller handset display, it can no longer safely be relied upon for pO2 monitoring. For example, if a diver calibrated their rebeather at Fish Lake without the proper procedure, the HUD would be off by 30%. There are, however, ways of calibrating the HUD correctly at altitude. 

Calibrating DiveCAN HUDs 

The newer Shearwater HUDs utilize the digital DiveCAN bus making them easier to calibrate at altitude (or with less than 100% oxygen) than the older analog HUDs. The diver interfaces with the DiveCAN HUD via a single button on the side of the HUD. This is used for powering on the HUD, calibrating the HUD, and acts as a wet switch. The DiveCAN HUD is connected via a cable to a circuit board located inside the head where the actual calibration is stored; the HUD itself is simply a display and an interface. There is a connector on the cable so that the HUD can be detached from the head. 

Courtesy of Sherwater.

In order to calibrate the HUD at altitude, the diver first detaches the HUD, then plugs the Shearwater controller handset (or another DiveCAN device such as a DiveCAN NERD) into the HUD connector, and then calibrates using the altitude mode and the pressure sensor in the handset. The resulting calibration is stored in the head. When the HUD is reconnected back into its original connector, it will display the pO2 correctly. In summary, the diver used the handset to calibrate the HUD port as well as the handset port. 

Another option is to use a pressure pot to pressurize the sensors to 1.0 before calibrating the HUD using the single button interface calibration procedure, but this is obviously not as convenient. The diver would need to calibrate the handset at ambient pressure, and then calibrate the HUD using a pressure pot with a pressure of 1 ATA.

Calibrating analog HUDs 

Calibrating Shearwater Analog HUDs at altitude usually requires a pressure pot. There is no way to tell the HUD that it is being calibrated using anything other than 100% oxygen at sea level. There are no connectors allowing the handset and HUD to be swapped for calibration. At lower altitudes, where the difference in pressure from sea level is small, the diver may be able to over pressurize the loop to mimic sea level pressure. After performing an altitude calibration on the primary handset at ambient pressure, the diver could then pressurize the loop until the handset reads 1.0 and then proceed to calibrate the HUD. 

Photo by Josh Thornton.

At higher altitudes, it is impossible to pressurize the loop sufficiently to reach 1.0 pressure. Ideally the rebreather head will attach to the top of a pressure pot. If such a pressure pot is not available, there are smaller pressure pots designed to accommodate only the oxygen sensors. These pressure pots are fitted with wires and connectors to enable the oxygen sensors to be plugged into the head. 

This allows the diver to isolate the oxygen sensors and pressurize them to 1 ATA to mimic 100% oxygen at sea level. With the sensors pressurized to 1 ATA (sea level) and flushed with 100% oxygen, the HUD can be calibrated. Care is needed to ensure the sensors are then placed back in the head in the correct order so that the calibration remains accurate. 

Although the removal of the sensors for placement in a pressure pot can be tedious, there is the added benefit of being able to test for current limitation and linearity above the intended pressure range of use. This same pressure pot calibration method can also be used on DiveCAN HUDs if desired. 

Calibrating with less than 100% oxygen

In many remote destinations, oxygen is produced with oxygen generators, which are often only capable of producing 92-94% oxygen. Calibrating a Shearwater DiveCAN HUD with less than 100% oxygen requires special procedures.

On the primary handset in “System Setup” the diver can edit the percentage of oxygen to whatever is in the calibration cylinder. So when set to 92% after a full flush, it will calibrate at 0.92 ATA (or slightly less depending on altitude/barometric pressure etc). The DiveCAN HUD can be calibrated using the same procedure as outlined above for altitude calibration. (Plug the handset into the HUD connector and calibrate using the appropriate oxygen percentage and using the pressure sensor of the handset).

Courtesy of Sherwater.

Calibrating an analog HUD once again usually requires the use of a pressure pot. However, after calibrating the handset the diver may be able to over pressurize the loop to achieve a pO2 of 1.0 inside the loop if the percentage of oxygen is not too low. With less than 100% oxygen, the diver will need to pressurize the sensors over sea level pressure until the previously calibrated handset displays a pO2 of 1.0. It may be easier to keep a bottle of 100% oxygen purely for calibrating the HUD to avoid these extra steps. 

Additional considerations when using less than 100% oxygen

Even after correct calibration has been achieved, there are dangers associated with using less than 100% oxygen. When oxygen is produced using an oxygen generator, which again yields only 92-94% oxygen, the remaining gas is mostly argon. Unfortunately, argon is extremely narcotic at depth, and with a density of 1.78g/l compared to nitrogen’s 1.24g/l, will cause a higher work of breathing.

Photo by Josh Thornton.

On a fully closed rebreather, at a constant depth, only “oxygen” is injected into the loop. However, if the injected oxygen contains argon, the fraction of argon in the loop will continually rise throughout the dive, as the body metabolizes the oxygen, leaving a residual of argon in the loop with each injection. To counteract this, periodic loop flushes need to be performed, any time less than 100% oxygen is being utilized.

Note from Shearwater Research: Note that the Shearwater user-adjustable default value is actually 0.98 vs 1.0, so handsets could read 0.98 after normal calibrations, not 1.00 depending on the value the user defines. 

Also, another method for altitude calibration is to calibrate the handset and HUD at sea level (or in a pressure pot on bench at home), then travel to the dive site (and just maintain previous calibration).  Remember that calibrations are NOT required before every dive, and in fact less frequent calibrations have benefits.

It is probably also worth mentioning the DiveCAN connectors are not intended for very frequent plug/unplug cycles, so it is best to keep these to a minimum.  Also a note to take care not to overstress and the connectors when doing the plugging/unplugging and to inspect for any damage or corrosion.

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Dive Deeper: 

He needed to calibrate! GUE explorer Martin McClellan was determined to conduct an extensive photogrammetric survey on the wreck of the 19th century steamship SS Tahoe, which rests intact on a steep underwater slope at a maximum depth of 470ffw (144mfw) beneath Glenbrook Bay in Lake Tahoe, Nevada, at an altitude of 6,220 ft/1,897 m. Pushing the Altitude: The Quest to Document the SS Tahoe


Michael Thornton started diving in 1998 and has been an addict ever since. He was certified OC Full Cave at age 15 and began teaching technical diving courses through the family dive store, Dive Addicts, Draper, UT in 2006. Michael is a rebreather, cave, and advanced mixed gas Instructor Trainer through TDI and IANTD. Recently he became a GUE Fundamentals instructor. He and his family launched SubGravity in 2014, which emerged out of a desire to safely explore and experience the underwater world in some of the most extreme and demanding environments imaginable. Michael is also a co-founder of TEKDiveUSA, the biannual North American advanced and technical diving conference.

Josh Thornton, a co-founder of TEKDiveUSA, has been diving for 23 years, and teaching for 17 of those. Josh took his OC Full Cave Course in 2004 and has been completely obsessed with cave diving ever since. He is the training director at Dive Addicts, and a CCR adv mixed gas Instructor Trainer and CCR cave instructor for TDI and IANTD. He is also a co-founder of SubGravity. 


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Risk-Takers, Thrill-Seekers, Sensation-Seekers, and … You?

It’s likely that many in our community no longer think of tech diving as a risky activity, or perhaps even appreciate how important taking risks may be to one’s personal health—let alone that of our species. Fortunately, InDEPTH’s copy editing manager Pat Jablonski dived deep into the origins, meaning, and benefits of regularly taking risks, and even offers a thrill-seeking quiz for your edgy edification. What have you got to lose?

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by Pat Jablonski. Title photo courtesy of Katelyn Compton Escott.

“Life without risk is not worth living.” – Charles Lindbergh

What defines a risk? What is involved in taking a risk?

Difficult questions to answer, because something that feels risky to one person might be yawn-worthy to another. Risk taking, unscientifically, is something you do that gets your blood up, raises your heartbeat, awakens your senses, and makes you hyper-aware of your surroundings.

Surely we can agree that the Covid pandemic has added an unexpected level of risk to everyday life. Add poor drivers, mass shootings, contentious politics, global climate change, and many are left believing that meeting each day is risky enough. But that’s not true for people who identify as risk-takers or thrill-seekers.

“Everyone has a ‘risk muscle’. You keep in shape by trying new things. If you don’t, it atrophies. Make a point of using it once a day.” – Roger Von Tech

There are many activities that go to the trouble of defining the level of risk involved with a specific activity, and while that’s not the purpose of this article, you should know that scuba diving ranks fairly high on the risky behavior scale–higher than skydiving and rappelling. And, cave/wreck diving or freediving isn’t on any risk scale we could locate. We can assume it’s up there—near or at the top.

Fock A. Analysis of recreational closed-circuit rebreather deaths 1998–2010 Diving and Hyperbaric Medicine. 2013 Volume 43, No. 2. With the caveat that they are “best guess numbers,” Fock concluded that rebreather diving is likely 5-10x as risky as open circuit scuba diving, accounting for about 4-5 deaths per 100,000 dives, compared to about 0.4 to 0.5 deaths per 100k dives for open circuit scuba. This makes rebreather diving more risky than skydiving at .99/100k, but far less risky than base-jumping at 43 deaths/100k. The current belief is that rebreather diving has gotten safer.

Divers are a fairly small niche group for many reasons. One of them could involve the degree of danger associated with the sport. Answer this: Do dry land people ever ask you why you would want to take such a chance with your life in order to go where you weren’t meant to go? 

It’s a reasonable question, albeit a hard one to answer.

Photo courtesy of Glen Kwan

“A life without risk is a life unlived, my friend.” – Big Time Rush

Kevin Costner’s Waterworld aside, humans have (yet) to be born with gills or webbed toes. Still, there you are. You’ve spent unmentionable amounts of money. You’ve carved out a whole day, or maybe weeks, away from your to-do list. You’re suited up and look like an alien. You’re on a quest to explore the aquatic world where you’re able to breathe only with a cumbersome apparatus. You’re planning to explore inner space! You’re going to delve into that amazing realm that’s off limits to most people. 

You may look all matter-of-fact, cool as a cucumber, another day at the office, but it’s a thrill, isn’t it? Inside, you’re a kid with butterflies in your tummy who’s getting away with something big and exciting. Okay, it’s true–you and your team are highly trained, your equipment is top-notch, every box is checked off, and you are behaving responsibly. However, you’d have to be in a coma to not realize that what you’re about to do is taking a risk. Who doesn’t know that people have died doing what you’re doing? Answer honestly: How much more exhilarating is the experience when you know it’s not a walk in the park? Our own Michael Menduno admitted that “the feeling of being more alive lasted for days” after a dive.

So, you’re a diver. Does that mean you’re a risk taker? A thrill seeker? A sensation seeker?

Let’s dive into that subject, first by taking a little quiz, shall we? 

Photo courtesy of SJ Alice Bennett

From A Death Wish to Life Is Precious

In the past, too many mental health professionals treated risky behavior like a disease in need of a cure, focusing on the negative side of risk, even using government funding to address risky behavior and stamp it out. 

Before that, Sigmund Freud might have even believed that thrill seekers had a death wish; in fact, it’s what was believed for many years. 

Modern-day science doesn’t support either theory.

“Only those who will risk going too far can possibly find out how far one can go.” – TS Elliot

For our purposes, we’re focusing on the positive aspects of taking chances, pushing boundaries, and seeking experiences that make life feel . . . more alive. Richer. Fuller. We want to examine what goes into the psyche of a person (like you?) who is enthusiastically willing to engage in an activity already identified as dangerous, possibly even by the people who are engaging in it, and hear what some experts on the subject have to say about such people.

Photo courtesy of Jen MacKinnon

The University of Michigan’s Daniel Kruger proposes that taking chances is a fundamental part of human nature going all the way back to our ancient ancestors—prehistoric humans who had to constantly put their safety on the line in their fight for survival. Think fighting off a wooly mammoth with a stick. Kruger believes we have consequently retained many of those same instincts today, and he believes that it’s a good thing. 

This writer, who is related to a major risk-taker, has always believed that heart-quickening experiences are essential for a well-lived life. I’m convinced and have long proposed that those pulse-pounding moments are often accompanied by a deepened understanding of and appreciation for one’s life—perhaps all life. And I’m happy to report that current science confirms that belief.

“If you are not willing to risk the unusual, you will have to settle for the ordinary.” – Jim Rohn

Dr. Kruger is one of the scientists who proposes that taking risks means “seeking that moment when life feels most precious.

This should not be news for you diving adventurers out there.

Nature vs. Nurture: Born That Way or Learned To Love Adventure

Another scientist, Marvin Zuckerberg, affirms the theory that risk taking is in our DNA. “Certain people have high sensation-seeking personalities that demand challenges and seek out environments that most people’s brains are geared to avoid.” I’ll go out on a limb and say that underwater caves or shipwrecks would qualify as environments most would avoid.

Dr. Cynthia Thompson, the researcher behind a 2014 study from the University of British Columbia, was early to look at the genetic factors that might make a person predisposed to participating in extreme sports, ones that are typically defined as activities where death is a real possibility. The results of her study revealed that risk-takers shared a similar genetic constitution, a genetic variant that influences how powerful feelings are during intense situations.

Photo courtesy of Steve Boisvert

Most scientists agree that personality is a complicated mix of genetic and environmental influences. The “nature vs. nurture” dilemma is alive and well. Dr. Thompson concluded that people who engaged in so-called high-risk sports were not impulsive at all, not reckless either. Instead, “they’re highly skilled masters of their discipline who take a very thoughtful approach to their sports.”

A study conducted in 2019 examined human boundaries, people who pushed them to their limits and beyond, and what made those people tick. Zuckerman labeled such people “sensation seekers” and defined them as “people who chase novel, complex, and intense sensations, who love experience for its own sake, and who may take risks to pursue those experiences.” Is that you?

“History is full of risk-takers. In fact, you could say that risk-takers are the ones who get to make history.” – Daniel Kruger

Other experts posit an alternate theory—one proposing that modern society in the age of seatbelts, guardrails, child-proof caps, safety precautions, laws, rules, and regulations has dulled the sense of survival. In other words, life has flattened out and no longer feels exciting, or risky. So, is one of the reasons we seek excitement because of boredom? 

Maslow’s Theory of Self-actualization

I don’t honestly know who was the first proponent of risk-taking being a positive thing, but the work of Abraham Maslow, the founder of humanistic psychology, was one of the first. Maslow became one of the most influential psychologists of the twentieth century, and he developed a theory of human motivation that advocates for “peak experiences.” Peak experiences are not attained without risk.

“One can choose to go back toward safety or forward toward growth. Growth must be chosen again and again; fear must be overcome again and again” – A Maslow

He proposed that, in addition to meeting basic needs, all humans from birth seek fulfillment in terms of what he called self-actualization—finding their purpose/being authentic. Self-actualization involves peak experiences—those life-altering moments that take us outside ourselves, make us feel one with nature, and allow us to experience a sense of wonder and awe. Maslow also believed that those who were able to have such peak experiences tended to seek them out rather than waiting for the next random occurrence. Hence the anticipation of the next dive?

“Do one thing every day that scares you.” – Anonymous

Photo courtesy of Adam Haydock

Out of Your Comfort Zone Into A World of Wonder

Psychologist Eric Brymer from Queenstown University of Technology in Brisbane, Australia, has spent years studying extreme athletes and has this to say: “They’re actually extremely well-prepared, careful, intelligent, and thoughtful athletes with high levels of self-awareness and a deep knowledge of the environment and of the activity.”

Recent research backs up what some extreme sports athletes have been saying for years, even if only to themselves.

“What participants get from extreme sports is deeply transformational, a sense of connecting with a deep sense of self and being authentic, a powerful relationship with the natural world, a sense of freedom,” says Brymer. “They get a strong sense of living life to its fullest as if touching their full potential.”

Brymer’s comments mirror what Maslow, the founder of humanistic psychology, said back in the 1940s.

We’re not advocating for taking stupid chances (such as diving without proper training, or necessary precautions) and we don’t believe anyone reading this article does that. We simply intended to focus on the scientific evidence that supports adventurers—people who get a thrill from an activity that offers—as a bonus; a chance to feel awakened from the mundane and thrust into a world of wonder. 

Risk-takers and sensation- or thrill-seekers chase unique experiences. Often, those experiences bring awareness of important issues or increase essential knowledge about the planet we share. Many people overanalyze and dither when faced with an unfamiliar situation; they shy away from unsettling circumstances. Risk-takers face the unknown and trust themselves to prevail. Learning to scuba dive, for example, pushes people out of their comfort zone, takes them into a realm foreign and mysterious. Diving forces divers to pay complete attention to a task, to focus with laser-like precision in order to conquer misgivings, and to attain a skill that few others have. Confidence comes with accomplishment. Leadership emerges. Fear is overcome. 

Sensation-seekers see potential stressors as challenges to be met rather than threats that might defeat them. With action, resilience develops. High sensation-seekers report lower perceived stress, more positive emotions, and greater life satisfaction. Engaging in extreme activities brings them peace. 

What does it bring you?

Dive Deeper

Bandolier: Risk of dying and sporting activities

National Geographic: What Makes Risk Takers Tempt Fate? Recent research suggests that genetic, environmental, and personal factors can make people take on risky—even potentially fatal—challenges.

Healthday: Taking Risks By Chris Woolston HealthDay Reporter


Pat Jablonski heads up the copy edit team for InDEPTH. She is a blogger, a writer of stories, a retired tutor, English writing teacher, and therapist. She’s a friend, a wife, a proud mother and grandmother. She is also a native of Florida, having spent most of her life in Palm Beach County. She has a B.A. in English from FAU in Boca Raton and an M.S.W. from Barry University in Miami. She learned to swim in the ocean, a place she thinks of as home, but she doesn’t dive.

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