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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.

Additional Resources: 

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. 


Education

Instructor Burnout Survey: Multi-Languages Below

Help Polish PADI course director Joanna Kannenberg complete her in-depth research into “instructor burn-out.” Instructors, please answer her (multi-language) survey.

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Header image courtesy of Joanna Kannenberg

English

IT CANNOT WORK WITHOUT YOU, dear diving instructors! I’m Joanna Kannenberg and diving is my passion. As a Course Director, I have been training diving instructors worldwide for over 20 years. The current global situation gives me the time for my long-planned doctoral thesis on burnout risks in our professional group. Many only see the sunny side of our work and only we know about the darker side.  

In order to make the largest possible well-substantiated study, I need as many completed surveys as possible! Please answer the questionnaires and also forward the link to your diving instructor friends / employees.  

Thank you for your help! See you under water ☺ https://burnout.webankieta.pl

Español

NO FUNCIONA SIN VOSOTROS, queridos instructores de buceo!

Soy Joanna Kannenberg y bucear es mi pasión. Como Course Director, he dedicado más de 20 años a formar instructores de buceo por todo el mundo. La actual situación mundial me ha dado el tiempo necesario para preparar la tesis doctoral que tenía planeada desde hace mucho tiempo. Mi tesis versa en los riesgos de “quemarse del trabajo” en nuestro ámbito profesional. Muchos solo ven el lado soleado de nuestro trabajo y solo nosotros conocemos el lado más oscuro. 

Para hacer el estudio más amplio y bien fundamentado, necesito  cuántas más encuestas completadas como sea posible. Por favor, responde a las preguntas de la encuesta y envíasela a tus amigo / empleados que sean instructores de buceo. 

¡Gracias por tu ayuda! Nos vemos bajo el agua ☺! 

(Las encuestas están disponibles bajo el enlace en inglés, alemán, ruso, polaco, español y árabe): https://burnout.webankieta.pl/

Deutsch

OHNE DICH GEHT ES NICHT, liebe Tauchlehrer/innen!

Ich bin Joanna Kannenberg und Tauchen ist meine Leidenschaft. Seit über 20 Jahren bilde ich als Course Director Tauchlehrer/innen weltweit aus. Die jetzige globale Situation gibt mir die Zeit für meine schon lange geplante Doktor-Arbeit über Burnout Risiken in unserer Berufsgruppe. Viele sehen nur die sonnige Seite unserer Arbeit und nur wir wissen, dass es auch die andere manchmal gibt. 

Um eine möglichst groß fundierte Studie zu erstellen, brauche ich von euch viele ausgefüllte Umfragen! Bitte beantworte die Fragebögen und leite den Link auch an deine Tauchlehrerfreunde/mitarbeiter. 

Vielen Dank für deine Hilfe! Man sieht sich unter Wasser ☺ 

(die Umfragen gibt es unter dem Link in: Deutsch, Englisch, Russisch, Polnisch, Spanisch und Arabisch): https://burnout.webankieta.pl/

Polski

BEZ WAS TO SIĘ NIE UDA, drodzy instruktorzy nurkowania!  

Nazywam się Joanna Kannenberg i nurkowanie to moja pasja. Jako Course Director od ponad 20 lat szkolę instruktorów nurkowania na całym świecie. Obecna sytuacja globalna dala mi szanse na zajecie się od dawna planowaną rozprawą doktorską na temat ryzyka wypalenia zawodowego w naszej profesji. Wielu ludzi widzi tylko słoneczną stronę naszej pracy i tylko my wiemy, że czasami jest też druga.  

Aby badanie było możliwie jak najbardziej wiarygodne, potrzebuję od Was wielu wypełnionych ankiet! Bardzo proszę o wypełnienie kwestionariusza, a także przekazanie linku znajomym / współpracownikom – instruktorom nurkowania.  

Dziękuję Wam za pomoc! Do zobaczenia pod wodą ☺ 

(ankiety dostępne pod linkiem w językach: polskim, angielskim, niemieckim, rosyjskim, hiszpańskim i arabskim): https://burnout.webankieta.pl/

Pусский

ЭТО НЕ ПОЛУЧИТСЯ БЕЗ ВАС, дорогие инструкторы по дайвингу!

Меня зовут Джоанна Канненберг и уже много лет дайвинг является моей главной страстью. В статусе Курс-Директора я обучаю инструкторов по дайвингу по всему миру более 20 лет. Нынешняя глобальная ситуация дала мне время для моей, давно запланированной докторской диссертации. Объектом исследования являются  риски выгорания в нашей профессиональной группе. Ведь многие видят только светлую сторону нашей работы, и только мы знаем обратную сторону.  

Для того чтобы сделать максимально обоснованное исследование, мне нужно получить как можно больше завершенных опросов!  

Пожалуйста, ответьте на эти анкеты, а также перешлите ссылку своим друзьям / сотрудникам, которые являются инструкторами по дайвингу.  

Сердечно благодарю вас за вашу помощь! Увидимся под водой ☺ 

(опросы доступны по ссылке на английском, немецком, русском, польском, испанском и арабском языках): https://burnout.webankieta.pl/

العربية

! ﻻﯾﻣﻛﻛﻧﻰ اﻟﻌﻣل دﺑوﻧﻛم ,ﻋازاﺋﻰ ﻣدﺑرﻰ اﻟﻐوص ﻻ
ﻧاﺎ ﺟوﻧاﺎ ﻛﺎﻧﯾﻧﺑﯾرج واﻟﻐوص ھو ﺷﻐﻔﻰ ﺑﺻﻔﺗﻰ )Course Director (؛ﻗاوم ﺗﺑدرﯾب ﻣدرﺑﻲ اﻟﻐوص ﻓﻰ ﺟﻣﯾﻊ اﻧﺣﺎء اﻟﻌﺎﻟم ﻻﻛﺛر ﻣن 20 ﺎﻋﻣﺄ .
ﻧﻣﯾﺣﻧﻰ اﻟوﺿﻊ اﻟﻌﺎﻟﻣﻰ اﻟﺣﺎﻟﻰ اﻟوﻗت ﻻطروﺣﮫ اﻟدﺗﻛوراه اﻟﻣﺧطط ﻟﮭﺎ ﻧﻣذ ﻓﺗره وطﯾﻠﮫ ﺣول ﺧﻣﺎطر اﻻرھﺎق اﻟﻣﮭﻧﻰ ﻓﻰ ﻣﺟﻣوﻋﺗﻧﺎ اﻟﻣﮭﻧﯾﮫ.
رﯾى اﻟﻛﺛرﯾون ﻓﻘط اﻟﺟﺎﻧب اﻟﻣﺷرق ﻣن ﻋﻣﻠﻧﺎ .وﻧﻌﻠم ﻧﺣن ﻓﻘط اﻟﺟﺎﻧب اﻟﻣظﻠم 
نﻣ اﺟل اﺟراء اﻛﺑر دراﺳﮫ ﻣﻣﻛﻧﮫ ﻣدﻋوﻣﮫ ﺟﯾدا ,اﺣﺗﺎج اﻟﻰ اﻛﺑر ﻋدد ﻣﻣنﻛ نﻣ

ﻻاﺳﺗطﻼﻋﺎت اﻟﻣﻛﺗﻣﻠﮫ 
رﯾﺟﻰ اﻻﺟﺎﺑﮫ ﻠﻋﻰ اﻻﺳﺗﺑﯾﺎﻧﺎت وﻛذاﻟك ﻋاﺎده ﺗوﺟﯾﮫ اﻟارﺑط اﻟﻰ  ﻣدرﺑﻰ اﻟﻐوص
ﺻادﻗﺎﺋك / وا اﻟﻌﺎﻣﻠﯾن
ﻛﺷرا ﻟك ﻋﻠﻰ ﺳﻣﺎﻋدﺗك ! اراﻛم ﺗﺣت اﻟﻣﺎء

ﻻاﺳﺗطﻼﻋﺎت ﻣﺗﺎﺣﮫ ﺗﺣت اﻟرﺑاط ﺎﺑﻟﻠﻐﺎت اﻻﻧﺟﻠﯾزﯾﮫ  ,اﻻﻟﻣﺎﻧﯾﮫ , اﻟروﺳﯾﮫ
ﻟاﺑوﻟﻧدﯾﮫ , اﻻﺳﺑﺎﻧﯾﮫ  وﻟاﻌرﺑﯾﮫ(, اﻻﺳﺑﺎﻧﯾﮫ  وﻟاﻌرﺑﯾﮫ

https://burnout.webankieta.pl



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