Hypoxia Experiment

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In 1997, members of the U.S. Deep Caving Team were preparing for a ground breaking project at Wakulla Springs. We had shiny new Cis-Lunar Mk5P rebreathers but little formal instruction. We were all test pilots in those days. tWe decided that we wanted to know what hypoxia felt like. Would we be able to detect it in time to bailout? Would we be incapacitated even after bailing out? Were our heads better than the audible and visual alarms that were supposed to protect us?

We set up a simple experiment in a classroom in Hudson, Florida. With an oxygen kit in hand, we intentionally cut off the oxygen supply to the rebreather and let the diver choose when to bail out. What we learned was that you should never try to repeat this experiment. In the face of hypoxia, you may not be able to revive the stricken diver.

We hope you learn that you should follow good protocols to prepare your rebreather including a proper pre-dive checklist. Prevention is the best action you can take. In the event that you experience an odd feeling on a rebreather, immediately bailout to open circuit gas or flush the counterlungs with breathable diluent. With fresh diluent* or open circuit gas you have bought time to figure out how to handle your emergency.

*Please note that CO2 emergencies can ONLY be solved with open circuit bailout. Diluent flushes do not solve CO2 emergencies.

Original footage supplied by Andrew Poole.

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Speaking Up for Safety

By | All Posts, Bahama Blue Holes, Cave Diving, Rebreather Diving, Sidemount Diving, We Are Water, Women Underwater | No Comments

Jill Heinerth, Dec. 10, 2016

Jill Heinerth serves as the documentarian for the Abaco Blue Holes Project. Her hybrid career includes teaching, photo-journalism, motivational speaking, consulting and pretty much anything that keeps her underwater. She is the Explorer in Residence for the Royal Canadian Geographical Society and a member of the inaugural class of the Women Divers Hall of Fame.

Kenny Broad found a hilarious video online about train safety. After a good laugh, we decided to try our hand at sending our own safety message out into the world. Our singing talents are rather weak, but our hearts are in the right place.

Cave and rebreather diving have been called the most dangerous “sports,” and unfortunately we have all lost our fair share of friends to lapses in judgment and poor choices before they got in the water. Years of accident analysis have shown that when divers break our “cardinal rules,” accidents happen.

In the 1980s, cave diving pioneer Sheck Exley determined that most fatalities were caused when at least one of these rules were broken:

  1. Get proper training from a qualified and active cave diving instructor before entering the overhead environment and then dive in conditions which are similar to or better than the environment you trained in.
  2. Run a continuous guideline from the open water so that in the event that visibility is lost, you will have a tactile reference to get you home safely.
  3. Always reserve at least two-thirds of your gas supply for the exit portion of your dive so that you can assist a teammate in the worst case scenario of a catastrophic gas loss.
  4. If diving deep, use an appropriate helium gas mix that leaves your head clear and your mind sharp.
  5. Dive with the appropriate redundant backup gear including at least three sources of light.

After more recent examination of cave and rebreather diving fatalities, we have added several more specific rules that mitigate the risks. These include:

  1. Personally analyze and properly mark the breathing gas in your cylinders.
  2. Complete a proper pre-dive check prior to diving a rebreather.
  3. Pre-breathe your rebreather for five minutes on the surface in a safe place and watch your electronic controls to determine proper function.
  4. Do not jump in the water if your rebreather fails a safety check.
  5. Change oxygen sensors frequently and pro-actively, and do not dive with fewer functioning sensors than the manufacturer’s standard for your unit.

Cave and rebreather training is available globally through training agencies and professional instructors that are good safety role models. If you choose to take on these activities, please seek out an active and experienced instructor who can take you through an industry recognized training program. The NSS-CDS, NACD, GUE, RAID, IANTD, TDI and other agencies can be found online.

So please forgive our lack of pitch and if you want to see a truly professional and humorous safety video, check out this one from our friends in railway safety.

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Sofnolime versus Spherasorb

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Finally some real data exists comparing the relative durations of Sofnolime 797 versus Spherasorb. Rosemary Lunn of The Underwater Marketing Company posted this to X-Ray Magazine describing recent scientific studies on sorb.

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Tech Diving with Aquanauts

By | All Posts, Grenada, Rebreather Diving | No Comments

Peter Seupel from Aquanauts Grenada prepares his rebreather on the bench of one of his well appointed  Newton dive boats. As he and his partner near completion of their gear, he beckons one of his surface support team to the back of the boat. “Get the rebreather checklist,” he asks the young man. Immediately, I feel comforted, noting that he is a diligent role model when it comes to technical diving. The staff member reads off a series of prompts and each diver double checks critical components. I often get a sense of a diving operation from the very first interaction. When an operator asks for my certification card and walks me through their liability forms, then I know that they care about the details. I know that nobody is exempt from the culture of safety. Seeing the owner including himself in that procedure tells me we are going to have a very good day!

We drop onto the BiancaC and quickly descend to her stern to photograph the large propeller. This is a drift dive, so we don’t need to worry about returning in the rather stiff current. We follow Peter to view a large hole in the side of the hull and masts that now lay on the sea floor. The visibility isn’t stellar today, but the BiancaC never disappoints. Her hull is a mosaic of color and texture and you could certainly spend hours just sitting in one place.

Our second dive of the day is on another wreck, the Shakem. We’ve heard that many divers find this to be their favorite site and we’re glad to have a bit more time on this shallower wreck.  Schools of butterfly fish and creole wrasse trickle down like rain and I am pursued by a small school that has found an affinity with my feet. The top of the wreck is covered with a red coral that is sprouting white blossoms that look like snow flakes. The intact wreck is in view from stem to stern. Is this visibility is lower than usual, I can’t imagine a perfect day!

When we return to the boat, Joe is sporting the biggest grin of the week. A wreck diver at heart, he gets excited about getting a little rust on his suit.

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Shearwater Petrel Calibration Tip

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Calibration Errors on a Shearwater Petrel

Q. I’m trying to calibrate my unit. The HUD shows 1.0 with green and yellow rows of lights. I have fully flushed with oxygen. The handset reads .97, .98 and 1.00 already. The millivolts on the first screen look right and are in range. What is it failing calibration? Looking at the failure screen the millivolts are off the charts, each reading over 200? What’s going on?

A. Check your System Settings. Ensure the Calibration O2 is the correct value. After a battery failure, numerous parameters in your computer such as “units” reset. Your cal gas might be registered at 0, rather than .98. Check that value. Once you reset it, the calibration will likely pass.

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More Tips for Traveling with Rebreathers

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Mask, fins, rebreather and you are ready to go, right? There are many small items that you might not think about that can ruin a trip in their absence.


Inquire about the local power supply. As an example, Mexico has the same current as the US, but many wall outlets do not have grounding plugs. You will need to purchase “cheaters” that convert your three-prong plugs to two-prong. Power strips are worth their weight in gold since wall outlets may be few and far between. In Europe, recessed wall plugs may not fit the fancy converter you bought at the airport. Those converters also often lack a hole for the grounding plug, making them impossible to use without the “cheaters” mentioned above. Many chargers are already rated for 120 and 240v power supplies. Do not waste converters on these devices as they are not needed.


Disinfectant may be challenging to take to your destination. Steramine tablets are very convenient for travel since they are dry and will not spill in luggage or take up precious weight allowance. If you have forgotten to pack Steramine, Betadine is relatively easy to find in foreign pharmacies (though a few people are allergic to Betadine). Dilute it in a tub to soak your breathing loop. If you are camping on an expedition, use one of your shipping cases as a bath. If you cannot find Betadine, Listerine will get your through a trip with reasonable cleanliness, but it does not kill all bacteria. Use proper disinfectants as soon as available.


You can never give enough credit to duct tape. It can repair an awful lot of damage in a difficult situation. I always carry some on trips, but rather than taking a heavy roll, I just wind a few yards around a business card for a small supply. A product called “Rescue Tape” is also fantastic. It is made of clear silicone self-binding tape. It does not have a sticky backing, but instead, bonds to itself in mere moments. It is extremely useful.


Clear plastic Ziplock bags allow you to neatly separate and pack gear so that it does not get lost during inspections. If you simply roll your DSV in your pajamas, the screener is likely to pick up the garment and send the sensitive device crashing to the floor in pieces. Slide delicate items into dive boots and fin pockets. Tape them in place or put them in a Ziplock bags if necessary.


Always make a photocopy of your critical personal identification like your passport and carry it separately in case of loss. Consulates can easily assist if you have back-up documents. Also hand-carry documentation about your rebreather and MSDS sheets for sorb or any other questionable items. Be prepared to share this information with screeners.


Place a friendly letter in your rebreather case that describes the device. Indicate on your letter that all parts are “safe for airline transport on passenger aircraft.” Invite the TSA screeners to contact you via cell phone and even offer up your seat assignment if they need it. Leave a photo of yourself wearing your CCR with a big smile in the case. It helps them to quickly understand what they are looking and let’s them know you are trying to be safe and compliant.

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Travel by Boat of Car with Your CCR

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When using a car and boat, your dive destination may be a lot closer to home, but there are still a few special tips to consider.

  • Bring extra bungee cords for boat travel. Many boats are specifically constructed for single tank divers. The benches may be awkward for a rebreather diver. Bring your own method of securing the rebreather to the bench or the floor.
  • Consider the orientation of your rebreather scrubber. If your rebreather is lying on its back, vibrating on a moving boat or in a car for hours, will your particular canister design be subjected to drastic settling? Some canister designs are equipped with springs that help to resolve settling issues, but if the orientation of the rebreather in relation to the settling forces is incorrect, you could get some channeling of material.
  • Fully assemble and check your rebreather before leaving the dock. You don’t want to be packing sorb on a bouncing boat and also don’t want to capture diesel fumes within the breathing loop.
  • Beware of extreme heat. If you gear is sitting sealed in a hot car for a long period of time, you can damage the oxygen sensors.
  • Beware of extreme cold. If your packed rebreather sits in a freezing car overnight, the moisture in the sorb can freeze causing damage to the canister and dusting in the material itself.
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New Video for Hollis PRISM2 Rebreather Shearwater HUD

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Jill Heinerth has created a free new video resource for users of the Hollis PRISM2 Rebreather Shearwater Heads Up Display (HUD). View here on YouTube.

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Rebreather Workshop in Dallas – January 24

By | All Posts, Cave Diving, Rebreather Diving, Underwater Photo and Video, Women Underwater | No Comments

Rebreather Fundamentals Workshop – Saturday January 24 from 8am – noon_JEH3825l

at Our World Underwater – Dallas

Classroom session: $75

Tickets available online at:


(space is limited)

Enjoy a half-day academic session with CCR expert Jill Heinerth. Using fascinating stories and a multimedia presentation, Jill will cover all the essentials of rebreather diving.

If you are looking to purchase a rebreather, this is an opportunity to learn how to be a wise consumer and make the choice that is right for you.

If you already own a rebreather and want a review of academics, new trends and the future of CCR diving, then this seminar will offer an opportunity to gather new information.

If you want to prevent yourself from joining the ranks of 20 or so people who perish each year on rebreathers, this workshop will inform you about accident analysis and prevention.

If you want a half day in a small group with an expert who will carefully answer your questions and help you find solutions, then this is your best opportunity to spend time with Jill.

If you want to see how rebreathers are being used in exciting expeditions around the world, this full-day road show will entertain and motivate you to participate in CCR diving.

The workshop is non-denominational… all training agencies and all manufacturers are welcome. All levels of divers are welcome from recreational through technical divers.


Rebreather operations and procedures

Basics, types, operation and essential care

Selecting a rebreather: comparing apples and oranges

Selecting an instructor

Understanding CE testing

Myths and misconceptions about rebreather diving

Carbon dioxide: dangerous and ignored

Reducing oxygen issues

Planning for the future: deep mixed-gas operations

Accident analysis and prevention: beating the complacency trap

Traveling with rebreathers: tips and warnings

Lots of time for questions!

Mixing Rebreathers with Open Circuit

By | All Posts, Cave Diving, Rebreather Diving, Women Underwater | No Comments

Madison Blue SpringWhen CCR divers and open circuit (OC) divers dive together, they are referred to as a mixed team. OC divers are often shy about asking a CCR diver how procedures may differ so it is up to you to ensure that gas management and emergency procedures are clear prior to entering the water.

1. Orientation

Show how the rebreather is donned and how it can be removed. Demonstrate how the wing is inflated and, if it is attached to an onboard cylinder, discuss how this limited supply could be easily exhausted during a rescue. Determine whether oral inflation of the wing by the buddy is possible. Discuss various warning lights, especially those that indicate life-threatening oxygen levels and discuss how developing problems can be recognized. Describe the significance of a vibrating mouthpiece, if applicable. Describe how and when it might be necessary to close the loop and why preventing a loop flood is critical for buoyancy.

2. Gas Planning

Inquire about the air consumption rate of the OC diver and plan appropriate gas volumes to ensure safe ascent using your bailout gas. Select bailout gas that is compatible with the OC diver’s decompression plans and plan decompression gases to accommodate all emergency scenarios.

3. Complete a Safety Drill

Describe what to look for during the bubble check, then rehearse gas-sharing scenarios prior to entering the cave. Determine whether sharing a long hose or passing off a bailout bottle will be a better decision.


If CCR divers strive to maintain a high level of conservatism and independence with their bailout gas, then safety and flexibility are benefited. Self-rescue is assured and buddy rescue of a CCR or OC diver is also probable. The goal of the orientation is not to teach the OC diver how to run a rebreather, only how to share gas and handle emergencies that may occur.

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