Accident Analysis – Near Fatal 5 Minute Pre-breathe

This incident is based on real events; however names have been changed.


  • Instructor – Mark
  • New CCR students – Frank and James
  • CCR crossover student and victim – Joe, experienced CCR diver and instructor diving a new rebreather for the first time

Setting the scene – This accident took place at a very busy beach on a beautiful and calm Sunday.  Air temperature was around 30° C / 86° F, water temperature around 27° C / 81° F, and the beach was crowded with swimmers.

Joe, an experienced CCR diver and instructor familiar with the site was assisting instructor Mark with logistics for the day of diving.   This was the fourth day of the new CCR students’ course but Joe’s first dive on the new rebreather.  It was a very hectic and stressful day prior to the divers gearing up as there were many complicated logistical considerations.

All rebreathers were assembled using the manufacturer’s build checklist in the parking lot at the dive site.  However, because of the extreme air temperature and exposure to the sun, the team decides to conduct their pre-dive checks and pre-breathe standing in shallow water.  Everyone dons their equipment and enters the water in the shallow area of the busy beach.

The accident – Frank and James begin using the TDI Pre-flight checklist to conduct their pre-dive checks and pre-breathe.  Joe remains somewhat distant from the group and conducts his pre-dive checks alone from memory (as an experienced CCR diver he felt a written pre-flight checklist was unnecessary).  It is quickly realized that the team is having difficulty communicating on the busy beach with the curious swimmers interrupting their checks, so they decide to surface swim out to deeper water away from the crowd as they continue their pre-breathe.

Approximately three minutes into their pre-breathe on the surface; Mark began visually checking each student and asking for an OK signal.  At this point, Mark noticed Joe was face down in the water and completely unresponsive.

Mark turns Joe face up in the water, inflates his BCD and removes the DSV from his mouth.   Joe is pale blue in the face and breathing very shallow.  As Mark screams, “Breathe Joe, breathe!” he slowly regains consciousness, his breathing strengthens and normal color begins to return to his face.

Once Joe appears to have recovered, Mark inspects his rebreather.  The PO2 display was reading close to 0.00 for all three oxygen sensors, the counterlungs were almost completely empty and the ADV shutoff was closed.  When checking the cylinder valves, Mark discovers Joe’s oxygen cylinder valve was completely closed.  As soon as the oxygen valve opened, the rebreather began injecting oxygen into the loop and bringing the PO2 back up to setpoint.

Joe fully recovered from the incident without residual effects from hypoxia.

What can we learn from this very near miss?

  1. Pre-dive checks and pre-breathe should always be conducted out of the water.
  2. Every pre-dive check should be conducted utilizing a written or electronic pre-flight checklist.
  3. Pre-dive checks should always be conducted as a team, in a manner in which teammates can verify each step as it is completed.
  4. CCR instructors must maintain control and verify equipment assembly and pre-dive checks with every student, no matter how experienced they are.

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14 replies
  1. Andy Robinson
    Andy Robinson says:

    All I would say is that you should consistently call the checklists “pre dive checklists,” not pre flight checklists. Diving is not flying. Those of us who are both pilots and divers may get confused, start running through our POH, and wonder where the pitot tube cover(s) went…

  2. Terry Ostby
    Terry Ostby says:

    Am a open circuit guy . I do things the same each and everytime I enter the water and before I enter the water . Not criticizing anyone or anything here . But it seems all or at least the ones that end up in print , the accidents involve experienced CCR divers who seemed to take short cuts or got complacent with their system or maybe thought something was no big deal . Complacency has no place in diving .It will kill you . The human is in an alien place when in the water . Just my observation.

    • Carlos Aguilar
      Carlos Aguilar says:

      You are so right, Terry. What happened was hypoxic blackout. That is the reason I won’t touch a rebreather. In this case, the diver was breathing on a closed system while his body was consuming the oxygen. The rebreathers was scrubbing out the CO2 so he wasn’t experiencing CO2 toxicity symptoms or the urge to breathe. There is no urge to breathe as oxygen levels drop. There is no room for error in a rebreather. People make mistakes all the time. It’s a terrible combination.

      • Terry Ostby
        Terry Ostby says:

        Yes Carlos
        I can never say I would never touch a rebreaher as they are an excellent tool . But in my case I have no need . The one thing you have to be is very metticulas in the way you do things. You are right there is no warning something is wrong you just go to sleep. Ea. dive has to be treated as if it is your 1st. dive . I do it on OC as I do a lot of solo dives and it just makes sense . So given the no margin for error on a rebreather it makes even more sense .After all it is your life your gambling on .

  3. Stuart Boreen
    Stuart Boreen says:

    Rule number one is the key. When doing a pre-breathe check in CCR diving, there is a possibility that something is wrong. In other words, you are doing something where you may lose consciousness and you must do it in a safe environment. Interestingly to me, this is the exact opposite of the case where airline security will confiscate your bottle of water and leave it in a trash bin at the screening site. They only do this because they know there is no danger in your bottle. If there was a danger, you would need another way of disposing of the water. Since there really is a danger in CCR diving you can not do your pre-breath in the water!

  4. Renee Power
    Renee Power says:

    Complacency kills. A checklist is great and we need to actually USE them and not have them in the truck. None of us are too good for checklists or completing other safety checks. Choose who you do this activity with carefully. If a person neglects a pre-dive checklist then what else have they ignored??
    I don’t believe rebreathers kill people. (I know there are legit malfunctions) People kill people.
    Complacency is rampant. I choose to be safe. No matter your gear configuration choice, no matter your agency and no matter your training level please be careful out there!!

  5. Patrick Vogler
    Patrick Vogler says:

    This accident only happened because of complacency. Rule number 1 in CCR diving is as everybody should know: “Always know your PO2!”. If he watched his PO2 he would noticed the closed valve after a few seconds, no matter he was in or out of the water. Another reason why I dive a mCCR.

    • Rod Abbotson
      Rod Abbotson says:

      Spot on, Patrick, I too dive an mCCR, for good reasons, once on the loop whether sitting in a dive centre, boat, or on the surface you should be monitoring your ppO2, in fact that’s a main part of the pre- breathe. Personally I get a lot of flack for not using a written checklist, but after over 2000 hrs of CCR diving I have never had a problem due to that. I have had two cell failures at depth a couple of times, that was OK as I didn’t vote myself out as an eCCR would. No monkey on the back for me!

  6. James Weber
    James Weber says:

    When I teach CCR, I teach my students to get a pulse and respiratory rate prior to the prebreathe and another post prebreathe. Since A higher PCO2 is your trigger to breathe, an increased level will have some physiological effects

  7. Romain
    Romain says:

    Ok . Oxygen not opened. One of the most common deadly mistakes. Make your CCR pre dive check SIMPLE, and straight to what is REALLY important:
    Oxygen up and working
    Diluant up and working
    Ppo2 monitoring reliable
    High and low pressure test ok
    All bail out tested
    Pre breath

  8. Tony
    Tony says:

    I don’t get it – how did they calibrate their units? Surely that alone would’ve told him that his O2 was closed?

    Let me guess – they did that when they built their units then turned off the gas? Fine – we do that ourselves when we set the units up the night before but then before we even get anywhere near a prebreathe we run through all of our checks AS A TEAM and the first thing will be another calibrate to be sure…which includes using the O2 MAV to fill the loop.

    Then our TEAM CHECK includes the visual checks as well as the oral confirmation to one another such as O2 is at xxx bar and is open (checking the valve)….Running xx/yy dil at zzz bar and is open (checking again). O2 MAC working, DIL MAV and ADV working. HUD and computers showing the additions of O2 and DIL as you’d expect.

    During our prebreathes we never use our gag straps – thanks but if something goes wrong I don’t want the mouthpiece fixed in my mouth!

    And. We do out checks before every. single. dive.

    My unit doesn’t have a brain so I tend to use my own.

    Experienced on another unit and apparently so arrogant that he didn’t feel the need to run through proper checks as a team.

    Even underwater we were trained to use the mantra buoyancy, team, task, in that order – what’s the point in trying to perform a task if you’re not neutral or a team member is in trouble.

    Once again it strikes me as the arrogance of an “experienced” technical diver got the better of him.

  9. Mike Etheredge
    Mike Etheredge says:

    Never put a rebreather on with the gas and electrics off. And only turn the gas and electrics off when you have taken it off.And Always know your PPO2.

  10. Patrik
    Patrik says:

    Diffrent vendors has diffrent predive checks.
    You can not pass a predivecheck to the stage where you are going to start breathe for 5 min with a poseidon system. It checks that your O2 cell is actually is working by shooting oxygen on the O2 cell to verify its functionality. And it will do so during the entire dive.
    It won’t pass pretest/prebreath with any valve closed. You won’t get to the point where you is told to start breath, by system startup, in the loop.

  11. Ruben Campuzano
    Ruben Campuzano says:

    Very valid and valuable observations!!!… a deep sea diver, a near miss and/or accident is a result from a sequence of events, like a chain link with a broken/missing one.


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