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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