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Rebreather PO2 Creep

Aug 12
Closed Circuit Rebreather units lined up for a class

Closed Circuit Rebreather (CCR) units lined up for a training session.


A NOAA Diver using a closed circuit rebreather (CCR) recently experienced a persistent creep in their PO2 while diving and, after trying a diluent flush, decided to bail out to open circuit as a precaution and end the dive. Although the problem could not be recreated later, an examination of the onboard oxygen monitoring log indicated a slow leak into the loop was probably caused by the oxygen manual addition valve. So, make sure these are tight and working well during the unit’s annual service!

Any time a rebreather diver experiences questionable conditions with a unit, it is advisable to bail out to open circuit and return to the surface to diagnose the problem. Your safety comes first!


Inspiration rebreather manual addition valve.

Dive Plan

Fish survey no decompression CCR dive to 60 feet between 45-60 minutes duration.


PO2 of rebreather “creeped” from 1.3 to 1.5 twice. After second occurrence, diver switched to open circuit and made a controlled direct ascent to the surface with buddy.


At a depth of 58 feet and at about 30 minutes into the dive, diver 1 finished their survey and was waiting for diver 2 to finish their survey. Diver 1 had added oxygen to breathing loop via manual addition. A few minutes later diver 1 noticed the PO2 in the loop had crept from 1.3 to 1.5. Diver 1 switched to bailout valve (BOV) on mouthpiece and conducted successful a diluent flush of the loop. Diver 2 noticing diver 1 was on open circuit (OC) approached diver 1 and asked what the problem was. Diver 1 indicated a PO2 increase in the loop but that the diluent flush had brought it back down. Diver 2 asked diver 1 if they should finish the survey and diver 1 signaled an okay. Two to three minutes later diver 1 noticed the PO2 had crept to 1.5 and went to their BOV. Diver 2 heard the OC bubbles and returned to diver 1 to assist. Because this was a second occurrence, both divers agreed to end the dive. Diver 1 switched from the loop to their off board open circuit bailout and both made a direct controlled ascent to the surface. On the surface in the boat, the divers were unable to replicate the PO2 increase in the loop. The buddy team dove twice more and the rebreather did not have any further problems.


The rebreather had been serviced in May 2015. The rebreather onboard electronics log was downloaded and it indicated a PO2 spike that was consistent with an O2 leak from the manual addition valve into the exhalation counterlung due to the gentle increase of oxygen within the loop. A solenoid failure would have been catastrophic and immediate in oxygen increase. A bench test of the manual addition valve was unable to replicate the problem, however the actuator button was found to have been backed off its normal position. This allowed for some play in valve stem which possibly allowed oxygen to leak into the loop. Whether the actuator button was not tightened properly after servicing or had backed off due to use is unknown. The actuator button was tightened to its correct position. Other manual addition valves on other rebreathers were also inspected and some were found to have actuator buttons also out of place from normal. These were also corrected and will be part of a periodic inspection.



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Reviewed: May 28, 2019. Contact us with page issues.

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