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Dive Report: 16/04/98

Hypercapnia incident report.

Thursday. April 16, 1998.
A wreck called the Koputai. A paddle wheeler off Bondi Beach, Sydney, Australia. In 78 msw (257 ft)

Calm sea with no current. 3 fully closed rebreathers divers, Scott Leimroth, Rodney Nairne and myself (Suzie Dudas). Took 6 minutes to descend keeping set point at 1.0 with Heliox. When I reached bottom I Ok’ed both buddys and we started the dive. Within a minute of reaching the bottom my head was realing and work of breathing was immense. Seemed impossible to reach Rodney only 5 ft away to give him a thumbs up. On the line I switched off my mouth piece and was handed off an open circuit reg by Rodney. With a 20 minute ascent Rodney controlled my buoyancy and vented my breather while I was in a semi-conscious state. Half way up the ascent (about 10 minutes later and at 90 feet) I was shaken awake and was able to control my bouyancy, breath off my own bail out and get out of the water on my own.

Cause of problem was inhalation one way check valve stuck part way open. It was unseated during over zealous rinsing of the loop, the high water flow against the valve forcing it through part of its retaining cage.

Unfortunately pre-dive checks failed to show up the problem. A noise test of breathing in and out to hear both check valves slap passed because one half of the check valve still slapped. (This is a known problem, alluded to in the last issue of the AARG Journal) In addition we crimp both hoses and vacuum and pressure test the valves; this test was unsuccessful for revealing the problem due to my particular hoses being impossible to crimp effectively. Lastly, 20 mins of pre-breathing the unit failed to give any warning or indication as I assume I was not working hard enough. (The unit was also breathed for 10 mins in the boat after the dive by Rodney however the failure was not detectable and only became clear on visual inspection of the mouthpiece). Subsequently I recommend a visual check which I will be doing from now on. (a small dentists mirror is handy for this).

I can only assume that I was building up CO2 for the entire 6 min. descent while not feeling it because my diluent add is upstream of the mouth piece and I was receiving fresh gas about every second breath. When I reached the bottom I *didn’t* have a gradual build up (such as in canister break through) which is the only reason I was unable to determine the problem. Normally any CO2 buildup in the loop is handled with a flush-thru and if necessary a switch to open circuit.

It is interesting to note that even upon switching to open circuit I felt no better and it took a further 10 minutes for me to regain control. I required assistance from my buddy during this time. I recommend a good buddy who knows your rig, sticks close and knows how to help with potential problems on a fully closed rebreather.

A succesful dive with no problems to 64 msw (215 ft) the following day with the same mix, and canister fill confirmed the cause of the incident.

Suzie Dudas

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