Cautionary Tales - Quick Disconnects

In my view the most important incidents are ones that can recur and so by discussing them, we can help save other divers from the same fate. I have chosen to recount 3 separate close calls, all caused by the same problem, all happening to close friends. The theme is the use of quick disconnects (QDs). The names have been changed to protect the innocent!

Incident 1

I was diving in a deep cave in Australia with a good friend Bob. Our plan was to explore a new section at over 100mfw so we were both wearing back mounted CCRs with sidemounted bailout cylinders. A shotline from the surface lake had additional bailout cylinders strung on it. Bob had his left bailout connected to his CCR by a Swagelok QC6 QD which was supplying diluent to his bailout valve and the rebreather’s automatic diluent addition valve (ADV) via a manifold.

Unbeknownst to Bob, his rebreather was slowly flooding during the early part of the dive. At 30m, we prepared to enter the first vertical section of the cave. After giving Bob a final OK signal, I headed down the shaft leaving him to follow. Bob rolled head down to follow, and suffered a caustic cocktail as water in the unit came up into this mouth. He immediately went into a head up position and switched his bailout valve over to open circuit. Unfortunately no gas was forthcoming! Back onto the loop to try and clear the flood and get some gas but only caustic water came from the mouthpiece. At this stage the rebreather was getting heavier and Bob started to sink. He grabbed for the regulator on his sidemounted tank but it did not come straight to hand. In a last-ditch effort, he sprinted up to the shot line and grabbed a bailout regulator. By the time I realised Bob was not behind me, the crisis was over and I ascended to find him happily coughing on the staged bailout cylinder above me.

Subsequent inspection of Bob’s equipment found that the QC6 has popped apart, despite initially working for the first part of the dive. A very close call.

Incident 2

Another cave, another good friend. Don and I were exploring a very long Australian cave with large crystal-clear tunnels. We had surfaced in a remote dry chamber, got out of the water for a period of time and were just starting the dive back out of the cave. Don was diving a manual sidemount CCR. A side mounted cylinder supplied both the ADV and BOV on the unit via a QC6. We both submerged to about 3m and started to scooter through a lake area towards the tunnel. As Don started to scooter he suddenly found he could not fill the loop from his ADV. As he sank further, he switched his BOV to open circuit to find he had no gas there either. Still sinking, he tried to fill the loop with his oxygen addition but the flow was insufficient to give him enough volume to breathe. I watched this evolve and had deployed a second stage to offer Don. Unfortunately, my helmet light shone in his eyes and he didn’t see it. Instead, close to panic and with no time to grab another second stage, Don aimed his scooter straight up and burst out of the water less than a meter from the overhead part of the tunnel. He was coughing and wheezing for nearly 30 minutes afterwards. Had we been 5m further into the tunnel he may not have survived. The cause? His QC6 QD has separated

Incident 3

Third friend, different cave! Sam was diving a manual sidemount rebreather using QC6 QDs to connect both his diluent (from a side mounted cylinder) and his oxygen (from a small cylinder attached to his CCR). On the surface, Sam successfully tested his oxygen manual add valve (MAV) before climbing down a ladder into the cave. On the surface, he noted his PO2 to be on the low side but decided to descend a few meters to get out of his buddy’s way due to the cramped conditions at the entrance. Once down to 6m, he stopped to recheck his PO2 and add more oxygen. But no oxygen was forthcoming. Sam checked the oxygen cylinder valve which seemed to be on. He put a hand on the QC6 which felt correctly aligned, bungied to the side of the rebreather. Unclear as to why he couldn’t top up his O2, but now looking at a PO2 of 0.19, he decided to ascend the few meters to the surface. He made an active decision not to bailout thinking he would be ok to ascend that short distance. Sam lost consciousness from hypoxia just below the surface and was pulled from the water by his dive buddies. He suffered no ill effects from his very close call. On inspection of his equipment afterwards his oxygen QC6 had separated.


The Swagelok QC6 has become extremely popular for technical diving applications. It is very robust, easy to connect under pressure and entrains only very small amounts of water when connected underwater. It has great flow characteristics when high flow is required at significant depths. I have listed the three most serious of many similar incidents I have witnessed or experienced due to QC6 disconnections. I believe there can be two causes. Firstly, a knock to the QD in just the correct spot can make the connection jump apart. This is easy to demonstrate yourself. Secondly, it is possible to connect the QC6’s, establish flow, and then find that the connection was not “locked” and the connectors subsequently separate. As a team we have developed the “tug test” to ensure the connect is secure. The call of “tug test!” is a common cry at the water just before descent.

It may be time to return to other products on the market that have a locking function, however my experience with the most popular of these is not perfect either. Whatever you use, check and double check, and don’t put all your gas supply eggs in one basket. And remember, all three of these divers are in my view, highly experienced, careful and competent guys. You never quite know how you will respond when you only have seconds to find a solution!


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