Gag Strap Setup Options?

[BlueTrin]

I'd have to search for it on scubaboard

I can pay in Likes, have an advance payment

 

[Underwater Tourist]

Do you mind sharing this data?

There was a case of a rebreather diver who survived because of a gag strap mentioned here:
But if you want a more detailer discussion, go thru this thread (from page 6):

Oxygen poisoning seizure > how to react ?

Hello all, we had an accident recently in the local community ; from oxygen poisoning. The diver was instructor, recognized as experienced, and had at least one experience buddy with him. They were diving a multi stage deep dive (80msw +). Looking at this, we're trying with some friend to...

scubaboard.com

 

[Dr Simon Mitchell]

Why not a simple regulator necklace bungee as the ones used for short hoses in OC?

View attachment 741353

Hello,

Unfortunately, it is not as simple as that.

The Drager, Narked at 90, Revo straps have subtle design features that almost certainly contribute to their effectiveness. Most importantly, there is a flange attached to the mouthpiece, and the strap is attached to the corners of the flange (not the mouthpiece per se) at a slight upward angle as Nick Butcher mentioned. The strap is springy silicone. This has the effect not only of pulling the mouthpiece into the mouth, but also 'cramming the lips' around the mouthpiece making less likely that there will be inward leakage if the diver becomes unconscious. This is where the homemade design above fails, and I would say the same for the original AP strap (they have released a modified version now) and the Apoc one that Brad Horn is promoting which lack the flange.

Do you mind sharing this data?

As you might imagine, the evidence is circumstantial. Human testing on unconscious divers will never be done. The closest we have come to that is 12 boys and their soccer coach with full face masks in the Thailand cave rescue. However, there is a paper from a very prominent French group who reported 54 loss of consciousness events in French Navy divers all wearing a Drager gag strap. Only 3 drowned. Based on a fair bit of experience / knowledge of loss of consciousness events underwater, I would have expected a higher proportion of drownings than that. There were other mitigating factors (e.g. highly trained navy diver buddies in most cases) but even so, I still think this constitutes a strong signal that the gag straps were effective. I have attached the paper.

I have also attached a paper by Paul Haynes, ex British special forces diver, who makes a strong argument for using mouthpiece retaining straps. Figure 5 in that paper shows the sort of gag strap I am talking about, but ironically, I think the user has installed it upside down. The strap is coming off the flange at a slight downward angle which is wrong! The picture below shows me wearing one properly.

Simon M

 

[mr_v]

I've been diving with a strap and had a chance to try a CCR w/o one. I can't believe how much comfort my strap (rEvo) offers. On a recent 90 minute dive I did not clinch my jaw muscles at all. My mouth was completely relaxed and there was zero water intake. It was an experiment, as usually I apply some pressure. Granted, I was conscious, but I could see how a strap could help an unconscious person.

When a tried a unit w/o a strap, I had to apply more jaw muscle pressure to stay comfortable. If you have tension headaches, perhaps a strap could help there, too.

 

[Brad_Horn]

The Drager, Narked at 90, Revo straps have subtle design features that almost certainly contribute to their effectiveness. Most importantly, there is a flange attached to the mouthpiece, and the strap is attached to the corners of the flange (not the mouthpiece per se) at a slight upward angle as Nick Butcher mentioned. The strap is springy silicone. This has the effect not only of pulling the mouthpiece into the mouth, but also 'cramming the lips' around the mouthpiece making less likely that there will be inward leakage if the diver becomes unconscious. This is where the homemade design above fails, and I would say the same for the original AP strap (they have released a modified version now) and the Apoc one that Brad Horn is promoting which lack the flange.

However, there is a paper from a very prominent French group who reported 54 loss of consciousness events in French Navy divers all wearing a Drager gag strap. Only 3 drowned. Based on a fair bit of experience / knowledge of loss of consciousness events underwater, I would have expected a higher proportion of drownings than that. There were other mitigating factors (e.g. highly trained navy diver buddies in most cases) but even so, I still think this constitutes a strong signal that the gag straps were effective.

G’Day Simon,

EN14143 has required a gag/crown strap/MRS/faceplate retainer to prevent the diver flooding when unconscious since 2003 on all rebreathers sold or manufactured in Europe. The Open Safety Apoc was simply the first CE certified rebreather to pass audit to the ratified standard and AFAIK the ALVBOV*** remains the only BOV sold with this critical piece of safety equipment as standard. As well as remaining the lowest WOB BOV in both OC and CC modes available on the market.

On the Apoc lip seal or flange as you refer to it. You have to look at the design holistically and not component parts in isolation. You would have noted from your examination of the Apoc, and specifically its unique integrated crown strap for this discussion, that the mouthpiece on the ALVBOV is designed to work with the jaws fully relaxed in normal operation; it is quite different to normal OC SCUBA mouthpieces, with a much thicker bite and internal lip seal or flange. This mouthpiece is an outcome of both its Functional Safety and lowest Work of Breathing design.

Interestingly, while logic suggests it when diving with a normal mouthpiece, there is no actual ‘evidence' that I’m aware of that an external lip seal or “flange" is actually required for a crown strap on a rebreather to be effective and prevent an unconscious diver drowning. Though comparing all 'CE certified' rebreathers against EN14143 and any reports of drowning might be evidence enough. And the origins of how and why Drager designed theirs exactly as they did is probably lost in even their own archives. Replications of this simply being copies without specific design review. But more than happy to be corrected on this.

***ALVBOV - Auto Loop Volume BailOut Valve. The Diluent MAV, ADV and BOV all being integrated in the one assembly with single 2nd stage faceplate and purge button integrating all three functions with only one LP hose required. Separate necklace triggers OC bailout if the ALVBOV is removed from the divers mouth; automatically closing the loop.

Minor point of correction regarding that French paper but you'll probably find that it was an AquaLung Gag Strap that saved those French Navy divers lives. There are some small but subtle differences in its design compared to the Drager Gag Strap off the LAR.

 

[Dr Simon Mitchell]

On the Apoc lip seal or flange as you refer to it. You have to look at the design holistically and not component parts in isolation. You would have noted from your examination of the Apoc, and specifically its unique integrated crown strap for this discussion, that the mouthpiece on the ALVBOV is designed to work with the jaws fully relaxed in normal operation; it is quite different to normal OC SCUBA mouthpieces, with a much thicker bite and internal lip seal or flange. This mouthpiece is an outcome of both its Functional Safety and lowest Work of Breathing design.

Hello Brad,

I certainly am looking at it holistically, and that is why I think your design is not optimal. In the picture you provided in your earlier post, I see what probably is a thicker bite. This could conceivably have a role in improving work of breathing at the human-machine interface, but I can't think of a reason why it would have any role in preventing loss of the mouthpiece, or preventing it from leaking in a loss of consciousness situation. The "internal lip seal" you describe seems more imaginary than real to me. The mouthpiece in the picture does not appear to have any more of an internal lip seal or flange than most other mouthpieces around. That's my take. People can look at the picture and judge for themselves.

Interestingly, while logic suggests it when diving with a normal mouthpiece, there is no actual ‘evidence' that I’m aware of that an external lip seal or “flange" is actually required for a crown strap on a rebreather to be effective and prevent an unconscious diver drowning.

Well, the French navy paper cited in this thread specifies that their divers were using a mouthpiece retainer with a "lip guard". Moreover, the likely enhancement of efficacy by a lip flange passes the face-validity test for any unbiased observer (which probably doesn't include a manufacturer whose device doesn't have one). Obviously we are never going to have definitive data that explicitly teases out the role of a lip flange.

Minor point of correction regarding that French paper but you'll probably find that it was an AquaLung Gag Strap that saved those French Navy divers lives. There are some small but subtle differences in its design compared to the Drager Gag Strap off the LAR.

Thanks. The paper states: "...using a strap to hold the mouthpiece in position, along with a lip guard...". If its not the Drager one, it must have been something very close to it.

Simon M