Aqualung Legend LX First Stage Failure at depth

[Diving Dubai]

Having thought about this, rather than be working as I should..

IF the nut holding the first stage to the yoke became loose, and I mean really loose. When you tighten the yoke screw all this slack would still be taken out. The ACD works by mechanical pressure being applied. As you tighten it, the centre is pressed back and the outer meets with the O ring completing the seal on the tank.

Apart from the yoke not being fitted correctly - not being in the dimple at first - when it did move to the dimple that would be enough to move the ACD back and closing it. However I'm still lost. Even this hypothesis sucks, because I would have though that if that were the case, you'd also lose the seal on the O ring and get a free flow (sealing faces not being tight).

This is really annoying me now. I keep sketching solutions and then realising I've been a fool. This whole ADC is designed to fail safe. Now clearly there has been an issue. but I still don't think it's the mechanical ACD valve.


Sometimes which components you have to accept a No Fault Found. What even you do you can't replicate the fault, and you can't find any apparent cause (and then afterwards the component performs flawlessly) When I worked on Aircraft engines and their associated systems in the military we hated this as you'd spend many hours having to investigate all manner of thing before being able to register a NFF. We preferred to find a malfunctioning component which we could change.


I'm no expert on the internals of 1st stages - but is it possible for lets a say, a piece of grit (for example) cause a problem at depth, but when the pressure is release this grit dislodges and the 1st stage works perfectly? Or just a small mis-match of tolerance between parts - say a new item having a size at the upper tolerance and another mating part at the lower end of it's tolerance?

Again happened to me on Aircraft engines, 2 perfectly serviceable parts would work fine with other components but not with each other.

Not a great help or reassurance I know. Sorry. I'll chat to my friend at the LDS this week when I collect my tanks. He's AL certified, and is considered a technical god by Rec & tech divers as well as the emergency services. He might be able to shed some light (or not)

 

[BoltSnap]

I'll chat to my friend at the LDS this week when I collect my tanks. He's AL certified, and is considered a technical god by Rec & tech divers as well as the emergency services

Are you talking about Stevie?

 

[Diving Dubai]

Are you talking about Stevie?

Yup

 

[Freewillow]

Stevie Wonder

 

[databob]

I guess a dive shop or Aqualung could pretty easily confirm or disprove the theory. Try 3 different tank pressures - 3K, 2K, 1K. Loosen the yoke nut a half turn, pull the first stage body away from the tank, breathe, see if air flows or not. Back the nut another half turn and try again. I had my issue at about 1500 psi. If a loose yoke nut can cause it (not so lose that it causes other obvious issues) then we know. I do not remember the yoke being sloppily loose. It looks like the ACD travel is about 1/8 inch. I think if the yoke were backed off more than that the tank would not have a surface to seal against. Could also be a no cause found - I work in computers and see plenty of those. Thanks again.

 

[mdb]

I guess a dive shop or Aqualung could pretty easily confirm or disprove the theory. Try 3 different tank pressures - 3K, 2K, 1K. Loosen the yoke nut a half turn, pull the first stage body away from the tank, breathe, see if air flows or not. Back the nut another half turn and try again. I had my issue at about 1500 psi. If a loose yoke nut can cause it (not so lose that it causes other obvious issues) then we know. I do not remember the yoke being sloppily loose. It looks like the ACD travel is about 1/8 inch. I think if the yoke were backed off more than that the tank would not have a surface to seal against. Could also be a no cause found - I work in computers and see plenty of those. Thanks again.

You are being very reasonable and informative.

I hope you have an "official" answer/opinion direct from Aqua Lung.

 

[Diving Dubai]

I guess a dive shop or Aqualung could pretty easily confirm or disprove the theory. Try 3 different tank pressures - 3K, 2K, 1K. Loosen the yoke nut a half turn, pull the first stage body away from the tank, breathe, see if air flows or not. Back the nut another half turn and try again. I had my issue at about 1500 psi. If a loose yoke nut can cause it (not so lose that it causes other obvious issues) then we know. I do not remember the yoke being sloppily loose. It looks like the ACD travel is about 1/8 inch. I think if the yoke were backed off more than that the tank would not have a surface to seal against. Could also be a no cause found - I work in computers and see plenty of those. Thanks again.

As I see it tank pressure has no affect, purely because this is mechanical. The ACD is pushed back as you tighten the yoke screw. It's hard to explain in text, but I'll try - before I attempt to make a sketch

Assume the nut holding the 1st stage was loose, almost completely off so you could see a gap between the nut and the yoke?

As you tighten the yoke screw onto the valve The outer part of ACD will come in contact with the O ring but the 1st stage will continue to be pushed forward (away from the valve) until it is stopped by the nut (say 1/4 inch) Tightening the screw more will put pressure on the ACD pushing it backwards. This would be okay and would work. The Screw on a Yoke has a good deal of travel

IF something caused the ACD to become stuck, jammed or bind on it's way back (poor servicing) then this could reduce the opening which with lower tank pressure and depth would have an impact I guess. However. This idea doesn't work. The ACD protrudes past the sealing face of the 1st stage. The ACD is a smaller diameter thn the internal diameter of the o-ring. If the ADC didn't get pushed back, the sealing face of the 1st stage wouldn't press against the O ring and you'd have a leak on the surface.

I have 3 sets of LX supreme regs all with ACD, none have ever had an issue - but I am interested in getting to the bottom of this for my own piece of mind. The more I think about it the more I feel it's something in the 1st stage and nothing to do with the ACD

 

[halocline]

As I see it tank pressure has no affect, purely because this is mechanical.

As I look at the drawing, there's a spring that pushes the ACD "out" meaning away from the body of the regulator. This closes the ACD. If you tighten the yoke screw sufficiently, you push the ACD "in" towards the body until it rests against a shoulder. This opens the ACD, allowing air to flow.

But, consider if you did not tighten the yoke screw sufficiently to seat the ACD flange against the shoulder in the yoke retainer. When you open the tank valve, air pressure would push the ACD "in" against the spring, opening the ACD and allowing air to flow. If nothing else changed, eventually air pressure from the tank would lower to the point where it could no longer compress the spring, and the ACD would push "out" closing off the flow from the tank.

There's a flaw in this scenario, though, and that is that when the air pressure from the tank pushes on the ACD to compress it, presumably it would also push the ACD off the tank o-ring, causing a leak. I suppose there is some very small distance the ACD could travel off the tank valve and still seal against the o-ring, but it's likely in the thousands of an inch range. Still, theoretically, tank pressure could affect the position of the ACD.

 

[Diving Dubai]

. Still, theoretically, tank pressure could affect the position of the ACD.

I thought the very thing. I've even dug out my yoke conversion parts and had a look. The actual moving part of the ACD doesn't sit on the tank O ring, just inside it the non moving part is the sealing face. As we know it doesn't take much to have an audible leak when you pressurise a full cylinder. This is the Fail safe. it the ACD isn't pushed back or provides a restriction in any way there is no seal. If there is a seal the ACD has to have been pushed back clearing the orifice.

I think that the fact that an ACD is fitted is a red herring to the problem. My knowledge of the internal workings and likely failure modes is limited to the ACD.

So if we ignore the ACD - what could the problem have been?

 

[Freewillow]

The whole story makes me think about a "comparable" problem that was related on a french speaking forum. I am not saying that the case is similar. i just want to add this grain of salt.

Here is the account of the incident - in French - A rough translation follows.

CRFI 2016/006 : Panne d’air inattendue
Préambule : Le récit et les recommandations qui en découlent ont pour objectif unique la
prévention des accidents ; il ne s’agit nullement de déterminer des fautes ou des responsabilités.
Récit du déclarant :
Homme 44 ans - MF1 - 800 plongées
Problème de panne d'air dû à un bloc 15l alu contenant beaucoup d'eau pour une raison inconnue.
Malgré le tube plongeur en bon état la quantité d'eau et les déchets sont venus obstruer le filtre de
mon détendeur sûrement quand j'avais la tête en bas.
Comme souvent à l’étranger, je disposais d’un seul premier étage, un détendeur Aqualung legend
en bon état.
Il s'est avéré qu'avec le système ACD1 le filtre d'entrée est très compact et se colmate vite.
Au final vers 50 bars une panne d'air franche a eu lieu ; heureusement avec un bon réflexe j'ai pu
aller vers un binôme et je m'en sors bien (un N1 du groupe moins expérimenté a eu le même
problème et a fait une remontée panique).
A noter que dans ces cas d'obstruction du filtre, la HP est instable et le problème peut être anticipé
car l'aiguille du manomètre fait le yoyo à l'inspiration à partir de 100bar environ (vu à la plongée
suivante avant de démonter le bloc).
Les leçons que j’en tire sont :
- Enseigner qu'un manomètre instable est un signe avant-coureur à surveiller qui peut
indiquer une obstruction du 1er étage avec risque de panne d'air ;
- éviter les causes pouvant mener à avoir de l'eau dans les blocs : gonflage de parachutes de
travaux sous-marins directement avec le bloc jusqu'à le vider par exemple mais je n’ai pas
pu connaitre la raison de la présence d'eau dans le bloc.

TRANSLATION:

CFRI 2016/006. Unexpected and sudden Air loss

Men: 44 years old “OWSI equivalent in French system” 800 dives

Air loss on a 15 L aluminum tank containing a significant amount of water for unknown reason .

Despite a dive tube – small tube inside the tank that prevent water and debris to enter the first stage – some

Deposits obstructed my first stage filter, most likely when my head was below my feet.

I had an well maintained Aqualung Legend as first stage. And I was on vacation.

It is noticeable that with the ACD, the entrance filter is rather small and compact, therefore can be

Sealed quite easily. At 50 bars, at the end of the dive, the air supply stopped sudently. Luckily I could share

Air with a fellow diver. An OW equivalent of N1 in the French system) had the same problem and rushed to

Surface. It is noticeable that before this happens, the pressure gauge needle show unstable pressure when

Inhaling. I could notice this during a second careful dive without dismounting the first stage.

Lessons to learn:

Inspect the pressure gauge in order to prevent and get warnings of a possible first stage obstruction.

Insure that there is no water in a tank.

For thos that want the link to the report, it is here post #112.