Scuba accident in a swimming pool close to my house

[ijdod]

@runsongas straight thread relies on the o-ring for teh actual seal, so leaking won’t be an issue. Diameter is also slightly different, iirc.

 

[SlugLife]

@runsongas straight thread relies on the o-ring for teh actual seal, so leaking won’t be an issue. Diameter is also slightly different, iirc.

The problem isn't whether or not it leaks, but rather damage caused to the threads or neck of the tank. If the wrong thread size did leak, that would be a good thing, because it would prevent the tank from being brought up to 3000 PSI, where it can act like a potential bomb.

The story referenced by this thread is one such case where forcing the wrong valve into the wrong tank was possible, and resulted in a tank-explosion.

 

[ijdod]

@SlugMug I could have worded that better, my point was it won't leak if you get to the point where the O-ring is in place (and I would hope nobody would try to fill a tank with a valve half unscrewed), so that tell won't happen. I'm very much aware of what this thread is about.

 

[runsongas]

@ijdod m25/NPS doesn't match up as well due to the thread angle mismatch, so it is harder to screw the m25 valve down far enough to get the oring to seal vs the situation with m25 and G3/4

 

[PfcAJ]

According to the advisory, the threads on the tank and valve are supposed to be marked. If you need a quick id there are any number of quick reference gauges available. If a set of threads is suspected of having been damaged from galling etc. There are special thread gauges that can be used to check if they are still in spec.

The point about hand-tightening is the most practical one.

Fingers are very sensitive, and should be able to detect most situations where threads are not engaging properly. Almost all threaded devices should be at least started by hand. If they don't go together with fingertip pressure something is wrong, wrong size, cross-threaded, corroded, etc. Modern tank valves only need to be hand-tight since they have a captured o-ring. The old 1/2"-14 NGT threads are the only semi-common ones that are not like that.

~50ft-lbs is what’s recommended. That’s more than hand tight.

 

[BoltSnap]

We have had at least two similar incidents here in Libya the last several years. One I personally know about and know the people involved, the valve separated from the tank as the divemaster was trying to pull it out from the truck. The valve flew out of the back door hitting a parked car across the parking lot causing extreme damage to the car door. The DM had hand injuries only without any long lasting deformities. He was lucky. The guy who put the wrong valve on the tank admitted that he had to "use force" to tighten the valve on the tank during the final turns as it got difficult to continue turning the valve on the tank. This conversation took place in front of me.

Typical of these type incidents, the valve/tank separation doesn't happen at the first tank fill, in this case it happened after 10 tank fills. The prior incident, the separation happened after 8 fills. Both incidents were M25X2 valve on G3/4 tank.

 

[OMyMyOHellYes]

~50ft-lbs is what’s recommended. That’s more than hand tight.

Yep. That's the spec.

Specs are necessary for some things. Not everything though. A sufficiently hand-tight valve is NOT unscrewing with virtually any operable pressure in the bottle.

OMMOHY

 

[PfcAJ]

Yep. That's the spec.

Specs are necessary for some things. Not everything though. A sufficiently hand-tight valve is NOT unscrewing with virtually any operable pressure in the bottle.

OMMOHY

I don’t disagree, but there’s quite a bit of variance between hand tight and hand tight. Orings can (and do) migrate.

In any case, that won’t be catastrophic.

 

[Bob DBF]

Yep. That's the spec.

Specs are necessary for some things. Not everything though. A sufficiently hand-tight valve is NOT unscrewing with virtually any operable pressure in the bottle.

OMMOHY

It's not a matter of unscrewing. The seal is the metal faces of the tank and valve, with the o-ring completing the seal. Hand tight may not compress the o-ring, leaving a gap between the faces, and the o-ring will eventually extrude. The torque spec insures that happens, although a lower torque will work. The seal is essentially the same as the valve to reg connection on the tank.

 

[Fibonacci]

Good IMCA summary off the DGE site:
https://www.divegearexpress.com/amfile/file/download/file/166/product/6641/