Changing the tank thread

[pisauron]

I’d like to point out that diving with composite cylinders can be a tricky business.

The main issue, as mentioned earlier, is corrosion. To be more specific, galvanic corrosion. Carbon fibres sit very high on the galvanic series, not quite as high as graphite, but still significantly higher than aluminium. It has a standard electrode potential of 0.55V, comparable to copper (Even slightly higher). Aluminium, on the other hand, sits much lower in the series, with a standard electrode potential of -1.67V, making for a terrible coupling. In this setup, carbon fibre acts as the cathode, while the aluminium liner becomes the anode, corroding rapidly if an electrolyte, such as water, connects the two. Even the cylinder valve can be effected, if the epoxy layer is not sound.

For this reason, composite cylinders require a buffer layer between the carbon fibres and the aluminium liner. This is often a glass filament epoxy layer, similar to what you see on the outermost surface. Current standards limit these buffer layers to a maximum of around 15% of the pressure load at minimum burst pressure.

Because of these risks, most manufacturers explicitly prohibit the use of their composite cylinders for diving. The only manufacturer that comes to mind with a purpose-built composite cylinder for diving is Luxfer.

That doesn’t mean they can’t be used for diving, as @rjack321 demonstrates well. But it does mean, that special care and thorough inspection are absolutely essential throughout the cylinders use.

The pictures you’ve shared are interesting because I don’t see an insulating epoxy layer between the carbon fibres. That said, pictures can be notoriously difficult to interpret when taken out of context.

How can we see an epoxy layer beneath the carbon layer and the aluminum? It's usually visible?

 

[rjack321]

How can we see an epoxy layer beneath the carbon layer and the aluminum? It's usually visible?

You can't.

Basically the manufacturer has to coat the aluminum liner shell with epoxy before wrapping it with the carbon fiber. Even with this layer, the aluminum liners want to corrode. This is one of the many reasons why carbon fiber tanks are basically not made for scuba uses.

 

[34109411]

Hello,

I recently found some strange tanks. On first glance it seems to be a cheap Chinese composite tank. We have 3 of them.

But once you try to fit it with a valve you run into problems. And we failed to find a valve for it. It's seems the manufacturer made a mistake.

The tank sticker says M18x1.5? So what! It's sure ain't that. Try M25? Nope. Try 3/4? Nope.

I took it to a shop specialized in bolts and they think it's might be M24...but don't have this in stock.

Suppose it is M24 - is there a valve for it? Never seen one yet. Do you know of any?

But what of I strip the threads and tap a new thread? The closest one seems to be M25x2... Will this make the tank dangerous for use? Do people do that?

Will love to find a solution with your help.

Thanks

why you don't get catalina at your dive shop ?

 

[pisauron]

why you don't get catalina at your dive shop ?

We have everything. I have found these in the basement.
Someone brought them couple of months ago without knowing what they are. Perhaps an airsoft player. But no one actually remembers.
I stumbled upon them and decided to try and solve the mystery - which now seems solved.

 

[Tanks A Lot]

How can we see an epoxy layer beneath the carbon layer and the aluminum? It's usually visible?

I should have been a bit clearer in my earlier statement, as this depends on how the cylinder is wrapped.

Carbon fibres are never wrapped around the neck, so if the cylinder has no paint and a clear epoxy coating, the insulating layer can often be distinguished. This layer is usually extended right up to the edge of the neck, as it’s easier to manufacture this way rather than stopping precisely where the fibres end. It often appears as a small ring on the face of the cylinder, directly around the liner.

The reason this layer is usually visible is that high-quality cylinders use different materials for the insulating layer and the outer shell. Cylinders designed for the SCBA market often have highly durable outer shells to withstand the heavy abuse.

In the pictures you posted, the outer glass filament epoxy shell is visible, though it appears rather thin and in poor condition. The thick layer underneath the shell seems to be plain epoxy filler, but I don’t see the insulating layer extending to the very face of the cylinder.

As someone already mentioned, this insulating layer isn’t always visible, but it often is, especially at the face. On hoop-wrapped cylinders, it’s even easier to spot.

 

[pisauron]

I should have been a bit clearer in my earlier statement, as this depends on how the cylinder is wrapped.

Carbon fibres are never wrapped around the neck, so if the cylinder has no paint and a clear epoxy coating, the insulating layer can often be distinguished. This layer is usually extended right up to the edge of the neck, as it’s easier to manufacture this way rather than stopping precisely where the fibres end. It often appears as a small ring on the face of the cylinder, directly around the liner.
View attachment 889904View attachment 889905

The reason this layer is usually visible is that high-quality cylinders use different materials for the insulating layer and the outer shell. Cylinders designed for the SCBA market often have highly durable outer shells to withstand the heavy abuse.

In the pictures you posted, the outer glass filament epoxy shell is visible, though it appears rather thin and in poor condition. The thick layer underneath the shell seems to be plain epoxy filler, but I don’t see the insulating layer extending to the very face of the cylinder.
View attachment 889906

As someone already mentioned, this insulating layer isn’t always visible, but it often is, especially at the face. On hoop-wrapped cylinders, it’s even easier to spot.

Thank you that clarified it very well.
Learned a lot about composite tanks from this thread.
Thanks

 

[OMyMyOHellYes]

Re-tapping is not a wise choice. I'm not comfortable doing it on "normal" aluminium cylinders and I would never attempt doing it on a composite cylinder. A composite cylinder already has a very thin liner to start with, taking any material off it is not a wise choice.

If you want my advice, call it a day and throw this one out. Whatever you might able to achieve is not worth the considerable risk you are exposing yourself and others to.

^^^^^^ What he said. ^^^^^^

 

[34109411]

We have everything. I have found these in the basement.
Someone brought them couple of months ago without knowing what they are. Perhaps an airsoft player. But no one actually remembers.
I stumbled upon them and decided to try and solve the mystery - which now seems solved.

don't use them.

 

[pisauron]

don't use them.

Why not? I plan to get me some new ones with correct threads and use them in my regulator testing station.

 

[rjack321]

Why not? I plan to get me some new ones with correct threads and use them in my regulator testing station.

Because those Chinese CF cylinders are nearly impossible to get hydrostatic tested and have a limited life span, takes a unique valve with a specific tiny Oring too.

Sure they are super light weight, but if it's just to sit under a workshop bench what's the point? A cheap old low pressure steel tank lasts for decades and you can find one used for cheap.