[barbegal]

From an engineering perspective these sorts of acrylic tanks are a nightmare to ensure they are safe. The seams are impossible to inspect rigorously so the only real way of checking that they are safe is to load them with an excess pressure ( https://en.wikipedia.org/wiki/Hydrostatic_test ). This can't be done with a tank of this size so the engineers can only real fall back on giving the tank excessive safety margins. But those safety margins cost hundreds of thousands of dollars for a tank this big so maybe they were reduced.

All it takes is one incorrect application of a seam bond and a small crack will form. The crack may take decades to propagate with no way of detecting it. When the seam opens up a bit the crack will then propagate the entire length of the seam extremely rapidly and the whole tank will fail. Ideally failures would be slow and predictable but this is impossible with acrylic.

These sorts of failures have occurred before https://www.plasticstoday.com/materials/when-acrylic-aquariu... and will keep occurring as long as this type of construction is used.

[weinzierl]

I just saw a documentary (in German) from 2003 about its construction. There is an interview with a guy from the American (Colorado) company which built the tank who boasted that only two companies in the world could build it and the other one declined because they deemed it impossible. Other interesting facts were:

- 15 segments of acrylic glass

- 12 segments for the outer cylinder, 3 for the inner

- all segments worth 4 million EUR

- 200 wall thickness

- shipped in a steel construction

They did not say it, but from the video it looks like the segments were assembled in at the destination but not in their final location.

EDIT: I misunderstood, looks like the outer cylinder was assembled in-place, the inner one on-site and then lifted inside.

For completeness, here is the video, but it is in German:

https://m.youtube.com/watch?v=E3KjGJ8AZzI

[petre]

Great engineering feat but it was a dumb idea to begin with. Just like sending people to Mars. Luckily nobody died, excluding the fish of course.

[steelbrain]

This is tangent from the topic at hand but I'm curious to pick your brain. Why is it a dumb idea to send people to Mars?

[petre]

For the same reason it was a dumb idea to put a dog in orbit without the means to return it back to Earth alive.

[pantalaimon]

The thing was just recently reopened after revision, here is an interview after the overhaul: https://www.domaquaree.de/de/Modernisierung-AquaDom.html

Interesting bit:

> It got a little tricky when taping off the glass envelope with foil to prevent possible cracks. Acrylic glass is a sensitive material that absorbs water over time. If it then dries out too quickly, cracks can appear even in the glass walls, which are up to 20 centimeters thick.

[bombcar]

Is there no way to detect cracks or is it just prohibitively expensive to do so?

I know that metal fatigue is a known thing that airlines check for and when they went to carbon fiber they had to develop tools to detect fatigue/cracks.

[mardifoufs]

From my limited knowledge[0], it does not seem like this would be an easy structure to scan for inspection. Acrylic can even used for the wedges that interface between the phased array (or single element) probe [1] because it's very good at not interfering with ultrasounds. Water is also a very good "coupling" material, which makes it even harder to get a useful scan since there would be no "backwall" reflecting back the ultrasounds on the other side of the acrylic.

There are other ways to scan for defects, but I'm mostly familiar with ultrasound phased arrays. Other NDT methods are also hard to use in the field after manufacturing.

[0] I work in NDT (non destructive testing), but I'm not a physics engineer and (work further up the stack from the actual probes/scanners). So this is an approximation, and I might be wrong.

[1] This random link is pretty useful to get an idea I think, but I'll also ask around our inhouse scientists if I get the occasion today!

https://www.olympus-ims.com/en/ndt-tutorials/transducers/pha...

[bombcar]

So it sounds like you'd need something on both sides of the "glass" kind of like this aquarium magnet but much larger/complicated: https://www.amazon.com/JRing-Aquarium-Cleaner-Aquariums-Clea... and slowly move it around the whole tank, repeatedly scanning as it goes.

So I'm going to put it in the "theoretically possible but nobody will do it" category for now. ;)

[jacquesm]

It's a much harder problem than that. Even if you did not find the crack just before taking the structure into use a crack could develop at any point in time after that and you're definitely not going to be emptying this tank on a regular basis for inspection.

So you either detect it before commissioning or it will eventually fail if there is a flaw.

[chmod775]

They had divers regularly going inside to clean it anyways. Doesn't seem like such an unlikely thing.

[jws]

Acrylic and water have virtually the same index of refraction, so if the crack forms on the wet side of the wall and wicks in water you will not detect it visually.

It forms the basis of some magic tricks involving walking on or supporting things in water. (I say some, the rest are totally magic and you should enjoy them.)

[pvaldes]

I just realized that water can be safely colored

[mc32]

Also, they focus on aesthetics.

They could have steel bands providing reinforcement which would be just slightly obtrusive at some angles, but boy, now it would have bands and well, it's not an uninterrupted glass [acrylic] cylinder.

[usefulcat]

The article specifically says that this particular aquarium was made of glass?

[gnabgib]

The article does, but "the AquaDom was a 25-metre-tall (82 ft) cylindrical acrylic glass aquarium"[0] Turns out that's just poor reporting, they could have been more specific. It was the largest (by volume) acrylic cylindrical aquarium in the world.

[0]: https://en.wikipedia.org/wiki/AquaDom

[IndySun]

Thank you barbegal. Given the height and water volume in this instance, do you consider the people involved took too huge a risk? I do believe acrylic is used in aquaparks and zoo aquatic pools. I skimmed instances of acrylic/water fails. Dare I ask, is acrylic construction a significantly cheaper option to solid alternatives?

[barbegal]

There are no real alternatives. Glass has mostly the same bonding issues but is more expensive and heavier. Putting acrylic or glass panels in a steel frame dramatically reduces the probability of failure but you lose the wow factor of having a single sheet of transparent material.

I don't think any installation of this size can be truly safe. It has to be constructed in-situ being too large to transport so you would have to come up with some way of pressure testing the tank in-situ.

[lqet]

Would it be possible to construct a seamless acrylic tube, maybe on-site?

[barbegal]

Not of this size. Certainly single pieces are used in other applications where safety is even more critical such as submersibles. But manufacturing a tube this big would be impossible.

[plastiquebeech]

Impossible to manufacture? Or financially impossible to build a factory for the purpose and larger infrastructure to accommodate transportation?

I'm curious if there is something about acrylics which makes it impossible to make very large single pieces.

[barbegal]

I guess you could transport it at immense cost. But you'd also need a machine capable of moulding or extruding it this big. Current maximum for plastic pipes is about 3.5m diameter https://www.agru.at/en/applications/agruline/grand-opening-r... so you'd need something which is capable of more than 3 times the diameter.

[CoastalCoder]

Could one pour liquid acrylic into a mold (like with a concrete foundation), rather than extruding it?

[yencabulator]

You're making me think of this industrial scale 3D printer: https://www.youtube.com/watch?v=vL2KoMNzGTo

[tdeck]

That's cool but it's all seams with 3d printing, so it seems like that would make the problem worse.

By the way, here's a big cool 3D printer that prints opaque acrylic: https://m.youtube.com/watch?v=b3CY3P28qyQ

[matheusmoreira]

What's the best material for a tank like this, if not acrylic? Normal glass? Not educated on this subject.

[BatFastard]

Ideally it would be extruded diamond, but that technology will not be available until early in the 26th century. Second would be clear aluminum, but again that will not be available for another 150 years.

[gspetr]

Just one question: Why did you give my homie Stephen Hawking the cold shoulder when he hosted a party[0] for time travelers in 2009?

[0] https://www.atlasobscura.com/articles/stephen-hawking-time-t...

[marcosdumay]

The entire problem is that acrylic is not plastic enough, so it fails catastrophically. Well, diamond has all the nice features for structure building, but it's even britter than acrylic. So your sci-fi diamond tank would fail in an even more sensational way.

[Reason077]

We have clear aluminium already in the form of sapphire glass (Al₂O₃). Presumably you're referring to a means of manufacturing it at a scale suitable for, say, an aquarium?

[4ad]

It's a Star Trek joke: https://www.youtube.com/watch?v=Wv5FGCeIP8I

[Tossrock]

Polycarbonate is stronger and more shatter-resistant (it's used for fighter jet canopies, among other things) but also substantially more expensive.

[iancmceachern]

Acrylic is the best material for this application we currently have. If we had a better material, we would use it.

[jvandonsel]

Transparent aluminum

[pvaldes]

maybe we could focus in shape also, not only material.

[nuc1e0n]

They should've made this tank with transparent aluminum.