I've not seen one of these yet, but it looks interesting. If you submerge it, does it fill with water through the diffuser plate?

Here's my guess as to how it operates based on look at the photo...

Once the container is full, the bubbles will "pull" water into the spiral as they pass through. It looks like the gas tube ID narrows near that opening at the bottom of the spiral. As the bubbles escape from that narrowing they will accelerate into the spiral going past the opening in the side of the spiral. This will create a lower pressure inside the spiral sucking water into it through that opening. You will then likely get a series of bubble-water-bubble-water-etc... going up the spiral.

This is similar to the concept that Python siphons use. Only in that case it uses water from the faucet as the motive phase rather than CO2 bubbles

It's an interesting concept because the CO2 should dissolve some in the spiral and any that doesn't dissolve should be turned into microbubbles as it passes through the diffuser plate. It also should work as a self contained bubble counter.

The big question is will it fill with water when submerged or if not, how do you get the water in.

Pls let me know how it works once you've got it up and running. I've been meanig to upgrade my diffuser as well and that looks like a cool device...