How is this done?

[moldrik]

Depends on how the tank is setup, it wouldn't be a big mess.

If you fill up the bottom tank, setup the vertical part, and vacuum the vertical part, you basically guarantee that if it fails, all the water would be sitting in the bottom tank. The water probably wouldn't splash either because of the vacuum that has been setup at the top.

If the vacuum is stable, it technically will hold, since the only way that water will get out of there is by new air coming in. And basically as the new air comes in, the same, small volume of water water would be pushed out.
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CD175