Airgen adds the Airgen Vent object which when turned on creates a breathable 'air' liquid, and the Airgen Tech which allows you to mostly negate the usual physics of said liquid.
The Airgen Vent:
This is the easy part. It's a vent you can place in the background which is interactable. When on it creates 'air' until the level of the 'air' reaches the vent.
It's crafted anywhere from 10 iron bars, or printable. The recipe should be known to all characters (it's merged into tier 1).
What the vent makes. It's an almost colourless breathable liquid (coloured mainly to help you keep track of it). It flows pretty slowly sideways to help slow down leakage when you open a door. It disappears if it flows where there's no background tiles (which should help keep things from getting too messy). It's otherwise a normal liquid, unfortunately, since there's only so much you can do with the liquids system as it currently stands. So it also muffles sound and 'disassembles' hydrophobic objects (like torches).
The Airgen Tech:
A special always-on (well... when you have it enabled in your inventory tech selector thing) tech that mostly negates the usual liquid physics of the 'air'. You still have the swim animation when jumping and reaching the surface of the 'air' will let you jump out of it, but otherwise movement is fairly close to normal.
It's crafted anywhere from 5 iron bars. The recipe should be known to all characters.
A Screenshot... of air... there's not much to see:
Q: Hey, someone already made this! Why'd you make this?
A: Two reasons really. First, I didn't actually look all that hard to see if it already existed because secondly, I kinda just wanted to make it myself to see if I could.
Q: Why does the 'air' have usual liquid physics? Why do I have to use a tech to fix it?
A: Unfortunately, the liquids system currently seems rather limited in how much you can alter things (or maybe I didn't see how to alter more... I'm not much of a programmer). Making the liquid breathable is a toggle in the liquid itself. But altering the physics on player movement requires scripting. That scripting already exists of course; tech alters your movement in a bunch of different ways. Unfortunately this means you have to have a tech enabled all the time you want the physics to be altered.
WARNING: Will almost certainly be incompatible with any other mod that uses custom liquids. I tried giving the 'air' a non-sequential liquidID, but the game didn't like that so I assume every mod with a liquid will be forced to use ID 8.