Edit: See my third post in this thread for the near-vacuum setup of this airlock, which is impervious to almost any realistically achievable high or low pressure, and is also very good at thermal insulation.
Sorry if this has been mentioned before - I haven't seen it anywhere and a brief forum search didn't turn anything up either.
I came up with a passive airlock design that keeps different gases perfectly separated without any power use or (to my knowledge) gas deletion. It utilizes the fact that gases separate due to gravity and that two gases can't occupy the same location at the same time.
As you can see the chlorine and oxygen on either side of the ladders will never mix. Spaces 5 and 6 will fluctuate, but unless the pressure difference between the gases is extreme, the chlorine and oxygen will never get up to 3 and risk spilling over into the other room. Furthermore, the hydrogen will never get below 5 or 6 and risk spilling upwards into either room, unless one of the rooms is very low pressure. I chose hydrogen to fill 1-4 because it is the lightest gas in the game, and therefore won't be displaced no matter what gases are being separated by the airlock. Also, the hydrogen is necessary to separate gases that have the same or similar densities such as oxygen and polluted oxygen. In the above example it is also necessary to prevent carbon dioxide from being exhaled into the chlorine room, which would eventually occur if 1-4 were filled with oxygen.
Building this airlock in an actual game is quite simple:
Simply pump hydrogen from an electrolyzer setup into the middle room up to an appropriate pressure, then deconstruct A and B and build ladders in their places.
Advantages of this design:
1. Zero gas bleed across the airlock
2. No power draw
3. No dup wait time for airlock cycling
4. No soggy feet or drenched debuff
5. Can separate rooms with substantial pressure differences. You would think that high pressures would compress the hydrogen into fewer spaces than 1-4, but testing revealed that a hydrogen pressure of 1000g was able to separate a chlorine room at 10kg from an oxygen room at 200g. Therefore, only the extreme pressures that are creating by polluted oxygen accumulating over polluted water, or natural gas produced by oil well back pressure, could realistically cause this airlock to fail. Edit: upon further testing, it turns out that even a pressure of 150,000kg will not push hydrogen at 1.1 grams out of space #2. High pressure will not realistically cause the airlock to fail.
Disadvantages/limitations of this design:
1. Slightly longer dup travel time because they have to climb and descend the ladders
2. Cannot thermally isolate rooms. Edit: see my more recent posts for the near-vacuum setup, which is extremely effective at thermally isolating the two rooms.
Enjoy! I hope you can put this design to good use.