Episode 9 of Season 4 opens on a colony that is already well along. The top bar reads Cycle 82, with 19 duplicants and about 108,000 kcal of food, and the name under the cycle dial is Comet; Episode 8 stopped at Cycle 120. Most of the hour goes to one puzzle: making the machines that clean the colony's water switch off once the clean tank is full. The answer runs through a NOT Gate. Along the way the coal runs out. When the stream stops it is Cycle 95, with 21 duplicants and about 72,000 kcal.
The tour
"I figured out ranching pretty well." Critters are kept in pens for the coal they give, a Sage Hatch among them, though "the little light bulb guys," the glowing bugs, got in the way. Coal is the main power source and keeps running low, so the plan is "hydrogen collection and burning" from the hydrogen pockets nearby. Food is "pretty well covered": extra eggs from the critters get crushed for omelettes. A Starvation alert still shows anyway.
A carbon dioxide area at the bottom of the base has become a fungus farm, since the upper farm "is starting to become too hot." The alerts list Duplicants have died: the main digger "suffocated over in the other area."
Fixers who will not fix
Cables break and stay broken. On the errand priorities screen Bubbles is a General Engineer, told to do nothing but toggle and fix things, and the status panel reads Pleasant Chitchat. "That's not what I wanted, Bubbles." A cluttered corner where duplicants keep standing around gets cleared out, with slime moved, a flat floor laid and compost set up.
Then: "so we're out of coal." Coal digging goes to priority eight, and a pump starts sending hydrogen toward the burner. "Not a lot, but some hydrogen coming down to burn." Later the commentary pushes carbon dioxide into that area to drive the hydrogen up, "sorta like fracking."
Hunting for a sensor
When the clean water tank is full, the machines "that convert bad water into good water" keep running. The Automation menu offers a Gaseous Element Sensor, a Hydro Sensor, a Signal Switch and more, but the signal needs to work the other way around: "what I want is a NOT gate." The Super Computer's panel shows Advanced Automation as the research in progress, and the research count soon goes from 30 to 31.
A liquid sensor goes in, and its card gives the catch away: Liquid Pipe Element Sensors detect "a specific liquid in a pipe." "Aha, they're pipe sensors." Nothing in the menus reads how high the water sits in a tank. The research screen lines up Smart Storage and Solid Transport next.
The workaround is a liquid pump placed at the top of the tank, with a bridge so nothing flows back. While the tank is low the pump has nothing to pull. Once the water reaches it, water runs through its pipe, and a sensor on that pipe can tell the machines to stop.
The NOT Gate
The signal comes out backward, so a NOT Gate from the Automation menu goes between the sensor and the machines, with fresh automation wire. "So if that thing is actually sending a signal saying, 'I have no water,' then that thing maybe will turn it on." The real test has to wait for the tank to fill, and the commentary only "hopefully will know once we actually fill up this tank."
The research count reaches 32. The Shipping menu now holds an Auto-Sweeper. One goes in next to a pile of debris, with a Storage Compactor for it to fill, but it needs a duplicant with a skill nobody has learned, so "we'll come back to that idea later." A fish release and a fish feeder go in for a fish farm.
Short on oxygen
Oxygen gets worse, and Suffocating joins the alerts. The Algae Deoxydizers sit at Awaiting Material Delivery while the commentary counts "six tons of algae." Too many priority-nine jobs may be why. A fireman pole goes in so duplicants can get down faster. The commentary walks through the oxygen room: three valves split what comes out into thirds, one for each of three floors. The plan for the hot upper farm is to swap its plants for mushrooms, which "can handle higher temperatures."
Where it ends
"Things are going pretty well. That is going to be the end of the stream." There are "a little bit suffocation issues," and the tank switch is still untested. The top bar reads Cycle 95, with 21 duplicants and about 72,200 kcal, and the alerts list six broken buildings and not enough oxygen.