Shahriar from The Signal Path shows a neat trick to make liquid oxygen using stuff you can buy locally or scavenge. First, he needs fairly pure oxygen gas. He uses an oxygen concentrator, a machine you often see in hospitals that pulls oxygen out of regular air.
It works with something called pressure-swing adsorption (PSA), where air goes through a special material (usually zeolite) that grabs nitrogen and lets oxygen slip by. The concentrator he had made oxygen that was pure enough for what he needed, even if it wasn’t as perfect as advertised.
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To turn that oxygen gas into liquid, it has to get super cold—down to about -183°C (-297°F) at normal air pressure. That’s where electricity helps out. Shahriar takes a Stirling-cycle cryocooler, which he got from an old superconducting RF filter, to make things chilly enough. This cryocooler runs on power to move a piston that squeezes and stretches a gas (usually helium) in a loop, pulling heat from the cold side and letting it out on the warm side. He added a fan to blow away the heat and a tube setup to send the oxygen gas from the concentrator into it.
As the cryocooler works, it cools down the oxygen gas coming in. The cold end gets to about 77 Kelvin (-196°C), colder than oxygen needs to turn liquid. After around an hour, the oxygen starts turning into a pale blue liquid and gathers up. Shahriar made at least 100 milliliters of it and checked it with regular tests to make sure it was right.
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