How does a scuba diver’s oxygen tank last for ~1 hour but an astronaut’s lasts ~6-8 hours when both tanks are around the same size?
In: Technology
First of all, except for very specific circumstances, you never use pure oxygen in scuba diving. It’s usually a regular air (21% oxygen) or slightly enriched air (up to 40% oxygen). The reason is that deeper than 6m/20ft breathing pure oxygen is toxic to central nervous system. The common symptoms are convulsions, seizures and death from drowning.
The deeper under water you go the faster the gas in the tank is used up. At 30m each breath consumes twice as much air from the tank as at 15m, and 4 times more than at the surface. That’s because to make a breath you need to inhale a certain more or less fixed volume of air which is the same at any depth, but since the pressure doubles with each 10m of depth the same volume of air holds different amount of air.
In space they would breathe air at the same or slightly lower pressure that we breathe at the surface of the earth because the space suits are pressurized to about normal atmospheric pressure. In contrast, without using specialized equipment you can dive to 40m where the pressure of water on you is 5 times more than at the surface.
Everything above is about a typical common scuba set up that you would see on vacation. It’s called an open loop.
Rebreathers are closed loop. They are popular among more advanced technical divers. It’s possible to dive for 6-8 hours with rebreather too.
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