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
You need about 2-3 liters of air to fill your lungs with every breath. The problem is that these are 2-3 liters at ambient pressure. If you breathe at a depth of 50 meters, the pressure is about 6 times higher than at sea level. So now you are using the equivalent of 12-18 liters of air per breath.
Normal air is 78% Nitrogen (inert), 21% oxygen and 1% the rest (important part is carbon dioxide).
With a normal scuba setup, each time a diver takes a breath and exhales, all of the gas is expelled. That’s why you see all of the bubbles leaving the diver’s mouth. This wastes a lot of air. Also, the deeper the diver goes, the more air is needed for each breath – the ocean is pushing on them and more air pressure is needed for the diver to breathe easily. Every 10m / 33ft of depth needs another “atmosphere” amount of air for each breath.
With a space suit, the air is fully recirculated in the backpack. With each breath, only a portion of the oxygen is consumed and replaced by carbon dioxide. The space suit scrubs out carbon dioxide, removes moisture (humans breathe moistly) and a tiny bit of oxygen is put back in to replace what the body absorbed. Also, a space suit provides less air in each breath due to lower air pressure because outside is the vacuum of space, not a lot of water pushing on them. In this way, a space suit can last many hours.
Back in the scuba area, there are more advanced “rebreather” systems that do not dump all of the gas out with every breath, but they are more expensive, more complicated and not common in recreational diving. When diving with a group, everyone would need a rebreather or the dive is limited in length, and no-one wants to sit and wait on the surface for 3 hours after the dive. Navy seals use rebreathers.
1/ it’s air not oxygen
2/ one is a closed system – astronauts and wastes very little the other is an open system and wastes a lot
3/ one is under pressure (diver ) one is not astronaut
So a closed system used by a diver at 1m would last the same as an astronauts
If we put both the diver and the astronaut at 100m they both would consume their air quicker as the air is under pressure so squashed by 10x so they need 10x more of it to fill their lungs and breath
The pressure under water compresses the air. The deeper you go, the less your air lasts. If you have access to a pool, take a tall empty glass and turn it upside down. Hold your breath and dive to the deep end holding the glass upside down. See the glass is no longer full of air. It is compressed. Go back towards the surface. It appears there is more air.
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