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
All else being equal, astronauts have equipment to scrub CO2 and recover additional oxygen, but divers usually just breathe out and it bubbles into the water through a valve. So astronauts are losing oxygen at a much slower rate. But if a scuba diver had a scrubber, as many military divers do, they could make the air last much longer.
Astronauts don’t really have air tanks, they have a rebreather system, which is much more efficient, divers just blow out bubbles with every breath, however most of that is just the same stuff you breathe in as you can’t absorb all the oxygen and nitrogen/helium isn’t processed at all, this is kind of a waste but at the cost of a very simple system.
Rebreathers basically have a system that turns carbon dioxide back into oxygen and filter the air so that they can keep breathing, this effectively lets you use all the air, and will therefore last longer
Also scuba divers are well… underwater, this means the pressure in their regular has to be the same as the pressure of the water around, and so higher pressure means more gas volume is used, and so even if astronauts had tanks, they would still last more because the pressure in space is lower
Diving air is usually either regular air at 21% oxygen or Nitrox with up to 40% oxygen, and 60% nitrogen. Astronauts on spacewalks with tanks of 100% oxygen. Divers can’t use 100% oxygen because it’s poisonous under high pressures, while space suits maintain comfortable low pressures.
In other words, they get 5x more oxygen per tank.
It’s the rate of breathing. I can go nearly 3 hours on a tank of air. I consume very little. The hour is how long dive boats take you out for but at shallow depths you can scuba until you’re out of air. On a deep dive your time is limited by the amount of nitrogen that will be in your body when you’re finished.
People are answering on the astronaut side, but I wanted to give an answer from the scuba side:
When a SCUBA diver breaths in on their tank, the air is pressurized to their depth pressure. Pressure under water increases by approximately one atmosphere every 10 meters you go down. So if you take a 4 liter breath of air at 20m depth, that uses 12 liters of air at standard atmospheric pressure. This is why it is very important to breath out while ~~assessing~~ ascending when diving, if you try and hold your breath the air will expand in your lungs and can cause serious issues.
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.
A rebreather absorbs the exhaled carbon dioxide from the breath and then recycles the air which still contains significant amounts of oxygen so that it can be breathed a second time enabling the air supply to last longer than would normally be the case. https://youtu.be/2_OL4sOyeEw
You made a false assumption. “Both tanks are around the same size”. A scuba tank is typically made up of air.
I don’t know what an astronauts stuff is made of but a rebreather requires several different components to work.
1. A small bottle of pure O2. These bottles can be very small, like the size of a spair air canister.
2. A bag to maintain positive pressure. This bag would hold the scrubbed exhalation that has already passed through a filter (scrubber) + additional O2 to make up for the loss of O2 your body utilizes on the previous breath of air.
3. A filtration system to scrub out the CO2 from exhalation. This is typically about the size of a lunch box and as far as I’m aware, must be this size because of the chemical processes occurring with the amount of breath exhaled from the average astronaut.
All these components can pack pretty tightly together in something the size of a small backpack.
Scuba divers operate on a “total loss” system. The tanks contain plain air, and they breathe in a breath, and when they exhale it gets released as bubbles. Also, Scuba divers are supplied air at the same pressure as their surroundings, pressure underwater increases by 1 atmosphere every 33 feet, so you go through your air faster the deeper you go. I have made tanks last almost 2 hours on shallow dives by breathing slowly.
Astronauts tanks contain pure oxygen, their suits and life support systems contain complex computers and instruments which monitor oxygen levels and co2 levels in the air in their suit. As oxygen gets low, they add more from the tank, and as co2 gets high, it gets scrubbed out.
Some very advanced scuba divers use a re-breather which is similar in concept to what astronauts use.
I haven’t seen a comment addressing the fact that astronaut suits usually supply an air pressure of 4.3PSIA. That a vacuum. Atmospheric pressure is 14.7PSIA or 0PSIG. The astronaut doesn’t need much pressure because the little osmosis that occurs in the lungs is enough from the oxygen concentration.
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