What is a rebreather? It is an underwater device that allows divers to reuse the air that they have breathed out, thereby being able to stay underwater much longer.
Most people, when at rest, breathe about 12 times a minute and take in about half a liter of air each time. If we are breathing at sea level, which most of us are, then that air we breathe in is made up of 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and the rest is made up of other gases. We don’t need the nitrogen and it sits in our lungs without doing anything before we breathe it out again. We breathe out the same amount that we breathe in. The oxygen, however, we obviously do need and it makes all the functions in our bodies possible. We absorb the oxygen into our blood through the lungs and we pull carbon dioxide out of our blood at the same time so it can be breathed out. Unfortunately, though, we are not very efficient with our oxygen use and we can only effectively use about 4% of what we breathe in. When we breathe out, the air is still 78.08% nitrogen, but it is now about 16% oxygen and about 5% carbon dioxide.
When people are diving underwater, they can breathe the same air that we breathe on the surface, so long as they don’t go too deep. Once people start going deep, the pressure increases and it becomes very difficult to breathe. The pressure of the water presses on the lungs and makes it very difficult to breathe in. To counter this, when people dive deeper, they have to use compressed gas. The gas is under pressure, which makes it easier to breathe it in. The problem with that is that when air is pressurized, the nitrogen starts to dissolve into the blood and this can cause complications such as the bends.
Regular divers who are not going too deep, carry a mix of gas that is similar to that on the surface, except with a little more oxygen in it. Even so, our lungs are the same and they can only absorb about 4% of the oxygen. They breathe it out with carbon dioxide. The exhaled air goes out through the respirator and rises to the surface as bubbles. This is a waste of oxygen because there is so much usable oxygen that is going up as bubbles, but there was nothing that could be done about it until the rebreather was invented.
A rebreather works by capturing the air that is breathed out, cleaning out the carbon dioxide, and recycling it so that it can be breathed again. This drastically increases the amount of available air and extends the length of time divers can stay under the water. There are two ways of doing it. The first is to have a scrubber in the pipe between the air tank and the respirator. The diver breathes in air and when they breathe out the air goes back down the same pipe and the scrubber pulls all of the carbon dioxide out. The second type of rebreather has two pipes, one for breathing in and one for breathing out. The pipe for breathing out goes through the scrubber to clean the air. There are different types of scrubbers, but a common one is sodalime. The sodalime is basic to start with, but as it absorbs more carbon dioxide it turns acidic. This is an easy way of seeing if it is saturated with carbon dioxide.
A rebreather can let people stay underwater for much longer than would be possible with regular SCUBA tanks. One thing that limits the amount of time a diver can stay down is oxygen exposure. Oxygen is very harmful to our bodies and if you are exposed to the pressurized oxygen for longer than about three hours, it can become dangerous. The scrubbers last for about three hours as well. If you are breathing a different mixture of gas or at a different pressure, the length of time can change. If this limit of oxygen didn’t exist, and the scrubbers lasted longer, you could probably breathe the air in a tank for up to ten hours or more.
Recent research is looking at other ways of getting the oxygen as well. One method uses liquid oxygen, which takes up less space. It is difficult to carry, though, and needs to be well insulated. Another method is by using chemicals that release oxygen when exposed to carbon dioxide. Potassium superoxide is one chemical that does this. The chemical would remove the carbon dioxide and produce the oxygen all by itself. The one problem with potassium superoxide is that it is highly volatile when exposed to water, which doesn’t make it a great idea to take it underwater. And this is what I learned today.
Image By PH2 (NAC) Jeff Viano, USN – http://www.defenseimagery.mil/imageRetrieve.action?guid=3ece497eba008b63dfa31213b5183855ba09ffee&t=2, Public Domain, https://commons.wikimedia.org/w/index.php?curid=32161777
Sources
https://www.padi.com/gear/rebreathers
https://www.quora.com/Why-can-normal-air-not-be-used-for-breathing-by-sea-divers
https://en.wikipedia.org/wiki/Rebreather
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8672270
https://en.wikipedia.org/wiki/Atmosphere_of_Earth