How a CC Rebreather Works
Image property of hippoconsulting
Rebreathers are jumping to the spotlight, as scuba diving gets mainstream divers ask for more. CCR is there to answer the pledges of the more adventurous divers, but how it works?
Rebreathers seem to be reaching his best moment, prices drop and friendly interfaces, redundant safety systems throw some light over the dark legend of closed circuit diving. Unlike open circuit diving during a rebreather dive you will be breathing the same gas all the time, there are some gas losses that we will go trough in a minute, but essentially you wear on your back an on the fly gas mixing station.
The average rebreather is composed of two tanks, counterlungs, 02 sensors, electronics, scrubber and 1 or 2 gas tanks, each of this parts is fundamental and needed for the correct operation, as you see the setup is different to the average dive system. The differences are many and designers are continuously searching for the best location of the different components, some expedition models allow sidemount disposition and 8 and 10 litres tanks.
Everything is crucial and malfunction of any can cause mayor disturbance in its use, to start with and the base of are the o2 sensors.
02 sensors are nothing but a battery that generates power oxidizing, that means that they have a limited lifespan and measuring voltage decay with the time. Oxygen sensor cells used in rebreathers are exposed to pressures of oxygen much greater than normally encountered in analyzing gas. As they age they become ‘non-linear’, meaning they are perfectly accurate for some conditions and very inaccurate for certain other conditions and this can have serious consequences for rebreather divers. Rebreather manufacturers recommend replacing the cells annually, long before they can become non-linear. The widely accepted absolute maximum useful lifespan of cells in rebreather applications is 18 months, with 12 months being the recommended replacement interval.
The o2 sensors accuracy and calibration allows the diver to stay in the optimal concentration of oxygen for the given depth, we cover this part in understanding NITROX, that tipically is 1.3 or 1.4 ppO2. And preventing the diver on exceeding 1.6 ppO2 that would become the mix toxic.
The scrubber is a canister where a chemical reaction undergoes. The content either prepacked or filled by the diver is a lime in different extrusions, balls, ticks, D shaped that comes coated with a reactive powder that absorbs carbon dioxide and prevents the diver of breathing carbon dioxide that is toxic even in very low concentration. Scrubber is a very sensible part of the process, if it is not compressed enough the lime will move and with the moisture of the breath will create tunnels where the resistance is lower, which means that air will flow more those channels and reduce the scrubber efficiency. Also, the reaction gets affected by the temperature and pressure.
There are 2 lungs, one for inhale and one for exhale, this way the gases go trought 4 posible locations, in the beggining they go into your lung, you breathe out and this same gas goes into the exhale lung, on the next breathe it will pass to the canister, where it will become scrubbed and analized by the cells, CO2 will be removed and O2 will be added in this step, on the next breathe it passes to the inhale lung, and from there to the lungs
Lungs have 2 possible locations, over the shoulder and backmounted, at the same time some rebreathers use backmounted coaxial counterlungs so when you expire you press the inhale lung, which will make breathing easier.
Oxygen injection can be done manually or automatically driven by the computer, some hibrid rebreathers allow both, the automatic addition is done trough solenoids, other mechanisms using solenoids are doorbells and tattoo machines for example, the principle is a piston acted by a coil that generate a magnetic field and displace the piece. these mechanisms are incredibly simple and reliables, the rate of failure is really low.