JUST HOW SIMPLE ARE REBREATHERS?
A Follow Up to the Rebreather Myths and Misconceptions Part I – Divers Deserve the Truth About Rebreathers by Jon Kieren:
In a previous TDI blog post we stated that rebreathers are not all that complex. This statement struck a chord with many of you and generated a lot of excellent feedback. Much of that feedback was accurate, and we appreciate your input. Because of your responses, we felt it important to put our statement into context.
The basic functions of a rebreather, the ones that keep you alive underwater, ARE NOT complex. However, you were right, many of the modern closed circuit rebreathers (CCRs) used in technical diving today are; there is no question about that.
So why would we make that statement in a TDI blog post if modern units are complex? Successful rebreather divers focus their diving practices on the simple aspects of the equipment. We feel it is important for a potential rebreather diver to first understand the basic functions of a rebreather. Once you understand those basic functions, you can build on that knowledge to understand the mixed gas rebreathers of today without becoming overwhelmed and intimidated.
Here’s what we mean
Rebreathers were the first commercially available form of SCUBA equipment, developed by Henry Fleuss in 1878. This simple system was used in 1880 to conduct work 300 metres/ 1000 feet inside a flooded tunnel, an area inaccessible to hard hat divers at the time. Modern rebreathers today still function under the same basic principles of Fluess’ 1878 model.
How a rebreather works
First, some VERY basic physiology: A diver metabolizes oxygen (O2) as a primary component for muscle and brain function. The byproduct of this metabolism is carbon dioxide (CO2). CO2 buildup in the bloodstream is the trigger that prompts us to breathe. Too little oxygen and the body does not operate, too much CO2 causes severe side effects.
A rebreather diver breathes from a mouthpiece which uses simple one way valves to direct gas flow. The mouthpiece is connected to a flexible container (counter-lungs) creating a closed “loop”. As the diver breathes, their body metabolizes the O2 in the loop, and produces CO2. The metabolized O2 is replaced by injecting either O2 or nitrox, and a chemical that absorbs CO2 located in the loop removes the CO2. This maintains a gas that is safe to breathe in the loop.
And that’s pretty much the whole foundation for rebreather diving. Pretty simple, right?
Where rebreathers start to get more complicated:
Remember the human body operates within a specific range of partial pressure of oxygen (PPO2), and the amount of oxygen that’s injected into the breathing loop is critical. Too much oxygen and the risk of CNS oxygen toxicity increases. Too little, and the loop becomes hypoxic and will not sustain consciousness. (for more information on PPO2, see the TDI Nitrox Diver course)
Oxygen rebreathersare the simplest forms of rebreathers because they only inject oxygen into the loop. This means that as long as they are only used within the maximum operating depth (MOD) of 100% oxygen (about 6 metres/20 feet and shallower), and the scrubber is functioning properly, the gas in the loop will always be safe to breathe. Because only 100% is ever injected into the loop, there is no need for electronic PO2 monitoring devices in the loop.
Semi-closed circuit rebreathers solve the depth limitation issues by using lower O2 content gasses (nitrox or trimix) to lower the PO2 in the loop and extend the depth limits based on the MOD of the gas used. Because the O2 content of the gas is lower than an oxygen rebreather, a higher volume of gas must be injected to replace the O2 that is metabolized. This excess gas is vented out of the loop, and is where the term “semi-closed” comes from. These systems are still simple, and because they only inject a single known gas into the breathing loop, basic calculations can determine the PO2 of the gas in the loop. While electronic PO2 monitoring is not required in semi-closed rebreathers, it is often used for additional safety.
Closed circuit rebreathers (CCRs) are currently the most commonly used for technical diving. Technology advancements have created reliable electronic PO2 monitoring and control systems which allow divers to use rebreather technology to its fullest potential. Instead of using a single gas source like oxygen or semi-closed rebreathers. CCRs use pure O2 to replace the metabolized O2 in the loop, as well as a diluent gas (air, nitrox, trimix, or heliox) that to dilute the loop’s PO2 and extend the depth limit to the MOD of the diluent gas.Because multiple gasses are injected, accurate PO2 monitoring and injection devices are critical to maintaining a safe breathing loop. Accurate PO2 monitoring is most commonly accomplished utilizingelectro-galvanic fuel cells(O2 cells), a wrist display, and heads up display. Multiple O2 cells and displays are often used for redundancy in case of a failure.Diluent injection is either automatic, using an automatic diluent valve (ADV), which fires when the loop volume drops to a minimal level upon descent, a manual addition valve (MAV), or a combination of the two. Oxygen injection can be handled in several different ways.
Manual CCRs (mCCR)utilize a constant mass flow orifice (CMF) or needle valve to constantly leak a small amount (just below that of the diver’s metabolic rate) of oxygen into the loop. The diver is then responsible for monitoring the PO2 on his/her displays and pushing a manual injection button to bring the PO2 up to the desired level (setpoint). MCCRs are the simplest form of CCR, but require constant input from the diver to maintain the desired PO2, and are often depth limited due to the CMF orifice.
Electronic CCRs (eCCR) utilize sophisticated electronics systems and an injection solenoid to control the PO2 in the loop automatically. The electronics monitor the O2 cells and determine when and what rate to inject O2 into the breathing loop in order to maintain the setpoint. Most eCCRs also allow for manual O2 injection by the diver as well.
Hybrid CCRs (hCCR) use a combination of a CMF or needle valve and eCCR technology. By utilizing both technologies, the hCCR leaks O2 into the loop maintaining a PO2 close to the setpoint, reducing the frequency that the solenoid needs to fire. While hCCRs incorporate the best of both worlds, they also carry the negatives of both as well. Many hCCRs are depth limited due to the CMF, and also carry the added complexity of the eCCR components.
Continued developments in technology means that CCRs continue to become more and more reliable equipment. Their minimal gas usage and extreme depth capabilities have made them standard equipment for much of the exploration conducted today.
While rebreathers can incorporate some complex components, the basic functions remain simple. Training, equipment preparation, and diving practices that focus on these basic functions makes a successful rebreather diver.