New to CCR?

down-the-chimney-schottCCR ADVANTAGES - AN OVERVIEW

  • Radically Reduced Gas Usage: consumption is influenced by your body's metabolism, not depth. Average use is only about 1 litre / 0.035 ft per min no matter if you are diving at 10 m / 33 ft or 100 m / 330 ft!
  • Best Breathing Mix: the optimal nitrox mix at every depth with dramatically increased no-stop bottom times.
  • Optimized Deco: if your dive does require deco stops, a properly working CCR will always provide the best mix for the current depth.
  • Silent, Bubble-Free Diving: get up close and personal with wildlife
  • Increased Comfort: warm, moist breathing gas (as opposed to cool, dry gas with standard open circuit diving).
  • Versatility & Adaptability: ready for anything from a shallow reef dive to extended range cave exploration, to deep wreck discoveries, SubGravity CCRs can be customized to meet your needs.

How does a rebreather work?

It can be a bit overwhelming when you first begin to look at rebreathers, especially if you do not completely understand how they work.  This short guide is intended to simplify things, and help guide you through the process of deciding whether closed circuit rebreathers are a good choice for you, and how to choose a unit that best fits your needs.  This page will start with a basic physiology review and then move into the basic functions of all rebreathers to help demonstrate how a rebreather works with our bodies to optimize our diving potential.  We will then discuss several different types of rebreathers, options, and what to look for when trying to pick a unit that’s best for you.  While oxygen only and semi-closed circuit rebreathers are commercially available, this page will focus solely on mixed gas, closed circuit rebreathers (CCRs).

Beginning with a bit of review will help provide some context when relating rebreather functions to how they relate to our bodies basic needs, and how they can work together to optimize our diving.

Oxygen, metabolism, and carbon dioxide are the foundation for our bodies most basic needs.  When we inhale, oxygen (O2) is transported through our lungs and to our tissues via the circulatory system.  This oxygen is then used as a key component in metabolism to create energy (needed for virtually every function of our body).  The byproduct of this metabolism is carbon dioxide (CO2), which is then transported from our tissues to our lungs and expelled when we exhale.  Seemingly simple, but it’s important to remember that our bodies operate within a narrow margin of error in regards to the O2 and CO2 exposure.  Too little oxygen (hypoxia) and our bodies cannot sustain the most basic functions, resulting in unconsciousness and eventually death.  Too much oxygen (hyperoxia), and we are at risk of CNS oxygen toxicity, resulting in convulsions (under water, this usually leads to drowning).  Too much CO2 (hypercapnia) can cause panic, disorientation, convulsions, unconsciousness, and eventually death. (There are even physiological problems with too little CO2 as well!)

As learned in basic nitrox training, our bodies respond to the partial pressure of the gases we are exposed to, so it’s not the fraction of the gas that is important, but actually the pressure of the said gas that is important.  Therefore, the inert gasses (those not used in metabolism) in the gas we breathe, such as nitrogen or helium, must be considered in diving applications as well.  As we descend, the partial pressure of all the gasses we are breathing increases.  This increases the dose of oxygen our tissues are receiving, and the higher pressure inert gas is dissolved into our tissues by diffusion.  As ambient pressure decreases on our way back up to the surface, we must allow enough time for these inert gasses to diffuse back out of our tissues safely to avoid causing large bubbles to form, potentially leading to decompression illness.  We can decrease the partial pressure of the inert gasses in the gas we are breathing by increasing the amount of oxygen in the mix (creating nitrox).  This requires a balancing act between safe oxygen exposures and minimal inert gas exposure.

If we think about the basic functions of a rebreather as an extension of our bodies, you will see that they are fundamentally quite simple.  At the core, all rebreathers perform 2 basic functions:  replace the oxygen that we metabolize, and remove the carbon dioxide that we produce resulting from that metabolism.  CCRs also add the advantage that they maintain a constant partial pressure of oxygen, essentially creating the optimal gas mix for every moment of the dive to balance oxygen exposure and inert gas exposure (remember the "best mix" calculations in your nitrox course?).  While all CCRs “scrub” CO2 from the gas (more on that in a bit), there are several ways that oxygen can be replaced.  There are advantages and disadvantages of each type of CCR, and it’s up to the individual diver to decide what is best for them.
Manual oxygen addition (mCCR):  An orifice continuously leaks a small amount of oxygen into the loop.  Typically, the diver sets the flow rate to be just below their individual metabolic rate.  Throughout the dive, the diver must manually inject oxygen to maintain the desired PO2.  MCCRs are simple, have slightly less reliance on electronics, and require constant input from the diver which potentially makes them more aware of what is going on in the loop.  However, they are often functionally depth limited to around 80-90 meters/ 265-300 feet, and the required constant input from the diver may be problematic on task loaded dives.

Electronic oxygen addition (eCCR):  The rebreather’s electronics control an oxygen injection solenoid to maintain the desired PO2 setpoint.  Both mCCRs and eCCRs utilize an array of oxygen sensors so the diver can monitor the oxygen content in the loop, however the eCCR’s computer reads the oxygen content and decides when it is necessary to inject oxygen automatically.  eCCRs require less constant input (compared to mCCR) by the diver which is advantageous on task loaded dives.  Some theorize that this lack of constant input from the diver can lead to complacency and reliance on the electronics, which can be problematic if there is an electronics failure and the diver does not notice. All SubGravity approved training programs include extensive training on CCR emergency procedures, which includes everything from minor gas delivery issues, to 100% electronics failures.

Hybrid systems (hCCR):  A hybrid CCR combines the continuous flow addition of the mCCR and the electronic injection of the eCCR.  By combining the continuous flow oxygen addition of the mCCR and the electronic solenoid injection of the eCCR, the hCCR is less reliant on the solenoid to maintain the desired setpoint.  This reduces power consumption of the eCCR, and gives the diver significantly more time to respond in the event the electronics fail to inject oxygen.  Some consider this option the "best of both worlds" while others dislike the additional parts and potential failure points.

All CCRs remove CO2 by utilizing a chemical CO2 absorbent that we call the “scrubber”, which is comprised primarily of calcium hydroxide and sodium hydroxide.  Scrubbers come in several different varieties, but essentially all work the same way.  The gas exhaled by the diver passes through the scrubber, where a chemical reaction takes place removing the CO2 from the breathing loop.  For the chemists out there, the overall reaction is as follows:

1) CO2 (g) → CO2 (aq) (CO2 dissolves in water - slow and rate-determining)

2) CO2 (aq) + NaOH → NaHCO3 (bicarbonate formation at high pH)

3) NaHCO3 + Ca(OH)2 → CaCO3 + H2O + NaOH ((NaOH recycled to step 2)

Axial:  A canister filled with CO2 absorbent which allows gas to pass in from one end and out the other.  Axial scrubbers are simple and typically more forgiving in the packing process.  Because the gas must pass through the entire scrubber from one end to the other, increased work of breathing may be recognized, especially on larger scrubbers.

Radial:  Like the name implies, gas flow in a radial scrubber radiates either from the outside in, or the inside out.  This shortens the overall distance that gas needs to pass through the absorbent material, benefiting work of breathing characteristics.  This also allows for overall larger scrubbers, increasing dive durations.  Radial scrubbers do require slightly more diligence in the packing process, however, with proper training and attention when packing, the advantages can be great.

 The mouthpiece of a rebreather serves several purposes.  Besides the obvious role of allowing the divers to breathe from the loop, the mouthpiece also directs the flow of gas around the loop.  Utilizing simple one way valves or “mushroom valves”, the mouthpiece allows gas to enter from the inhale side of the loop as the diver inhales.  The inhale mushroom valve then closes and seals the inhale side of the mouthpiece as the diver exhales, opening the exhale mushroom valve and allowing gas to pass from the mouthpiece into the exhale side of the loop.  There are two categories of mouthpieces, dive/surface valves (DSVs) and bailout valves (BOVs).

DSVs:  Simply allow the diver to switch from dive mode to surface mode, which opens and closes the mouthpiece to the ambient atmosphere. DSVs are simple, streamlined, and lightweight.  However, they require the diver to remove the mouthpiece and switch to an offboard bailout regulator to access open circuit gas in case of an emergency. (Show image of DSV)

BOVs:  Also allow the user to open and close the loop to ambient atmosphere.  However, when the mouthpiece is closed, it functions as an open circuit regulator.  This allows immediate access to a known open circuit gas without requiring the diver to remove the mouthpiece.  Incorporating an open circuits regulator into the mouthpiece makes BOVs more complex and slightly bulkier than a DSV, however.  (Show image of BOV)

To breathe effectively, a rebreather must incorporate some sort of flexible container which the diver breathes in and out of.  We call these “counterlungs”, and they come in several varieties.  Where the counterlungs are positioned can have a significant effect on the hydrostatic work of breathing of the rebreather.  Essentially, the farther away from the divers lungs, the more of an impact the counterlungs will have on the work of breathing.

Over the Shoulder Counterlungs (OTSCL) drape over the diver’s shoulders and down their chest.  Historically offering the best work of breathing characteristics, OTSCLs also take up a considerable amount of valuable real estate on the diver's chest and can be bulky and cumbersome.  OTSCLs also require use of a T-piece, which can limit head movement. (Show image of OTSCL)

Back Mounted Counterlungs (BMCLs) can be mounted in the case of the rebreather behind the diver, or run down their back directly in front of the backplate.  Traditional BMCLs were mounted behind the diver's backplate and wing in the case of the rebreather.  Traditional BMCLs also do not require T-pieces on the diver’s shoulders, which allows for greater flexibility of the loop hoses and typically allow for slightly better head movement.  While very streamlined, this places the counterlungs far above the diver's back and can cause an increase in work of breathing as the diver inhales.  (Show image of unit with in-case BMCL, KISS or rEvo)

Newer generation BMCLs offer a mix of both OTSCLs and traditional BMCLs.  Mounting to the diver's backplate in front of the wing, these configurations place the counterlungs along the diver's back which both frees up the chest area and offers similar work of breathing characteristics of OTSCLs.  However, this type of BMCL requires a T-piece similar to OTSCLs. (Show image of defender/x Counterlungs)

In summary, while the overall function of a rebreather is quite simple, there are many variables to consider in the specific design you choose.  These variables and options really come down to user preference and what is important to you as a diver.  It will take some time for you to sort through these options and decide which direction you would like to go, but there are many resources available to help you along the way.  Feel free to reach out to individual manufacturers and ask for their input and reasoning for why they design things the way they do.  Don’t be afraid to contact instructors and divers asking them for their input as well.  Just remember, much of this input may be somewhat biased, so you will need to sift through some of it to get the feedback you feel is valuable to you.  Most importantly, get in the water!  There are many demo events held throughout the world, offering divers the opportunity to try out several rebreathers.  Also, most instructors are willing to offer more personalized demo experiences when requested.

Which CCR is Right for Me?


Defender CCR

One of the most challenging aspects of becoming a CCR diver is deciding how to choose which CCR unit to purchase and train on. In a perfect world, each prospective CCR diver would have the opportunity to train on and dive each potential unit for several months before committing to a purchase. Unfortunately, this is not possible for most people. Perhaps the most important thing is for the buyer to do research with regards to anticipated application, manufacturer and distributor’s reputation, build quality, after purchase support, technological advances, access to quality training, and reputation of the explorers using these units.  Try dives are great, but keep in mind that without direct comparison between multiple units, as a new CCR diver it may be difficult to accurately analyze which unit may be just correct for you. Additionally, keep in mind that most instructors specialize in one particular brand unit or another and will almost always recommend the unit which they own and teach on. Instructor advise is great, but is almost always prejudiced towards the unit(s) on which they teach. The same principle applies towards other CCR divers. Almost all divers feel strongly about the unit for which they have just paid $10,000+ dollars, so keep an open mind and make sure that your prospective unit ticks as many boxes as possible for what you are looking for. Unfortunately, there is no single magic unit which works for virtually every diver out there. Different people have different needs and requirements. Do you homework and speak to several different credible people about a prospective unit. Find a qualified instructor on that unit and if possible, do a try dive or 2 or 3!