HOW REBREATHERS WORK: PART 1
The Closed Circuit
Recycle – Replenish – Re-use
As an experienced open circuit diver, you’ll be very aware that the bulk of your gas is lost, bubbled away, every time you breathe out and that this waste increases dramatically, the deeper you go. With fully closed circuit rebreathers like the AP Inspiration, all of your exhaled gas is retained within the system in a closed loop. It is then filtered, refreshed and recycled back to you to breathe again. Bubbles escape only during ascent and mask clearing.
HOW REBREATHERS WORK: PART 2
The Constant PO2 Setpoint – elegant gas mixing on the fly
Diving an AP rebreather is essentially like diving with a small nitrox blending machine on your back – one that constantly produces the best gas for whatever depth you are at. How does it do this? By using the elegant mechanism of Constant PO2.
On open circuit scuba the gas mix, or fraction of Oxygen, is constant (approx. 21% O2 in air) at all depths while the partial pressure of Oxygen (ppO2 or PO2) is variable with depth – i.e. 0.21 bar at the surface, 0.42 at 10m, 0.63 at 20m and so on.
With APD CCRs it is the opposite case: the PO2 is held constant (at a Setpoint) by the rebreather controller and consequently the mix or fraction (FO2) of oxygen is variable as the diver changes depth. How so? Read on…
The AP controller maintains a constant PO2 at the chosen PO2 Setpoint throughout the dive (usually 0.7bar at the surface and for descent and 1.3 for the rest) by adding a fresh squirt of oxygen when the pressure of O2 falls below this setpoint (due to metabolism by the diver or when ascending). As a result, the FO2 (or O2 percentage) varies with depth.
This can be better understood if you imagine a diver ascending from 50m…
HOW REBREATHERS WORK: PART 3
A Tale of Two Cylinders – The Diluent Gas
The Inspiration is a complex piece of kit – but ingeniously simple at heart. There are two onboard cylinders. One contains pure oxygen, the other a diluent gas – usually Air.*
The diluent gas has several important functions: injected to maintain counterlung volume on descent, compensating for squeeze (injected either automatically via the Auto Diluent Valve (ADV) or manually by the diver if no ADV is fitted); it is also used for BCD and dry-suit inflation. Only small quantities are used – typically 30-40 bar per dive.
BEFORE YOU DIVE DEEPER
Closed circuit diving offers big advantages but it’s not for everyone. Things you need to consider…
Is it for you? Be honest with yourself about your willingness to:
- Invest the time, energy and money into training and new kit?
- Be prepared to understand that you are a novice again and to learn new things?
- Be self-disciplined enough to learn, practice and re-practice new skills way beyond the initial training course – to turn skills and drills into habit and instinct?
- Be personally disciplined enough to consistently carry out maintenance routines and pre-dive checklists, to be able to “Always Know Your PO2” throughout the dive.
- Build your experience gradually, staying within your training limits?
- Gain a thorough understanding of your rebreather, how it works, how to build it and re-build it?
- Are you the type of diver who allows themselves to become complacent about kit, dive discipline and diving rules over time?
- Are you the type of diver capable of planning ahead, to think through all the possibilities and make provision for them… what could go wrong, what would I need to do?
- Are you the sort of person who skips the instruction manual and looks for the quick-start guide?
- Have you got the networks, buddy divers, boat cover etc with the necessary skills to understand rebreathers and assist you in appropriate ways if required?
- Do the rewards justify the risks for the type of diving you want to do?
- Are you prepared to do the research and source a unit that has been CE certified, i.e. independently tested to meet internationally agreed performance standards
- Are you prepared to do the research to find an instructor with the appropriate experience (and ongoing CCR practice), the reputation and teaching skills to instill what you need to know to keep you alive – not just someone to get you the badge?