Rebreather Diving – A Look At The Basics
Once the preserve of military, scientific and extreme technical divers, in recent years closed circuit rebreathers (CCR’s) have entered the sport diving mainstream. A bewildering number of options and different machines present a mass of information for the potential new CCR diver which is difficult to assimilate without some base knowledge. With this article I hope to clarify the basic operational aspects of CCR as well as highlighting the positive and negative aspects of this diving system.
What is CCR? In order to understand the fundamental difference of CCR we first need to clearly define open circuit scuba (OC). At its simplest, we have air contained in a cylinder with a set of regulators attached, we inhale the air from the cylinder through the regulators, and exhale into the water column, producing bubbles with every breath cycle.
If we want to go deeper and stay longer we need to use different gas mixtures, most divers are familiar with nitrox and the benefits it provides for extending no stop dive times at moderate depths, but technical divers take this further by using multiple cylinders/gases for any given dive and switching between these cylinders/gases during the dive in order to have a breathing medium which is safe and efficient (from a decompression standpoint) for the given depth.
This is because OC SCUBA has a constant fraction of gas (Fg or FO2 if we are talking about oxygen, which we usually are), a cylinder filled with Nx32 has 32% oxygen and 68% nitrogen in it no matter what depth it is being breathed at, however the pressure of gas will change dramatically between the surface and the maximum operating depth (MOD, the maximum permissible depth of the gas). Breathing from this cylinder at the surface while sat on the boat will give a pressure of oxygen (PO2) 0f 0.32, at 40 meters, the PO2 will be 1.6, while the FO2 of the gas in the cylinder will remain 0.32 or 32%.
Assuming we breathe 18 liters of air per minute at the surface (this is called our SAC rate, surface air consumption, and it varies widely between individuals), at 30 meters we are subject to 4 atmospheres of pressure (ATA-atmospheres absolute), meaning that our breathing gas must also be delivered at this pressure, so our gas consumption at 30 meters will be 18 x 4 = 72 liters per minute. Now our SA80 cylinder, which contains 2200 liters of gas, will last 2200/72 = 30.6 minutes at this depth.
CCR is radically different, it provides a constant PPO2 and a variable F02, so it is diametrically opposite to OC. In order to explain how this happens, it is helpful to learn some new concepts which describe the CCR system.
The loop is the unit’s extension of our lungs; it is where our exhaled gas goes, is scrubbed of carbon dioxide and replenished with oxygen before we inhale it again. It consists of a mouthpiece containing one way valves which direct the flow of gas in a constant direction, flexible bags, called counterlungs, which inflate as we exhale and deflate as we inhale, scrubber material which has a chemical reaction with carbon dioxide, binding it to itself and removing it from the breathing gas, oxygen sensors which measure the pressure of oxygen (PO2), a handset/display for system information/controls and usually 2 devices, 1 automatic and 1 manual, to add oxygen to the system to keep it at the desired PO2.
The loop is fed by 2 cylinders, 1 of these cylinders is filled with air (at the start, later, as you progress down to the depths the air is replaced with trimix), this is the diluent which is providing the gas to fill your lungs/the loop, just enough for 1 breath, as this breath is continually recycled. The second cylinder contains 100% oxygen, this is added to replace the oxygen you metabolise during the dive, typically in the range of 1 liter per minute.
With CCR we can choose our PO2, the most efficient way to do this is to have 2 settings, called low and high setpoint, our low setpoint is what we use on the surface and is typically 0.7, equating to 70% oxygen at 1ATA, our high setpoint is what we use at depth and is typically 1.3 (unachievable at the surface). How the unit provides this varies according to 2 distinctly different design concepts, which are covered in detail in my next article, ‘solenoid or constant mass flow, which one’s for me?’.
Suffice to say, we now have a dive system which is providing us with a constant PO2, only requires 1 breath of diluent (providing we do not waste gas by constant mask clearing and depth changes, expanding gas must be vented from the loop during ascent just like a BCD) and uses approximately 1 liter of oxygen per minute regardless of depth, (oxygen consumption is independent of depth but dependent on workload).
This is a massively more powerful tool than OC, providing an ideal, warm and moist gas mixture at every depth (longer no deco limits/shorter decompression, less thermal stress, more comfort), using comparatively tiny quantities of gas and near silent diving (bubbles make a lot of noise) enabling us to get up close and personal with the marine life.
Despite the growing popularity of CCR and attempts to automate systems to make them easier to dive, all these benefits come at the cost of increased complexity of use and higher cost of purchase. CCR will always be a more complex and less forgiving system than OC, so will never completely replace it. Compare a family saloon car to a highly tuned racing car, the saloon will be far easier to drive and much more suitable for day to day use, but the racing car will be able to do things the saloon car cannot.
Experienced divers willing to invest in the time it takes to master the new skill set required for CCR diving will find this a hugely rewarding experience, opening up a new world of possibilities.
If you would like to find out more about closed circuit rebreather diving, then contact the Deefer Diving team and enquire about CCR courses on the proven and popular AP Inspiration and KISS units.
