There is an old adage among engineers who design safety systems, it goes like this; 'Three is one and two is none'. In simple terms this statement asserts that if you have three systems designated to perform the same single function (wherein two are dedicated as back up), count on two of those three systems failing in any given situation. The rationale is that as that these systems are all typically interconnected to the overall system and subject to the same factors that might cause them to fail. Therefore, if you start with only two systems and both fail you are left with none. I am often asked how I manage contingency planning and balance my redundancy when it comes to decompression diving, specifically my decompression gases. My response for many results in alarm bells in their own dive plans and configurations.
At the entry level of decompression diving (with less than 10-15 minutes decompression) your contingency plan is less complex with many options. For example a dive to 150fsw (45msw) on air for 15 minutes requires a total of 7 minutes decompression using an accelerated deco gas of EAN75*. Should the diver lose his/her decompression gas, completing the deco stop on their bottom mix only extends the stop time by 4 minutes. However, as your depth, time and/or gas mixtures increase so do the physiological implications if you miss or even mishandle your decompression stops. Take the previous example to the same depth but with a 30 minute bottom time. In this scenario a 75% oxygen decompression gas gets you out of the water with 31 minutes decompression however, if you lost your deco gas, completing this dive on your bottom mix adds almost 20 more minutes on top of the 31*. You would need to consider if a) you have enough bottom mix to complete this dive and b) can you complete this dive from a physiological perspective (thermal, urination, etc)?
In the world of contingency planning there is a universal approach known as the PACE concept. The PACE approach to contingency planning is a method used to ensure redundancy and reliability in critical systems by identifying and preparing multiple layers of backup options. The acronym PACE stands for:
Primary: The first and preferred solution, which is typically the most efficient or reliable in regular operations.
Alternate: A secondary option that can be used if the primary solution fails and usually of equal quality and reliability to that of the primary.
Contingency: A less efficient but functional fallback if both the primary and alternate solutions are unavailable.
Emergency: The final and least desirable option, used only when all other options have failed.
Origins of PACE
The PACE methodology is believed to have originated within military and emergency response planning contexts, particularly in communication and operational logistics. These fields demand a high level of readiness and resilience in the face of uncertainties, where a failure in primary systems could result in mission failure or life-threatening situations.
Over time, the PACE approach has been adapted for broader applications in various industries such as information technology, engineering, and business continuity planning, where critical systems require multiple layers of redundancy. It is the fundamental approach to every aspect of my dive plan. In essence, PACE is a proactive approach to contingency planning, ensuring that critical systems maintain functionality even under adverse conditions. It promotes resilience and preparedness, helping to prevent total system failure by offering a tiered structure of options.
The Most Overlooked Aspect of Contingency Planning
So getting back to the decompression aspect of our dive plan management, how do we incorporate true decompression redundancy? Many well equipped and well trained technical divers will explain that their back options options for a failed or lost decompression gas lie somewhere between going back to their travel gas (if carried), using their bottom gas (if not hyperoxic) or going to a buddies system if they are able to share it (or manual mode, onboard OCB/off-board diluent for CCR divers). All fall somewhere in the PACE spectrum but I assert none of them a really appropriate alternates. In short, having a fully redundant reliable alternate decompression gas delivery system that does not compromise your planned profile and does not rely on another diver is absolutely mandatory. Anything less is irresponsible and severely lacking in competency.
The trick then is to set up a system that is good balance between redundancy and equipment configuration (not over-geared), simple and reliable. For most dives wherein my profile requires up to 30 minutes of decompression** I carry a deco bottle with the following features:
A tank H or Y valve. This is a valve that provides two independent first and second stages.
The H or Y valve allows two divers to share decompression gases simultaneously.
Each of my second stage regulators are equipped with shut-off valves to prevent inadvertent loss of gases.
My LP hoses and second stage cases are green to avoid confusion at depth during gas switches or going to bail out at depth. I also have covers on my mouth pieces serving as an additional reminder when deploying reminding the diver that they contain a potentially hyperoxic mix.
My tank is clearly marked as a deco tank.
In the photos above you can clearly see how two totally independent oxygen delivery systems are set-up, labelled and stowed. Note the second stage shut off valve in the middle image.
So let us run this through our PACE algorithm for a dive requiring less than 30 minutes decompression and then consider, is this still an acceptable series of back up systems? Options for both Closed Circuit Rebreather (CCR) and Open-Circuit (OC) divers have been provided.
Primary - For CCR divers we have an on-board oxygen delivered via the controller/injectors and for OC divers, decompression stage tank with a quality first and second stage on the left H-valve post (see picture above).
Alternate - For CCR divers one can manually drive their oxygen and for OC divers they can still use their decompression stage tank with quality first and second stage on right H-valve post.
Contingency - For CCR divers one can plug in their off-board decompression tank which also carries full open circuit system which serves as an emergency option. For OC divers the contingency plan may include one of the following options - Self-Reliant: travel gas, second deco bottle or bottom mix and External Options: another divers deco gas (H/Y valve) or a staged back-up deco gas.
Emergency -  CCR divers can bail out and use their off-board decompression stage tank which is equipped with a quality first and second stage on H-valve (which then offers two more options should one regulator fail). For OC divers the contingency plan again will likely include one of the following options listed above - Self-Reliant: travel gas, second deco bottle or bottom mix and External Options: another divers deco gas (H/Y valve) or a staged back-up deco gas.
Note that in the example above the CCR diver had many more contingency options available to them. The emphasize being that closed circuit systems, although more costly on initial set-up, not only provide safer more affordable gas mixes but more redundancy as well. Read my blog on the merits of CCR if you are interested in learning more about the advantages of Rebreather diving.
The above scenarios are, of course, based on responses to failed gas delivery systems (i.e. regulators or CCR injectors/electronics). Additional or alternate contingencies must be considered for instances where a diver loses their entire oxygen (deco) gas supply. Even in the above PACE scenario the Emergency Option for open circuit divers (deco on travel (nitrox) gas or bottom gas) could likely lead to very lengthy hangs doubling or even tripling deco times in the water.
What a diver chooses to carry with them on a decompression dive will ultimately be determined by the divers training, attitude, skills, knowledge and of course the equipment available to them. However, nothing supersedes sagacity which is the ability to combine knowledge, experience, insight and perception; and use that attribute to assess really how effective one's configuration will be when something goes wrong. In my years immersed in accident analysis and risk mitigation, one thing has proven to be consistent which is that small error or failure usually begets bigger ones proving that 'two usually means none' and that is not cool.
*IANTD Air Decompression Tables with EAN75% Decompression
**Note: that this particular plan is generalized for a profile requiring up to 30 minutes of decompression. For longer decompression dives or dives in overhead environments, current, solo diving etc, the plan would be adjusted to ensure it is appropriate for the environment and the activity.
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