Redundancy and PO2 Monitoring in eCCR

I feel there is some misunderstanding or marketing of rebreathers. Redundant PPO2 monitoring is not for continuing the dive but to abort the dive and exit the water or cave on the rebreather if the backup system is working properly.by Lamar Hires

The number one rule of rebreather diving is always know your PO2. This is true for any type of rebreather, but for eCCR is especially important since electronics are involved and PO2 maintenance occurs without human interaction. We could go into sensor health and calibration but for the sake of this article let’s assume these are a given. The failure on a rebreather of any type is an abort dive signal especially on an eCCR.

I feel there is some misunderstanding or marketing of rebreathers. Redundant PPO2 monitoring is not for continuing the dive but to abort the dive and exit the water or cave on the rebreather if the backup system is working properly. I hear and read statements like “a back up system allows the diver to continue the dive” this is not an accurate statement; a backup system allows the diver to stay on the rebreather during the abort, the exit and/or decompression. This is no different from technical open circuit diving; backup systems are to safely exit.

There are a number of theories or options on redundancy and back up. These include: (1) More cells are better, as exemplified with the four cells option on the rEvo. (2) One cell is sufficient, with validation. This viewpoint represents the opposite extreme. The Poseidon Mark VI Discovery incorporates this philosophy, using the diluent composition and depth to periodically confirm via software that the cell is functioning properly. As a backup, they have incorporated a second cell as well. (3) Adding a fourth cell monitored by an independent computer. Many rebreathers may be modified in this manner, using CCR compatible computers like the VR-3, Hydrospace Engineering Explorer, or Liquivision X-1. This single cell supplements the traditional three cells designed into the rebreather. (4) Incorporating a fourth cell, with overlapping three-cell voting logic. This approach uses the original three-cell configuration as the primary information source, with an additional fourth cell coupled with two of the three original cells to provide a second three-cell redundant configuration for voting and confirmation purposes.

With so many options what is sufficient? When does it become too much? I have explored this, and for the past three years I have used the approach that, “less is better.” My cave diving background has ingrained in me a definition of redundancy as a backup solution that is separate from the primary system, with no shared components. A back-up light is a completely separate light, not sharing any common components such as a light emitter or battery source. With this thought process I have been using a single controller on my CCR that handles all the primary systems and a 4th cell integrated dive computer that displays a PO2 reading of a cell that shares no common components; the cell, wires, and monitoring display are all independent from the rebreather system. The only commonality is the location of the independent cell near the three rebreather cells driving the solenoid.

With a system failure on the primary controller I fly the rebreather manually, using the single cell monitoring on my backup computer during the exit of the dive. I have many options to stay on the loop and maximize my gas supplies. During an emergency abort I fly manually at 1.0 atm, because even a 25% error factor still keeps the PO2 in a safe range. If you are concerned about a single cell reading, you can use a high PO2 bailout gas and fly semi-closed mode, running the range of the gas from 1.2 atm down to 0.8 atm, and then flush the loop to bring the circulating gas back up to 1.2 atm and repeat the process. I find this extreme but it can be done very safely.

When manually flying the CCR you are in complete control. Running the PO2 at a lower set point gives you more range when adding oxygen and breathing it down. At the 10 and 20 foot decompression stops I will run the PO2 up since I was running it lower on the dive. Even with the loop filled with 100% oxygen you cannot exceed 1.6 atm at 20 foot, so even without sensors the risk of an oxygen seizure is low. I do this routinely at decompression to validate the cells for high readings.

Everyone has to gauge their level of comfort on a rebreather… but remember backup systems are to abort the dive. They only work if you know how to use them. My preference of a backup system is to utilize an independent fourth cell and PO2 monitor, and fly the rebreather manually. The number one rule for CCR diving is “know your PO2.” With any failure the first option is bailout. The option to stay on the rebreather is a personal decision, and the skills necessary to do so must be practiced on a regular basis.