The importance of both regularly changing O2 cells and carrying out in-dive validation was evident made on a recent mixed gas dive in Anilao. The dive in question was done using a JJ-CCR with 3 cells that were just over 16 months old – normally I’d only use cells under, or very close to, 12 months – however due to a ‘technical issue’ (aka forgetting to bring the counterlungs for my HH unit!) I chose to use the available JJ over the other option of a 6 hour round trip to Manila to pick-up the HH counterlungs. I should also point out here I am certified to use a JJ and it is a unit I’m familiar with :-)
The dives were originally scheduled for July with some our Trimix course graduates, however we were forced to reschedule when typhoon “Falcon” (International name “Chan-hom”) passed close to the East coast of the Philippines, bringing with it rough conditions to the West coast. Even with the dive entry direct from shore we opted it was better to delay the dives to another time rather than push through with a long dive while the waves were getting bigger by the hour.
Picking the right conditions for the dives and finding a time in which we were all available (myself, a course director based out of Cebu and an instructor based out of Manila/Anilao) brought us to Friday 14th to Sunday 16th August to complete the dives and extend the Acacia deep line a little further.
All gases were already blended and ready from the previous planned dives in July and we had a perfect window with tides to complete the dives. Given the circumstances and the fact that I’d previously used cells up to 18 months on different units with no problems I felt the risks involved with using the older cells could be countered with the caveat that I’d perform regular validation and verification checks on the dive and bail-out if I did not feel comfortable with any results.
Having completed all pre-dive checks on the JJ all seemed fine, the unit was already reading 0.21/0.22/0.21 on all cells before calibration, during calibration the 3rd cell seems a little slow to react compared to cells 1 and 2, checking the mV reading it was showing just under 43mV for O2 which seemed a little low (the other 2 cells were reading around 49 and 48mV) – given the slow reaction I decided to recalibrate the unit and found the same results, all cells also returned down to linearity when exposed to air (0.22, 0.21 and 0.22 as I recall). Another set of pre-checks followed and the unit seemed ready to dive.
The output from the primary handset is shown below:
During the first dive our planned depth was 110m, the aim being to confirm the line was still in place and ready the end spool for a extension down to 130-140m the following day. With the lower mV reading on cell 3 I wanted to check linearity at a PPO2 of 1.4 and above so confirmed all 3 cells could reach 1.6 PPO2 at the start of the dive (line 0 on the shearwater output). During the initial descent (between lines 0 and 1 on the shearwater output) I noticed the third cell was consistently a little lower than the other two, but just inside the 20% deviation needed to be voted out by the Shearwater electronics – I normally fly all rebreather in manual mode with the electronics set as a back-up at 0.7 or 1.0 PPO2 in most cases so this wasn’t of immediate concern, however I wanted to check it stabilised for decompression info during the dive.
The plan was to use scooters to make the initial swim easier (and quicker for the divers using OC) from shore to the steep drop-off (around 100m distance from shore), we dropped the scooters at the top of the steep drop off at a depth of 60m, this gave me a chance to validate the cell readings – adding O2 to spike the PPO2 to above 1.4 now resulted in the 3rd cell, previously showing the 20% lower PPO2, now jumping to a PPO2 of 1.7 while the other two cells showed 1.4 (line 1 on the Shearwater output) – I was worried at this point that it could be that the other two cells were faulty and the 3rd cell the most accurate, however adding diluent to bring the PPO2 back down showed the 1st and 2nd cells reacting much faster while the 3rd cell didn’t seem to change, responding very slowly and much less than would be expected for the amount of diluent added to the vented loop. By this time I was almost certain the third cell was at fault and cells 1 and 2 were giving me correct reading, to confirm this I vented the loop and filled with diluent through the descent to 110m, checking the PPO2 did not fall below the diluent PPO2 for that depth (I was using a 9/65 diluent so checking the PPO2 didn’t fall below 0.1 x the pressure works as a quick check – the wonder of metric units!).
Through the descent cells 1 and 2 behaved as would be expected, but yet the 3rd cell continued to react slowly and tended to ‘stick then drop’ rather than a constant drop of PPO2 that occurred each time diluent was added on cells 1 and 2. This well and truly confirmed the 3rd cell faulty, I just wanted to confirm the 1st and 2nd cells were working as planned. To err on the side of caution I kept the 3rd cell reading below a PPO2 of 1.6 and let cells 1 and 2 drop to a PPO2 of 0.8/0.9 – the solenoid was set to fire were set to kick-in at a PPO2 0.7 and given the depth being over 100m at this point I wasn’t too worried about the lower PPO2 until the cells could be verified (my back up computer was set to a constant PPO2 of 1.0 until the ascent). Once we had completed the adjustment of the spool and set-it up for the next dive I made a diluent flush on the ascent to check the cells responded, the PPO2on cells 1 and 2 rose slightly to just over 0.9 (very slightly lower than the actual diluent set-point at that average depth), while the 3rd cell showed the PPO2 increasing well above 1.6 when additional diluent was added. I was now confident cells 1 and 2 were giving good information, and that cell 3 was giving completely wrong information – by this time the 3rd cell was voted out (as it had been since the 60m check on the descent) so the primary handset was controlling everything as planned.
The ascent continued as planned (with one delay at around 65m to adjust and re-secure the existing line placement), I wanted to keep an average PPO of 1.2 above 60m on the ascent so did this manually using the readings from cells 1 and 2. The 3rd cell was still way above the readings on cell 1 and 2, until I carried out another cell check at 36m by adding O2 to spike the PPO2 above 1.4 (line 3 on the Shearwater output), at which point cells 1 and 2 spiked as planned while the 3rd cell only increased slightly in PPO2 (bringing it back into 20% of the average and therefore no longer being voted out). The next few deco stops then see the PPO2 of cell 3 drop slower than in cells 1 and 2, leading to cell 3 hovering just under 20% above the cell 1 and cell 2 reading – ie not voted out(between lines 3 and 4 on the Shearwater output).
To make sure the output of cells 1 and 2 wasn’t voltage limited I spiked the PPO2 back up to above 1.4 at 24m (line 4 on the Shearwater output) which brought all 3 cells close to 1.6 again, after this point the cell reading stayed quite close, until the cell 3 PPO2 suddenly dropped at around 73 minute of run time, I did another small PPO2 spike to check cells 1 and 2 weren’t frozen but found both responded as expected while cell 3 had almost frozen at a lower PPO2. The lower starting mV reading for cell 3 meant that once again it was just hanging in there at 0.25 below the other cells and was not voted out again until around 10 minutes later when it dropped more than 20% outside the cell 1 and 2 readings.
By the end of the 6m stops on both computers I now had two quite different decompression profiles, my primary handset was showing over 40 minutes (I think it was 43) at 3m while the back-up (also a petrel but set to a fixed PPO2 of 1.0 initially then 1.2 after 60m on the ascent) was showing just 23 minutes. Given the problems with cell 3 and the fact the cells were older I thought it was best to finish off the dive on OC O2 which also brought down the total time needed at the final 3m stop (I switched to OC bail-out on 100% O2 a line 7 of the Shearwater output, the majority of the OC stop was then done at 4.5m as you can see). After switching to OC I vented the loop via the counterlung OPV dump (line 8 on the Shearwater output), the JJ ADV is very sensitive and a little diluent entered the loop as I was venting - as soon as this happened I noticed that the reading for cell 3 increased suddenly while cells 1 and 2 dropped as expected. Even during the OC part of the dive you can still see that the 3rd cell reading jumps around a lot.
We finished the dive by clearing both computers on OC bail-out (both were petrel controllers dived at GF 25/85), I was just a little surprised how much impact the faulty 3rd cell appeared to have on the decompression times – this seems to be an ‘unlucky’ profile as the 3rd cell seemed to have a funny habit of landing about 19.5% away from the other cells so didn’t quite get voted out.
Obviously there is a very high chance that all of this could probably have been avoided by having fresh cells after 12 months, however the way the cell behaved was something I had not seen before while ‘standard’ pre-dive checks and a validation PPO2 of 1.6 at 6m at the end of the dive wouldn’t have shown anything out of the ordinary. The fact the cell behaved normally, then gave an artificially high reading at depth and an artificially low reading at mid-range before returning to near alignment in the shallows and finishing off with any slight change in PPO2 (increase or decrease) being exaggerated was a new one for me.
In summary two lessons learned, don't use cells past the recommended 12 months and secondly never under estimate the importance of in-dive cell validation through out the dive - hitting 1.6 at 6m does not validate the whole dive!
