152m Song Hong – 02/05/2014
We arrived at the dive site around 0900 on Friday morning prepped and ready to see just how deep Song Hong sinkhole really was. We had spent the previous 3 days conducting warm up dives to 100m plus. Staging some shallow bail out cylinders along the way ready for any bail out scenario we may encounter from depth. The previous day was the day we were meant to go to 152m but due to a dry suit malfunction, we called the dive. It turned out a blessing in
disguise as the remaining team members with proper thermal attire were able to then stage our flush gas/bail out trimix gas on the 50 metre line, meaning all the cylinders were in position for the next days attempt to 152m.
After a final check on the equipment, validating the gases we will be using, and a quick boost to fill an O2 cylinder, we entered the water where we went over the dive plan one more time so it was very fresh in our minds. Wearing a dry suit, I prefer to get in the water to cool my head down and then take the time to don our deep bail out cylinders. With this type of diving, it’s imperative to be extra attentive to each detail of the dive and I find taking the time, keeps the nerves at bay and lessens the likelihood of omitting any relevant and important steps in getting back home safely.
Once the checks were completed, we descended to 6m for the final cell verification, and a final check that all our equipment was functioning properly. The cells were reading correctly but the pee valve in my suit was uncomfortable so needed to ascend to adjust that. Once back in the water, we gave the nod to the support diver that the time starts now and we descended following the cookies that our team had placed a few days before. We got down to the 21m mark, verified our 50/20 mixes with an
OK from our primary light and proceeded following the line till we arrived at our 18/48 mixes on the 50m line. The time we saved on the descent by not having to stage these cylinder here was priceless as it really made for a smooth journey down to depth. At the 60m mark, as discussed previously in our pre dive briefing, my team member moved to the side of the line and I moved ahead to lead us both down to 150m. We had previously both been down to 120m together so we were relatively familiar with the bottom contour to that point. Beyond was unknown, so based on our descent rate from the 120m dive we were able to determine roughly how much time we’d need to get from 120m to 152m. Due to the angle of descent, we were able to switch to the high Set Point of 1.1 at a shallower depth than normal, 80 m, knowing a O2 spike due to rapidly increasing ambient pressure would be unlikely.
Validating tie offs as we descended, we made it to 152m well within our planned time, mainly due to the fact we didn’t need to stage on our descent. As you can see from the attached profile, it was very smooth indeed. We had planned to arrive and turn the dive after 14mins, we arrived in 11mins allowing us valuable time to actually check out the landscape in one of the deepest caves in the world.
What we were greeted with was simply amazing; a real lunar landscape, the bottom filled with granite boulders and sand throughout the chamber. We ventured over to the 150m tie-off to check the angle of descent further into the cave for future reference, made a mental picture of it before turning to my team mate and agreeing at 13.30 minutes into the dive, lets turn it and start the long swim back.
Swimming back a little slowly for my liking, I signaled to speed it up a little. We arrived back at 90m well within our run time. At our first scheduled stop of 90m we both took turns to flush out the onboard 7/73 mix with our off board 12/55 mix. We then moved at a rate
of 3metres/min to our next waypoint of 50 meters where our 18/48’s were waiting. As we were a little early arriving at the 50m line, our support diver wasn’t waiting for us so we proceeded to don and flush with the 18/48’s. An efficient flush is important as we try to avoid an ICD hit by effectively diluting the new mix through the old before venting the excess through our mask/loop mouthpiece. Once we are satisfied the new mix is the one we are breathing in the loop, and have flushed out the old mix, we switch to the new gas mixes on our Shearwater Predator handsets and Petrel backups. This technique allows for a more efficient deco as we are adding a higher PPO2 and flushing out the inert gas that we simply do not want to be breathing while decompressing.
Once our support diver Chris had arrived, we both handed him our deep bailout mixes of 7/73 and 12/55 plus the off board S40 air filled cylinder I was carrying for my dry suit inflation. To maintain my buoyancy at that point, I plugged in my wing inflation cylinder I had mounted on my rig. We had only our 18/48’s with us and the release of the strain on our shoulders was very appreciated.
From that point, we moved to 45m where we conducted a nice long deep stop allowing several ceilings to clear and offering some much needed time for our bodies to release those micro bubbles of inert gas.
We then followed the ceilings on our handsets to commit our deco obligations up to 21 meters where we had one more flush to complete. From our 18/48’s we flushed with at 50m, to the 50% that was waiting for us, having been staged on the line the previous day. This went without incident and we moved up slowly following our ceilings to the habitat where we made the most of the metal platform at 12 meters by taking the weight off and laying down giving the body a much needed rest. At this point the loop was at PPo2 of 1.4 so
we needed to keep an eye on the CNS loading. We also enjoyed some chocolate bars and fluids from a camel pack, bought down to us from our very helpful support diver, Chris.
Making the most of the platforms at 12m, I stayed there until my 9 meter ceiling had cleared, before moving up to that level, and so on till we were at the last stop. I have found from previous deep diving on my Rebreather, that if I stay at the stop below my ceiling, my body feels a lot cleaner at the end with minimal after effects. I find it a technique that works well for me, but may not for others. The way I see it, if you are breathing a high PPO2 of 1.4, then if you stay a little longer at those deeper stops, it’s allowing those micro bubbles to stay smaller and more time for them to leave the body. Food for thought.
Overall, it was a successful dive and we will be planning to go deeper into the cave system in the next few months.