Its early november when a group of divers visit the Lot region in southern France for some cave diving. Cars loaded with tanks and gear they arrive at the guesthouse and unload all but the necessary gear for the next day. The tanks for the next day are again analysed, labeled and stages marked with their corresponding MOD and divers initials.
After a good night of sleep the group travels to the Ressel, one of the most popular caves in the region. Weather is good, the parking lot empty and the river very calm - a perfect cave diving day. The plan is to dive with trimix 18/45 backgas, a EAN32 travel stage and a EAN50 deco stage. On the way in, diving through the deep tunnel, the 32% is breathed, the deco gas dropped at 21 meters and a switch to backgas at 30m where the 32% is dropped too. On the way back the 32% is clipped on, but not used and at 21 meters the 50% deco gas is picked up and deployed for the exit out of the Ressel again.
The dive starts without any problems, although visibility in the first minutes isnt perfect. At the second T, at a depth of 21 meters, the 50% deco gas is dropped first and just after that, before the shaft descents to 30 meters, the gas switch is made and the 32% is dropped too. Due to an error of the first diver who thought the shaft would drop too far for the gas to be breathable, the switch is made already before the descent. Now diving on backgas the divers decent further to a maximum of 48 meters and turn the dive at almost 600 meters into the cave.
On the way back the travel gas cylinder is picked up as planned and just beyond also the EAN50 deco gas is clipped on and deployed. All is well and the divers do a very conservative deco along the way to the exit. At 6m some time has to be spend waiting for the last stop to clear and after which the final meters back into the river are swum.
Back at the surface all divers are happy with the dive and enjoyed the trip into the deeper section of the cave. The rest of the day is spent relaxing in the French guesthouse. A couple more dives are made during the following days and the divers return home safely and happy with their experience.
Its a couple of days after the trip when some of the tanks are to be topped up. One of the stages marked with an MOD of 30m and an analysis label showing 32-ish% of O2 is tested for pressure and oxygen content before being filled. The analyser is connected and the valve opened when the O2% starts to rise on the display quickly from 21% to 30%, 40%, 45%... and stabilizes at almost 50% O2 Another look on the labels and a check with a different analyzer verified the mistake: the tank labeled as EAN32 contained EAN50.
This being the tank dated for the day of the deep Ressel dive, meant it had been used as travel gas and the premature gas switch at 21 meters might have saved the day. Had the tank been used to the pre-planned point at 30 meters, its PPO2 would have been 2.0 after slowly increasing on the way in over half an hour of swimming. Another analysis showed that the other tank this diver had been using as deco gas was indeed marked as EAN50, labeled at a MOD of 21 meters and contained not 50%, but 32% O2. There was only one possibility: the tanks were swapped when analysing and labeling!
Discussion afterwards with the involved divers lead to the point of failure: both stage tanks were analysed immediately after another, oxygen content written on tape and applied to the wrong tanks. Because all equipment was assembled and tanks were analysed on the night before the dive, no repeated analysis at the dive site was performed. All in all, the mistake had gone unnoticed and the dive had been performed with EAN50 on the way in and EAN32 as deco gas. Conservatism saved the day and a lesson was learned: dont analyse more than one tank at a time and dont get distracted while analysing!
After a good night of sleep the group travels to the Ressel, one of the most popular caves in the region. Weather is good, the parking lot empty and the river very calm - a perfect cave diving day. The plan is to dive with trimix 18/45 backgas, a EAN32 travel stage and a EAN50 deco stage. On the way in, diving through the deep tunnel, the 32% is breathed, the deco gas dropped at 21 meters and a switch to backgas at 30m where the 32% is dropped too. On the way back the 32% is clipped on, but not used and at 21 meters the 50% deco gas is picked up and deployed for the exit out of the Ressel again.
The dive starts without any problems, although visibility in the first minutes isnt perfect. At the second T, at a depth of 21 meters, the 50% deco gas is dropped first and just after that, before the shaft descents to 30 meters, the gas switch is made and the 32% is dropped too. Due to an error of the first diver who thought the shaft would drop too far for the gas to be breathable, the switch is made already before the descent. Now diving on backgas the divers decent further to a maximum of 48 meters and turn the dive at almost 600 meters into the cave.
On the way back the travel gas cylinder is picked up as planned and just beyond also the EAN50 deco gas is clipped on and deployed. All is well and the divers do a very conservative deco along the way to the exit. At 6m some time has to be spend waiting for the last stop to clear and after which the final meters back into the river are swum.
Back at the surface all divers are happy with the dive and enjoyed the trip into the deeper section of the cave. The rest of the day is spent relaxing in the French guesthouse. A couple more dives are made during the following days and the divers return home safely and happy with their experience.
Its a couple of days after the trip when some of the tanks are to be topped up. One of the stages marked with an MOD of 30m and an analysis label showing 32-ish% of O2 is tested for pressure and oxygen content before being filled. The analyser is connected and the valve opened when the O2% starts to rise on the display quickly from 21% to 30%, 40%, 45%... and stabilizes at almost 50% O2 Another look on the labels and a check with a different analyzer verified the mistake: the tank labeled as EAN32 contained EAN50.
This being the tank dated for the day of the deep Ressel dive, meant it had been used as travel gas and the premature gas switch at 21 meters might have saved the day. Had the tank been used to the pre-planned point at 30 meters, its PPO2 would have been 2.0 after slowly increasing on the way in over half an hour of swimming. Another analysis showed that the other tank this diver had been using as deco gas was indeed marked as EAN50, labeled at a MOD of 21 meters and contained not 50%, but 32% O2. There was only one possibility: the tanks were swapped when analysing and labeling!
Discussion afterwards with the involved divers lead to the point of failure: both stage tanks were analysed immediately after another, oxygen content written on tape and applied to the wrong tanks. Because all equipment was assembled and tanks were analysed on the night before the dive, no repeated analysis at the dive site was performed. All in all, the mistake had gone unnoticed and the dive had been performed with EAN50 on the way in and EAN32 as deco gas. Conservatism saved the day and a lesson was learned: dont analyse more than one tank at a time and dont get distracted while analysing!
