I'll go through the thread and see what I missed - thanks. My friend mentioned swimming away from the wall in a down current situation - perhaps this is also mentioned in the thread. When I first started diving I read every accident and incident report here, and read Diver Down and other books - just so I could figure out what NOT to do, what TO do in situations that I might not otherwise think about - and how to be prepared. I remembered that you are to swim away from the wall, but it's good to have that reinforced because I'm guessing my lizard brain would be saying "JUST GRAB ONTO SOMETHING AND CLIMB TO THE TOP" and it doesn't sound like that's the best course of action. I guess a comparison would be what to do in a rip tide, or how to handle your car when sliding on snow/ice. Doing what seems natural isn't always the 'right' thing to do in order to survive, and if/when something like that happens you have to be able to override panic and recall that info in the moment. That's a tall order since this isn't a skill we can practice like mask-removal or air sharing.
Oxygen Toxicity risk with Nitrox?
- Thread starter Lriemann
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The dive described here raises two very common safety issues that I think are more important than the specific question of how much risk of oxygen toxicity the OP exposed himself to:
1. Safety margins. Too many divers tell themselves that it's okay to exceed limits because there are safety margins built into tables, MOD's, and computer algorithms. Lots of people exceed this or that limit without dying, their "logic" tells them; therefore, it's okay to rely on some alternate consideration (e.g., "My computer has a conservative algorithm," or, "I'm in better shape than most recreational divers") to replace the textbook or planned limit with what ends up being no hard limit at all. Once you've decided that your planned limit isn't really a limit, you're totally winging it and choosing to accept unevaluated risks.
The whole point of safety margins is that we do not know when our physiology and the characteristics of our particular dives will put us in a position where we need some or all of those built-in safety margins to keep us from experiencing oxygen toxicity or a DCS hit or some other adverse outcome. Therefore, we want to keep the entire safety margin available to us for those occasions when we need it. We'll never know if or when that safety margin actually prevent an incident, but we nevertheless want the whole safety margin in the bank for us on every dive.
2. Dive planning on deep dives. OP's question was about oxygen toxicity, but the more pressing safety issue to me was lack of dive planning. When I teach the deep dive in the AOW course, I require my students to perform a number of cognitive and motor skills tests at depth in order to convince them that, irrespective of whether they "feel" narced, their brains do not function as well at 30 meters as they do at the surface. I endeavor to hardwire into their consciousness that they will alter the dive plan on a deep dive only in a more conservative direction because they know their judgment will be impaired at depth. I teach them that any "great ideas" that occur at depth to add challenges to the dive plan cannot be trusted and must be ignored. Diving deep with a non-existent plan and then deciding while narced to go deeper than whatever vague pre-dive intention was used as a substitute for a plan are much more serious safety concerns than whether recreational divers should use a max PPO2 of 1.4 or 1.6.
1. Safety margins. Too many divers tell themselves that it's okay to exceed limits because there are safety margins built into tables, MOD's, and computer algorithms. Lots of people exceed this or that limit without dying, their "logic" tells them; therefore, it's okay to rely on some alternate consideration (e.g., "My computer has a conservative algorithm," or, "I'm in better shape than most recreational divers") to replace the textbook or planned limit with what ends up being no hard limit at all. Once you've decided that your planned limit isn't really a limit, you're totally winging it and choosing to accept unevaluated risks.
The whole point of safety margins is that we do not know when our physiology and the characteristics of our particular dives will put us in a position where we need some or all of those built-in safety margins to keep us from experiencing oxygen toxicity or a DCS hit or some other adverse outcome. Therefore, we want to keep the entire safety margin available to us for those occasions when we need it. We'll never know if or when that safety margin actually prevent an incident, but we nevertheless want the whole safety margin in the bank for us on every dive.
2. Dive planning on deep dives. OP's question was about oxygen toxicity, but the more pressing safety issue to me was lack of dive planning. When I teach the deep dive in the AOW course, I require my students to perform a number of cognitive and motor skills tests at depth in order to convince them that, irrespective of whether they "feel" narced, their brains do not function as well at 30 meters as they do at the surface. I endeavor to hardwire into their consciousness that they will alter the dive plan on a deep dive only in a more conservative direction because they know their judgment will be impaired at depth. I teach them that any "great ideas" that occur at depth to add challenges to the dive plan cannot be trusted and must be ignored. Diving deep with a non-existent plan and then deciding while narced to go deeper than whatever vague pre-dive intention was used as a substitute for a plan are much more serious safety concerns than whether recreational divers should use a max PPO2 of 1.4 or 1.6.
The subject of the thread was not downcurrents; what I was referring to was the fact that oxygen toxicity does not happen immediately after a MOD violation. It takes time.
As long as we are talking about it, though, many times a downcurrent is the equivalent of a waterfall. It can be impacted by the terrain. Just as a waterfall is created by water moving through a channel and over an edge, current moving through a channel on the reef and over the edge can cause a downcurrent, and in such a case,moving laterally along the wall can get you out of the channel. That channel can create a funnel effect.
As I said earlier, I think the risk of down currents in places like Cozumel is exaggerated, in part because of false reports. Here are two examples.
- A dive shop owner, one of her DMs, and her boyfriend had a serious incident including significant DCS (she eventually died, and the DM was crippled), and they reported that they had been caught in a down current and taken to 300 feet and beyond. People were panicking--I'm never going to Cozumel if that kind of thing can happen! Well, it turned out to be a total lie to hide the fact that they had intentionally dived to 300 feet, at which point the dive shop owner either passed out or was so narced that she went to 400 feet before being turned around by the DM. Since they were only diving single tanks, they had to surface without doing any decompression stops.
- In a very widely viewed video, a young and inexperienced boy was wearing a GoPro camera on a helmet, and he went out of control on his descent and only stopped descending when the father got to him and put air in his BCD. It was blamed on a downcurrent, but that was BS. The camera shows the boy was plummeting for sure, but it shows him plummeting past divers who were somehow not being affected at all, and his father had to swim hard to get to him and turn him around. Turning him around was accomplished by adding air to the BCD. It sure looked to me like the overweighted youth had simply forgotten to add air to the BCD and panicked. Once again, everyone commented on how dangerous it is to dive in Cozumel because of those nasty downcurrents.
Thanks for this. After taking the time to read through the thread I feel better about the amount of time I could spend below my MOD if I was caught in a downcurrent - and less worried about a downcurrent in Cozumel. I'm a rule follower. I will always watch my computer and do my best to avoid a deco stop (yes, I had an accidental deco obligation, but I blame it on a conservative backup dive computer while my primary was not conservative - I should have been watching both but I was not) - and I won't intentionally go anywhere my MOD on a dive.
Alaskan Scuba Dude
Banned
In March 2000, six students and an instructor dived using open-circuit scuba in a narrow pass and were swept by a strong current to a depth of 90 metres' sea water. Three died and four were injured, which makes the incident the worst diving accident in New Zealand history. The group was on an officially-sanctioned course with many factors contributing to the final tragic events. The dive is described and the medical response examined. The legal consequences are reported and their implications for diver training and employment are discussed.
Keep reminiscing about romantic nostalgia and how you have rubbed shoulders and corresponded with whomever...
Reference ?
When I plan a nitrox blend I plan it to the sand even I don't plan to go that deep, just a extra margin of safety just in case.
Have I violated 1.6, yes.....
Planned the dive dove the plan. We knew that we had severe current close to the surface so ascent and descent on the line.
Did the dive while down surface surge came up so when we hit the 20 foot stop and switched to pure O2 all of sudden we were getting tossed up and down like rag dolls, 15 to 25 feet.
So switched back to backgas. Changed grip on line to maintain 20feet and back on O2 changed stop to account for that less than a minute of tossing and finished stop.
Wasn't over 1.6 for more than a few seconds.
Have I violated 1.6, yes.....
Planned the dive dove the plan. We knew that we had severe current close to the surface so ascent and descent on the line.
Did the dive while down surface surge came up so when we hit the 20 foot stop and switched to pure O2 all of sudden we were getting tossed up and down like rag dolls, 15 to 25 feet.
So switched back to backgas. Changed grip on line to maintain 20feet and back on O2 changed stop to account for that less than a minute of tossing and finished stop.
Wasn't over 1.6 for more than a few seconds.
All I can find on this is the very brief abstract of an analysis that you quoted in you post. That abstract is frequently cited, but the full analysis is unavailable.
The incident happened in French Pass, and this is what Wikipedia says about it:
French Pass has the fastest tidal flows in New Zealand, reaching 8 knots (4 m/s).[1] When the tide changes, the current can be strong enough to stun fish.[2]
Here is what Wikipedia says about the incident there:In 2000, student divers taking part in a drift dive during the local ebb flow were separated from their surface float and caught in a whirlpool. This dragged them into "Jacob's Hole", a 68-metre (223 ft)[12] deep depression south west of the pass. The depth of this descent resulted in multiple fatalities. The group appears to have been drawn deep into the hole and then returned to the surface again. A dive computer record of one of the survivors show a depth of up to 89-metre (292 ft). According to the coroner's report, the accident occurred on a falling tide, so the current was flowing from the south west to the north east.
I am not sure of the point of your post. Are you saying that we should never dive nitrox because we might be swept to 300 feet in a whirlpool, we should never dive in French Pass, New Zealand (one of the most dangerous dive sites in the world during a current switch), or that something like that can happen anywhere else in the world as well?Similar threads
