Scuba diver dies while exploring popular shipwreck, a third tragedy in the Florida Keys
- Thread starter cd95806
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
That is a cool table I have never seen before. Thank you. I noticed the date (1991) and wondered if there was more current research or data. I looked at the DAN site and, along with other info, DAN has the same data as the table.
The DAN article makes a distinction between open circuit and closed circuit exposure. There's these two bullet points:
- The U.S. Navy uses 1.3 ATA as the maximum limit in its closed-circuit rebreathers. Very long exposures, however, may put the diver at risk for some lung toxicity symptoms.
- The NOAA limit for nitrox diving at 1.6 ATA is 45 minutes for normal diving and 120 minutes for exceptional exposure diving.
For open-circuit scuba, the “green light” region is any oxygen partial pressure of 1.4 ATA or less (about 82 feet or 25 meters on a 40 percent oxygen mix). If you don’t exceed this level, the other limitations of open-circuit scuba diving will limit your exposure time to lengths where CNS oxygen toxicity is unlikely.
Between 1.4 and 1.6 ATA (99 feet or 30 meters on a 40 percent mix) is the “yellow light” region. The possibility of oxygen toxicity at 1.6 ATA is low, but the margin of error is very slim compared to 1.4 ATA. Individual variation, an unplanned depth excursion that causes an increase in oxygen partial pressure, and the possibility of having to perform strenuous exercise in an emergency raise the possibility of oxygen toxicity to levels where you should exercise caution. Levels of 1.5 to 1.6 ATA should be only for conditions where you are entirely at rest, such as during decompression. The dive team must still prepare for the possibility of an oxygen convulsion at these levels.
Above 1.6 ATA is the “red light” area. Recreational divers should not exceed this level. Even mild exercise may put divers breathing high-density nitrox mixes at increased risk. Open-circuit scuba divers can achieve durations likely to get them into trouble at these levels. Diving using these high partial pressures of oxygen should be for trained professionals who can weigh the risks and benefits and have the necessary training and support structure in place if an oxygen convulsion occurs.
I'll never forget an interaction I had on a dive boat, when I was a relatively new diver, many years ago. It was with a very experienced instructor... the kind of guy that everyone in my region knows by just his first name and holds himself out as an authority on all things scuba. I was using nitrox, went to set my computer to 1.4. This guy -- I will never forget this -- told me (and he wasn't joking) that if I set it to 1.6 the computer would "let me go deeper".
I thanked him for the advice and when he wasn't around to "help", I set it to 1.4. I'm naturally distrustful of anyone on a dive boat that too enthusiastically offers unsolicited advice, and of course years later I fully appreciate how terrible his advice was. But there are "experienced instructors" out there that sometimes offer terrible advice to new divers, and it's reasonable that new divers would follow the advice.
I get what you are saying and it's still misleading advice by the instructor given to @yle . A diver's NDL will not be impacted by the PO2 setting on their computer. The NDL is the primary limiting factor on a computer along with ascent rate. I think more experienced divers ought to mentor less experienced ones either when asked a direct question or when unsafe practice is overheard or witnessed.
But it is an incorrect statement because it includes the phrase "computer will let me". My computer does not "let" me to do anything and it does not prevent me from doing anything. My computer gives me information. I make my decisions.
My greatest concern from that interaction with the instructor was that he believed computers decide what divers can or can't do.
I included in my post his exact quote, that if I set my computer to 1.6 it would "let me to go deeper."
Maybe I'm unusual, but I have always believed (and have always taught my students) that computers don't "let" us to do anything, just as they don't prevent us from doing anything.
The instructor in my story genuinely believed -- i.e. the intent of his advice to me -- that if I set my pp02 to 1.4, then my computer would not "allow" me to go beyond the corresponding MOD... but if I set it to 1.6, then the greater MOD was fine because the computer allowed it. There was also a healthy dose of machismo included with his advice: the insinuation was that only newbie divers use 1.4, but the "real" divers use 1.6 (This didn't faze me, I already knew this guy was kind of a tool.)
I had enough common sense to understand the slippery logic he was using: that my computer would "allow" me to go as deep as I wanted if I just chose whatever MOD I wanted and then set the pp02 correspondingly high.
I thought that I would share a first hand experience of an oxygen toxicty event, so that someone may benefit.
Several years ago, I was doing a decompression dive to 180 feet on x17/20 (17% oxygen, 20% helium). On ascending to 110 feet (1.4), I switched to my low o2 deco gas which was x32. I was very slightly negative on the ascent and was about to kick gently up to the first decompression stop. Shortly after switching to the x32, I had some minor issue that I wanted to resolve. While solving the problem, suddenly, I saw fireworks in front of my eyes (visual disturbance, oxygen toxicity symptom), saw that I was at 130 something (1.6), and immediate switched back to the x17/20. Within a few seconds, the visual disturbance went away and I resumed the ascent after staying on the x17/20 for several minutes.
What happened was, as I was solving the minor problem, I had stopped kicking and was actually, very slowly descending. In addition to the fireworks, I recall being very agitated at the time (irritability another CNS toxicity symptom).
I routinely do 1.6 ata for deco and was actually surprised that this had happened. I attribute the increased sensitivity to the activity that I was undergoing to resolve the minor problem.
Please be familiar with the VENTID symptons. There is no guarantee that you will have a symptom or that you will recognize it if it does occur. But, if you do know the symptoms and you do have a sympton, you might at least know what is going on and ascend enough to recover. I was fortunate enough to have the x17/20 and to be familiar with the symptoms and also to have a symptom prior to a CNS convulsion.
Several years ago, I was doing a decompression dive to 180 feet on x17/20 (17% oxygen, 20% helium). On ascending to 110 feet (1.4), I switched to my low o2 deco gas which was x32. I was very slightly negative on the ascent and was about to kick gently up to the first decompression stop. Shortly after switching to the x32, I had some minor issue that I wanted to resolve. While solving the problem, suddenly, I saw fireworks in front of my eyes (visual disturbance, oxygen toxicity symptom), saw that I was at 130 something (1.6), and immediate switched back to the x17/20. Within a few seconds, the visual disturbance went away and I resumed the ascent after staying on the x17/20 for several minutes.
What happened was, as I was solving the minor problem, I had stopped kicking and was actually, very slowly descending. In addition to the fireworks, I recall being very agitated at the time (irritability another CNS toxicity symptom).
I routinely do 1.6 ata for deco and was actually surprised that this had happened. I attribute the increased sensitivity to the activity that I was undergoing to resolve the minor problem.
Please be familiar with the VENTID symptons. There is no guarantee that you will have a symptom or that you will recognize it if it does occur. But, if you do know the symptoms and you do have a sympton, you might at least know what is going on and ascend enough to recover. I was fortunate enough to have the x17/20 and to be familiar with the symptoms and also to have a symptom prior to a CNS convulsion.
The computers decide what to do or not to do based on their programming and the mathematical model they use. No one believes that the computer will stand in your way to stop you from doing what you want to do. If you are following what the computers says of what to do and when to do it, etc. then the computer is the controller of its part of the dive. If you chose not to follow its instructions or the information it gives you, then you are on your own. At any rate, this is discussion is semantics and is silly. The end result is that setting your computer to a PPO2 limit of 1.6 will enable you to have a deeper MOD than setting it to 1.4. The instructor you are referring to did understand what the PPO2 limit does and how it effects your diving depth, MOD.
Similar threads
