ianr33
Contributor
1.6 is a pretty standard maximum for pO2 during deco. Thinking is you are relatively shallow (low gas density) and will not be exerting yourself. Most divers will not use this for longer than about 15 minutes at a time.
Nitrox table vs computer question
- Thread starter Lizard Leg
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1.6 is a pretty standard maximum for pO2 during deco. Thinking is you are relatively shallow (low gas density) and will not be exerting yourself. Most divers will not use this for longer than about 15 minutes at a time.
waterpirate
Contributor
The risk assesment for a given po2 is a personal one best addressed with a qualified instructor or mentor. Takeing the "online offerings" really does not address this. My risk assesment would start with your dive count if accurate. How much experiance do you have diving below 60fsw? My advice to you is to dive to depths that the po2 is not an issue untill you get some more experiance and mentoring. Then move into deeper water with a better foundation of understanding and experiance to support you.
Eric
Eric
If I were doing the dive and only 32% was available the planned may depth would no longer be 120. For any reason. 110 max. Mix for the bottom or adjust the planned depth. I personally use 1.3 for working portion of dives now. Add more risk factors - heavy exertion, cold water, etc then that gets more conservative. Getting ready to teach a TDI nitrox class in about a half hour. It is something we spend a bit of time on. Best mix, adjusting mixes and MOD's for conditions, and keeping track of the relative risks and adjusting for them.
Its interesting you do this Jim. While in a class, I had brough up the idea of adjusting down from 1.4 in the Great Lakes for the "less than ideal" conditions. It was glossed over an "unnecessary" conservatism (not that it was a wrong approach, but more as "belts and suspenders")...
It is, however, something I adopted...
It is, however, something I adopted...
Interesting thread...
Here are a few random thoughts, in no particular order.
MOD means Maximum Operating Depth, not Optional Operating Depth. When a real Nitrox diver sets MOD, it means something. i.e. if the MOD is 111 feet, that's the floor. <-- notice that that's a period, not a comma
The original Nitrox course taught by Dick Rutkowski from NOAA was two full days of classroom, delving deeply into all things physiological and physical known about oxygen under pressure, not only as a breathing gas, but how to handle and mix it to get Niotrox in the bottle. Many of today's watered down courses fail to infuse a sufficient appreciation for the dangers of oxygen toxicity, and barely give any passing acknowledgement to the dangers of oxygen handling, if any at all. This isn't necessarily the fault of the instructors... their own training is all too often shoddy.
The setting for max safe PO2 is highly variable and depends on many factors including time, temperature, workload, prior exposure, dive profile, OTC and/or prescription drugs and others. Even then the max safe PO2 for a given diver is highly variable from day to day. Few divers these days have any training in factoring these considerations into a dive plan and should strictly adhere to the recommended maximum PO2 of 1.4ATA (or less) when computing MOD. Indeed, most of today's Nitrox divers don't even have any idea what an OTU is...
As for the OP's original questions, (1) a computer will usually give the diver longer bottom times and shorter surface intervals than the tables, and (2) with a planned depth of 120ft, and a PO2 max of 1.4, a Nitrox mix of 30% O2 or less is required. If the diver elects to consider maximum PO2's of more than 1.4 under any circumstances, due diligence dictates taking the "long course" (Advanced Nitrox or Tech Foundations or equivalent) at a minimum.
Rick
Here are a few random thoughts, in no particular order.
MOD means Maximum Operating Depth, not Optional Operating Depth. When a real Nitrox diver sets MOD, it means something. i.e. if the MOD is 111 feet, that's the floor. <-- notice that that's a period, not a comma
The original Nitrox course taught by Dick Rutkowski from NOAA was two full days of classroom, delving deeply into all things physiological and physical known about oxygen under pressure, not only as a breathing gas, but how to handle and mix it to get Niotrox in the bottle. Many of today's watered down courses fail to infuse a sufficient appreciation for the dangers of oxygen toxicity, and barely give any passing acknowledgement to the dangers of oxygen handling, if any at all. This isn't necessarily the fault of the instructors... their own training is all too often shoddy.
The setting for max safe PO2 is highly variable and depends on many factors including time, temperature, workload, prior exposure, dive profile, OTC and/or prescription drugs and others. Even then the max safe PO2 for a given diver is highly variable from day to day. Few divers these days have any training in factoring these considerations into a dive plan and should strictly adhere to the recommended maximum PO2 of 1.4ATA (or less) when computing MOD. Indeed, most of today's Nitrox divers don't even have any idea what an OTU is...
As for the OP's original questions, (1) a computer will usually give the diver longer bottom times and shorter surface intervals than the tables, and (2) with a planned depth of 120ft, and a PO2 max of 1.4, a Nitrox mix of 30% O2 or less is required. If the diver elects to consider maximum PO2's of more than 1.4 under any circumstances, due diligence dictates taking the "long course" (Advanced Nitrox or Tech Foundations or equivalent) at a minimum.
Rick
iopturbo
Registered
It's just one of those things you don't know your limit until it's too late so it's better to not find out. I am going to try and get in a chamber in the next few weeks and see what my threshold is. We do a lot of deep dives around with high work loads(spearfishing) so I'm curious what I can handle.
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Lizard Leg,
Yes, do the nitrox training.
We do 70+ minute dives to 100+ feet using a nitrox computer.
The important thing to understand is the nitrox computer is tracking both your nitrogen and O2 load.
Yes, I have hit 1.6 ppO2. However, when I do I know that I can maintain my buoyancy and stay within my Max Operating Depth ( MOD ).
Also, realize O2 toxicity is a function of depth & time.
The maximum single exposure limits recommended in the NOAA Diving Manual are 45 minutes at 1.6 bar, 120 minutes at 1.5 bar, 150 minutes at 1.4 bar, 180 minutes at 1.3 bar and 210 minutes at 1.2 bar.
One question I have for you is: Why are you planning a dive to 120' with less than 24 dives under your belt?
Maybe it is just a math exercise.
We have most of the divers that we train become nitrox certified right away. However, they dive within their skill limit. They are not even close to their MOD.
After the day of diving my computer usually shows around a 10 - 20% O2 load. At 50%, you should be concerned.
Dive 32% at around 60' - 80'. You will be well within the MOD and within your experience " comfort " level.
You still will have the benefit of less nitrogen loading. It's nice to look at your nitox computer and see a No Stop time of 50 minutes, instead of approaching Deco.
In a chamber study done at 3.1ATA PO2 until OXTOX symptoms occurred, subjects were exposed every day for 30 days. Time-to-symptoms varied from 5 minutes to over 2½ hours!
The takeaway here is that seeing "what I can handle" in a chamber won't give a diver a very useful data point, and since, when it comes to OXTOX, as TS&M says,
it's best to err towards conservatism, then go a few more standard deviations in the conservative direction on top of that.
Rick
Here's a good article on the subject, for those who'd like to delve a bit deeper...
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OP
Lizard Leg
Contributor
It is basically a math exercise to try get my understanding up. The dive planned is the deep dive portion of our AOW, with an instructor we love, diving the Thunderbolt Sept. 30. Dive is planned for air, so very minimal bottom times and he has said max will probably be about 110'. However, knowing that the hard bottom is 120' (might want to go look at the prop!) I was trying to calculate, with my SAC and with various tanks, my estimated bottom time, both on air and nitrox, allowing for a 1 minute stop at half max depth, and 3 minutes at 15, with a 30fpm ascent, with both 500psi at surface and rule of thirds.
It's pretty much an exercise to better understand how to plan my dives. The more I dive and the more experience I gain the easier it will be to get accurate plans I know.
If I was planning this as an actual dive, I would be diving EAN30 (121MOD, PPO2 1.39, EAD 102.57) or I would be diving air. I don't like pushing limits too far but i was just curious from hearing about other divers who regularly push the 1.4 - not into 1.6 or higher, but they crack the edge a little bit here and there. If we had the dive profile planned for a descent straight to 120fsw for 5 minutes, then ascending to 110fsw for a few minutes, then to 100, then to 90, etc. I was trying to get a clearer understanding of the physiological difference of 5 minutes at 1.48ppo2 then 30 minutes at 1.39 and lower, vs 35 minutes at 1.39 or lower.
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