Repetitive Nitrox Dives and O2 Clock

[jiveturkey]

Ok, I've done a bunch of searches and can't really find what I'm looking for. I did my nitrox cert a year ago but haven't had a chance to do much nitrox diving so I'm still a newbie. My diving is purely recreational.

This past weekend I did some dives (EANx34) and was reviewing my materials. Not that I was even close, but it says in the Padi manual that you cannot exceed 100% O2 exposure in 24 hours. What about 25 hours? How long do you have to wait to dive again after you get to 100% from repetitive dives? I realize that it's tough to get to 100% exposure but it's certainly possible if you were doing some liveaboard diving of something of that nature.

I've found some references on SB to NOAA recommendation but can't find anything specific on their site. Does anyone know a link? I've also seen some discussions on here regarding the half life of o2. Any links for more information on this?

 

[captndale]

Ok, I've done a bunch of searches and can't really find what I'm looking for. I did my nitrox cert a year ago but haven't had a chance to do much nitrox diving so I'm still a newbie. My diving is purely recreational.

This past weekend I did some dives (EANx34) and was reviewing my materials. Not that I was even close, but it says in the Padi manual that you cannot exceed 100% O2 exposure in 24 hours. What about 25 hours? How long do you have to wait to dive again after you get to 100% from repetitive dives? I realize that it's tough to get to 100% exposure but it's certainly possible if you were doing some liveaboard diving of something of that nature.

I've found some references on SB to NOAA recommendation but can't find anything specific on their site. Does anyone know a link? I've also seen some discussions on here regarding the half life of o2. Any links for more information on this?

The usual recommendation is to use a 90 minute half life for your O2 CNS exposure. That would mean, for example, that if you ended your first dive with 60% you would be at 30% after a 90 minute surface interval.

 

[neil]

Capt. Dale is correct on the half-time usage, but if you are just going by NOAA guidelines, which the PADI course uses, here's the rules:



"A series of repetitive dives may be conducted without a normoxic interval between dives if the sum total of the O2 partial pressure duration limit for all the dives does not exceed the Maximum Single Exposure Limits.


If one or more dives within a 24-hour period have reached or exceeded the limits for a normal single exposure, the diver must spend a minimum of 2 hours at a normoxic (air) PO2 before diving again. If one or more dives within a 24-hour period have reached the Maximum Total 24-Hour Day Limits, the diver must spend a minumum of 12 hours at a normoxic PO2 before diving again."

Remember there are TWO limits: the single exposure limit and the 24-hour maximum exposure.

The PADI materials don't seem to give any guidance on reaching O2 clock limits, maybe I missed it

Hope that helps.

 

[ScubaSixString]

Ok, I've done a bunch of searches and can't really find what I'm looking for. I did my nitrox cert a year ago but haven't had a chance to do much nitrox diving so I'm still a newbie. My diving is purely recreational.

This past weekend I did some dives (EANx34) and was reviewing my materials. Not that I was even close, but it says in the Padi manual that you cannot exceed 100% O2 exposure in 24 hours. What about 25 hours? How long do you have to wait to dive again after you get to 100% from repetitive dives? I realize that it's tough to get to 100% exposure but it's certainly possible if you were doing some liveaboard diving of something of that nature.

I've found some references on SB to NOAA recommendation but can't find anything specific on their site. Does anyone know a link? I've also seen some discussions on here regarding the half life of o2. Any links for more information on this?

The 24 hour thing might also have to do with PULMONARY O2 tox. (the irritation of the lungs type)

The half life of oxygen might keep you from CNS ox tox but the lungs might not respond well to repeated exposure to high levels of oxygen. Also remember the 24 hour thing is a rolling limit. if you have dive 1 ending at 9am that takes up 15% of your clock. Dive 2 ending at noon taking 30% and dive 3 ending at 3pm that takes up 65% (unrealistic, but this is not to scale ) at 9 am, you'd have a clock with 15% remaining.

Thats my take on it anyway

 

[BigJetDriver]

Pulmonary oxygen toxicity is extremely hard to get into with recreational divers. The Lorraine-Smith Effect, as it is also known, takes very lengthy exposures for the diver to begin to suffer from it, such as saturation diving with too much O2 in the mix, or long exposures in the pot with no air breaks.

In a discussion with other re-breather divers of note, it was pointed out that RB divers had done at least six days straight at diving levels of 1.3 ppO2 continuous, with spikes to 1.6 without suffering any ill effects. (Please note: This does NOT mean continuous exposure. They obviously got out of the water and took lengthy air breaks.)

Just a bit of oxygen trivia!

 

[padiscubapro]

Pulmonary oxygen toxicity is extremely hard to get into with recreational divers. The Lorraine-Smith Effect, as it is also known, takes very lengthy exposures for the diver to begin to suffer from it, such as saturation diving with too much O2 in the mix, or long exposures in the pot with no air breaks.

In a discussion with other re-breather divers of note, it was pointed out that RB divers had done at least six days straight at diving levels of 1.3 ppO2 continuous, with spikes to 1.6 without suffering any ill effects. (Please note: This does NOT mean continuous exposure. They obviously got out of the water and took lengthy air breaks.)

Just a bit of oxygen trivia!

I agree wholeheartedly.. but rember o2 tolerance is an individual issue... On a certain trip where I was conductng training I was typically doing 6 hours a day at a 1.3 (or above) which is well above recommended NOAA limits for 2 weeks straight.. I made sure surface intervals were at least 90 minutes (or longer depending on the next planned dive) and had no signle exposure over the single exposure limit..
Also between the last dive of the day and the first of the next day was at least 12 hours which leave 8 halftimes for oxygen.. so even at 100% exposure your clock is theoritically reset..


The only side effect that I had was hyperoxic myopia which lasted 2 or 3 days after diving was stopped(the improved vision was a welcomed side effect ;0 ), and didnt start until about the 12th day of high po2 exposures..

It quite hard for an OC recreational diver to ever approach these limits.. Decomprssion divers itsa different story..

On expeditons where very long oxygen expusres are required with AIR breaks most people can tolerate exposures much higher than the "accepted"limits..

I am not advocating people ignore the limits, just pointing out they can be "broken" if one knows the risks, and had the proper support and equipment in place..

 

[ArcticDiver]

Ok, I've done a bunch of searches and can't really find what I'm looking for. I did my nitrox cert a year ago but haven't had a chance to do much nitrox diving so I'm still a newbie. My diving is purely recreational.

This past weekend I did some dives (EANx34) and was reviewing my materials. Not that I was even close, but it says in the Padi manual that you cannot exceed 100% O2 exposure in 24 hours. What about 25 hours? How long do you have to wait to dive again after you get to 100% from repetitive dives? I realize that it's tough to get to 100% exposure but it's certainly possible if you were doing some liveaboard diving of something of that nature.

I've found some references on SB to NOAA recommendation but can't find anything specific on their site. Does anyone know a link? I've also seen some discussions on here regarding the half life of o2. Any links for more information on this?

Been implied, but not explicitly stated: The 24 hours is a rolling 24, not a calendar 24. That is you count back 24 hour from the present moment. When you do your calculations that way it is obvious that it is really difficult to exceed limits.

But, the real measure is just how each person reacts to specific oxygen doses on specific days. So, it pays to work up to higher doses and pay attention to how your body reacts under different circumstances. Then be very conservative.

 

[jiveturkey]

Why isn't padi more specific on this? Like them or not, their manuals are usually very good. The info they give on the o2 clock to very vague.

So I guess the 90 minute half life is a good way to judge and do calculations. Thanks guys.

HOwever, can I prorate the half life with an hour surface interval?

 

[BigJetDriver]

From the IANTD CNS Repetitive Chart, if you have hit 100% on your CNS O2 clock, a 30 minute surface interval will put you at 83%, a 90 minute SI will put you at 49%, and 6 hours will drop you to 7%.

This is for information only. The numbers are not linear in nature. Please get the chart and use it. If you don't, as the "Amish Diving Society" folks like to say: "You will die!" :11:

 

[jiveturkey]

But, the real measure is just how each person reacts to specific oxygen doses on specific days. So, it pays to work up to higher doses and pay attention to how your body reacts under different circumstances. Then be very conservative.

Is this an issue for me at this point, doing only non-deco dives with mixes under 40%? I haven't noticed my body reacting in anyway really.