Nitrox on boat with air refill

[packrat12]

Stuartv,

This is where experience is important to understand what you are reading. Yes, no one has said it cannot be done. But....

Let us look at this from a logical point. The Max NOAA PPO2 is 1.4 for recreational diving. Period. Any contingency planning that goes above needs to have the mix altered. For the sake of argument I will ignore ½ O2 wash out.. I use .7 ft3/min as a normal calculation for air consumption for the average person. All of my numbers are rough.

PPO2 @ 1.4 = 150 minutes exposure. For sake of argument, we will do 80% = 120 minutes.
Highest mix for recreational = 40%
Shallowest depth of 1.4 = 80’ (1.4 @ PPO2=40)
Total dive time required = 120 minutes.
Max NDL at 80: 74 min
After a 6hr SI NDL at 80: 46 min
Gas required = 3.4ATA * .7 = 2.4 ft3/min at depth * 120 = 288 ft3 = 3.75 (80ft3 tanks) without any reserve to fully drained.

This is one heck of a dive! Over 4 -80's in one recreational day diving.... whew! Still does not reach CNS of 100%, only 80% which is recommended. Does not include the CNS wash out either. The above is very sloppy but gives an idea of the extreme profiles that must be done to achieve CNS O-tox while on a recreational profile. I will accept that there can be errors also and will very much entertain that fact. This is a very rough calculation.

 

[stuartv]

Stuartv,

This is where experience is important to understand what you are reading. Yes, no one has said it cannot be done. But....

Actually, I think some folks did say it couldn't be done (in the passive/aggressive format of "oh, yeah? Prove it. Give us an example."). Thus why they said the OP doesn't need to worry about it.

Anyway, why would you think I don't understand what I read? Did you read the passage in the book? If so, you should probably refute the example Steve Lewis gave in the book, not try to explain it to me. And if you didn't read the full passage in the book, then, again, why would you suggest that I don't understand what I read?

Regardless, please don't shoot the messenger. I didn't say one way or another whether I think it can or cannot be done. I simply quoted from a book written by a (presumed) expert. It's on page 86, if anyone cares. I am not trying to argue with anyone here. I am simply providing info so that if the OP wants to read a "published" opinion to compare and contrast with the Scubanati's opinions given here, he, and anyone else who stumbles into this thread, has a specific reference that they can look up.

Finally, to address the specifics you gave: One, a night dive followed by 4 dives the next day doesn't seem unreasonable to me. Especially if you're on a liveaboard. And, two, I'd feel better with 5 deep (and thus, shorter) dives than 4 or 5 dives to 80 feet or less. I'm not sure where the suggestion comes from that dives to 80' or more are somehow more taxing than shallower dives. At least, it doesn't happen that way for me. The depth itself doesn't really seem to make any difference to how tired I am afterwards.

So, I'd say 5 dives to 100+ feet within 24 hours is totally within the realm of possibility for a recreational diver.

Further, if I'm not mistaken, calculating your daily (24 hour) CNS limit does not allow for "washout", either (excepting for a special case of diving to a ppO2 of 1.6atm). So, not sure how your comment on that is relevant.

Thus, 5 dives in a 24 hour period that each give you 20% of your daily limit puts you at 100%, and it would then be recommended to sit out a full 24 hours. And it puts you way over the max recommended cap of 80% - beyond which, Steve Lewis says that many divers think one should sit out for 12 hours. I note that a 36 minute dive with ppO2 of 1.4 yields 20% of the 180 minute daily exposure limit. That's 5 dives in 24 hours to a max of 95' each dive for 36 minutes each (total dive time, mind you, if you're using tables to calculate your O2 exposure) on EAN36, and you're at 100% of the 24 hour limit.

In other words, I think telling the OP to not worry and to not bother getting the details and calculating his O2 loading himself is balderdash. It's the worst kind of Trust Me diving. "Trust me, you don't have to worry about CNS OxTox if you're diving within Rec limits." I think it's great that, in his situation, he gave it some thought and attempted to verify his safety.

 

[Freewillow]

The opp original worry was the fact that the nitrox bottles were filled with air. And for this there were no worries. Now we are 17 pages later and the "problem" is not solved. Too many "expert scientists" on this forum .

 

[JohnnyC]

There are 3 types of limits we need to track, oxygen toxicity in the form of PO2, CNS oxygen toxicity which we track in terms of a percentage, and Whole Body Exposure, or OTU's, which are tracked on a points scale based on per minute at a given PO2.

OxTox is easy.

CNS toxicity has a 90 minute half-time washout. If you hit 25% on your first dive, after 90 minutes you recalculate your CNS percentage based on 12%. It's exactly the same concept as recalculating an NDL based on residual nitrogen time.

Whole Body, calculated in OTU's, has no provision for washout. It is used for repetitive DAYS of diving. It is best illustrated by using the REPEX table, considered the standard for multiple repetitive dives over repetitive days. It begins with 850 "points" for the first day of diving, and progressively diminishes the allowable exposure per dive and per day so that you have accumulative exposure tracking.



If unfamiliar with each of these and how they individually effect oxygen exposure levels, once should not try and provide an opinion. This is not directed at anybody in particular, but without understanding the differences between these, it is very easy to convince yourself that you're aware of all of the aspects of oxygen exposure on the diver. Simply reading a book does not impart this level of expertise. It is important to be aware of which limitation will be the limiting factor for your diving.

This document will help with understanding OTU's, which seems to be the most misunderstood concept, although I'm not sure why. It's dirt simple.
http://tecvault.t101.ro/REPEX0.PDF

 

[scubadada]

It's too bad so much bad advice is given on SB. It's actually quite easy to exceed NOAA 24 h O2 exposure doing recreational dives, I do it pretty frequently. The NOAA table and Oceanic computers do not utilize the O2 elimination half life of 90 minutes. Do 4 dives per day, 5 dives in the 24 hour window and you only have 5 hours at a pO2 of 1.0. Diving EAN 36, even EAN 32 this is not difficult. Fortunately, I am able to put this into perspective to plan my dives.

 

[NAM001]

No statement is true 100%. There are always unspoken assumptions. Like "It cant be done." is assumed to be without really working at it, One could prob ably get anything done if dedicated to do so. One could do 12 hours at 60 ft but that is the 1 of a million divers that would try that.

It is probably more akin to saying you cant break your leg walking. I would say that is true unless there is something going on out of the normal. Saying the statement is false because the cousin of the plummer that fixes my grandmothers exhusbands house broke his leg wallking down the street when he was hit buy a car, is not just reason to say the statement is false. There is too much of this kind of argument on SB.

Actually, I think some folks did say it couldn't be done (in the passive/aggressive format of "oh, yeah? Prove it. Give us an example."). Thus why they said the OP doesn't need to worry about it.

---------- Post added November 18th, 2015 at 02:59 PM ----------

Well put....

There are 3 types of limits we need to track, oxygen toxicity in the form of PO2, pulmonary oxygen toxicity which we track in terms of a percentage, and Whole Body Exposure, or OTU's, which are tracked on a points scale based on per minute at a given PO2.

OxTox is easy.

Pulmonary toxicity has a 90 minute half-time washout. If you hit 25% on your first dive, after 90 minutes you recalculate your CNS percentage based on 12%. It's exactly the same concept as recalculating an NDL based on residual nitrogen time.

Whole Body, calculated in OTU's, has no provision for washout. It is used for repetitive DAYS of diving. It is best illustrated by using the REPEX table, considered the standard for multiple repetitive dives over repetitive days. It begins with 850 "points" for the first day of diving, and progressively diminishes the allowable exposure per dive and per day so that you have accumulative exposure tracking.

View attachment 220082

If unfamiliar with each of these and how they individually effect oxygen exposure levels, once should not try and provide an opinion. This is not directed at anybody in particular, but without understanding the differences between these, it is very easy to convince yourself that you're aware of all of the aspects of oxygen exposure on the diver. Simply reading a book does not impart this level of expertise. It is important to be aware of which limitation will be the limiting factor for your diving.

This document will help with understanding OTU's, which seems to be the most misunderstood concept, although I'm not sure why. It's dirt simple.
http://tecvault.t101.ro/REPEX0.PDF

 

[packrat12]

I do not recall a single post from anyone that said you could not exceed CNS limits recreationally. Most of us have said it is very difficult to achieve. I have on several occasions stated that Hi PPO2, many dives with hi PPO2 and just many dives may be able to exceed limits. Also, I have said that you would know it, e.g. you are pushing very hard.

With experience, you will see that in your recreational diving, you will probably never exceed your CNS limits. If you do, you will be doing very unique diving. For the dive presented here, an experienced diver would note that few if almost any dives are square profile. Most dives average a PPO2 less then PPO2 at max depth. Whether you count it or not, 1/2 O2 is used by divers without issue. Doing back-to-back NDL dives to 80' with 40% is highly unlikely to be repeated much, much less who sits out 6 hrs between dives???

Since you are interested in pursuing technical, you may want to run models yourself. I have yet to see a real world run exceed CNS limits while remaining recreational. You probably can present them but again, look at the real world here.

"Further, if I'm not mistaken, calculating your daily (24 hour) CNS limit does not allow for "washout", either (excepting for a special case of diving to a ppO2 of 1.6atm). So, not sure how your comment on that is relevant." ..... Please tell me that you understand the difference between CNS & Pulmonary O-Tox..... Please tell me you understand the risk differences between them..... From this comment, it does not look like it. Your Petrel and most technical modeling programs do use CNS wash out BTW so once you go technical, you will be using it. The OTU (Pulmonary) does not wash out and is a rolling 24hr period. It also presents different effects.

Even on a live aboard, it is highly unlikely that the captain will dive you to max PPO2 on 5 repetitive dives. You will more than likely do a stair step group of dives and multi-levels within them. But yes, a liveaboard is probably one of the few instances to watch for CNS, I can also see extreme spearfishing.


And as freewillow has repetitively pointed out, the OP was going from a nitrox mix to an air mix on subsequent dives without having an analyzer. Our response is STILL accurate, you can, in recreational mode, ignore the CNS/OTU risks, especially in the real world!

 

[scubadada]

No statement is true 100%. There are always unspoken assumptions. Like "It cant be done." is assumed to be without really working at it, One could prob ably get anything done if dedicated to do so. One could do 12 hours at 60 ft but that is the 1 of a million divers that would try that.

5 hours at a pO2 of 1.0, not difficult, as difficult as it may be, think about it

---------- Post added November 18th, 2015 at 04:15 PM ----------

I do not recall a single post from anyone that said you could not exceed CNS limits recreationally. Most of us have said it is very difficult to achieve. I have on several occasions stated that Hi PPO2, many dives with hi PPO2 and just many dives may be able to exceed limits. Also, I have said that you would know it, e.g. you are pushing very hard.

With experience, you will see that in your recreational diving, you will probably never exceed your CNS limits. If you do, you will be doing very unique diving. For the dive presented here, an experienced diver would note that few if almost any dives are square profile. Most dives average a PPO2 less then PPO2 at max depth. Whether you count it or not, 1/2 O2 is used by divers without issue. Doing back-to-back NDL dives to 80' with 40% is highly unlikely to be repeated much, much less who sits out 6 hrs between dives???

Since you are interested in pursuing technical, you may want to run models yourself. I have yet to see a real world run exceed CNS limits while remaining recreational. You probably can present them but again, look at the real world here.

"Further, if I'm not mistaken, calculating your daily (24 hour) CNS limit does not allow for "washout", either (excepting for a special case of diving to a ppO2 of 1.6atm). So, not sure how your comment on that is relevant." ..... Please tell me that you understand the difference between CNS & Pulmonary O-Tox..... Please tell me you understand the risk differences between them..... From this comment, it does not look like it. Your Petrel and most technical modeling programs do use CNS wash out BTW so once you go technical, you will be using it. The OTU (Pulmonary) does not wash out and is a rolling 24hr period. It also presents different effects.

Even on a live aboard, it is highly unlikely that the captain will dive you to max PPO2 on 5 repetitive dives. You will more than likely do a stair step group of dives and multi-levels within them. But yes, a liveaboard is probably one of the few instances to watch for CNS, I can also see extreme spearfishing.


And as freewillow has repetitively pointed out, the OP was going from a nitrox mix to an air mix on subsequent dives without having an analyzer. Our response is STILL accurate, you can, in recreational mode, ignore the CNS/OTU risks, especially in the real world!

Wrong, see above, too many experts on SB. BTW, I believe in the O2 elimination half life and take that into consideration. Maybe someday I will have a computer that takes that into consideration.

 

[NAM001]

Your comment is proof of my post. Who does 5 hours at depth on one day. Not the average joe recreational diver.

5 hours at a pO2 of 1.0, not difficult, as difficult as it may be, think about it

---------- Post added November 18th, 2015 at 04:15 PM ----------



Wrong, see above, too many experts on SB. BTW, I believe in the O2 elimination half life and take that into consideration. Maybe someday I will have a computer that takes that into consideration.

 

[packrat12]

Wrong, see above, too many experts on SB. BTW, I believe in the O2 elimination half life and take that into consideration. Maybe someday I will have a computer that takes that into consideration.

What part? Remember that this post really started out with a recreational diver that had nitrox and went to air fills on the boat. It has evolved into repetitive high PPO2 nitrox dives. Never, have I not stated that high number of dives cannot exceed limits but that you would know you were pushing them. 5 hrs is an awful long time underwater for 24 hrs. I would definitely call that pushing. A bunch of shorter dives at higher PPO2 should be a flag also. It is difficult is what most of us have said.