Another (legit) nitrox ?

[scubadobadoo]

What happens when your computer fails or the battery dies and you have to start diving tables or off a rented computer? Now the OTUs are no longer being calculated by the computer. It's never a good thing to rely on a computer without being able to work the numbers...

I think you misunderstood me. I am not condoning a reliance solely on the computer and I am not sure how you inferred that about my statement anyway. It seems as if you answered your own assumptions about my post and then decided to randomly quote me.

In fact I agree with you 100%.

"Concentrate on HOW you dive and not WHAT you dive with," I believe I said. The "WHAT" in this case is the computer.

Is this more clear? .......If you are going to trick your computer by setting it to air when diving nitrox, then why even use one in the first place. Use your brain instead. It's cheaper and more reliable.

 

[Steve SDI/TDI HQ]

Might be possibly in theory if you dive a lot. Seems pretty difficult in practice though. Do you have a series of no-deco dives that exceed the limit? I believe it's not possibly with ean32, might be possibly with ean36 if you have something like 6 dives at the no-deco limit over a 18 hours period.

Hi Raymond:

It is possible in more than theory. I believe that many, many divers using nitrox on multi-day, multi-dive profiles come close to exceeding or in fact exceed NOAA daily limits for CNS toxicity.

I think there are many situations where the total bottom time over a 24-hour period approaches 180 minutes... the NOAA daily limit for an oxygen partial pressure of 1.5 and 1.4 bar. That's only three hours. Sport dives on live-aboards of one hour plus are the norm in my experience. Doing three in a day is not uncommon. In fact, three dives a day is conservative. Add to this the practice of hanging off the back of the boat sucking surface supplied oxygen for a few minutes after every dive and our punter is getting awfully close.

My opinion is based on observation. I am not pulling this out of a hat. High daily doses that approach NOAA's limits are real not imagined.

As an aside... basic nitrox certified divers are not restricted to 32 or 36 mixes, and in any event, it is the oxygen pressure and time of exposure that should concern us. Oxygen pressure is a function of depth and fraction of oxygen. Citing a mix without associating it to a depth and time renders the example meaningless.

 

[raymond phule]

About my calculations.

I took 32 and 36 because I found the NOAA tables on line. I looked at the depth with a partial pressure of 1.4 (or close). I also later did the calculations for ean40 and then is it likely that it is possibly to get into problems.

3 hours bottom time at ppO2=1.4with ean32 is similar to 6 dives to the NDL at 30 min. This doesn't seem to be possibly using any combinations of surface intervals ( a higher number of dives but shorter dives). Similar calculations with ean36 makes it possibly but you need to dive most of the day and night. I assumed dives to ppO2=1.4 would be most problematic.

3 hours diving is possibly but not all the time with a high ppO2 according to the tables.

A this depends also on how you calculate the CNS clock. Seems to be different ways around. Applying a half time of 90 min ( I believe my computer uses a half time of 60 min) is going to result in much lower values and I dont believe it is possibly to get close the limit with ean40.

Breathing oxygen is ofcourse also going to change things.

 

[Steve SDI/TDI HQ]

About my calculations.

I took 32 and 36 because I found the NOAA tables on line. I looked at the depth with a partial pressure of 1.4 (or close). I also later did the calculations for ean40 and then is it likely that it is possibly to get into problems.

3 hours bottom time at ppO2=1.4with ean32 is similar to 6 dives to the NDL at 30 min. This doesn't seem to be possibly using any combinations of surface intervals ( a higher number of dives but shorter dives). Similar calculations with ean36 makes it possibly but you need to dive most of the day and night. I assumed dives to ppO2=1.4 would be most problematic.

3 hours diving is possibly but not all the time with a high ppO2 according to the tables.

A this depends also on how you calculate the CNS clock. Seems to be different ways around. Applying a half time of 90 min ( I believe my computer uses a half time of 60 min) is going to result in much lower values and I dont believe it is possibly to get close the limit with ean40.

Breathing oxygen is ofcourse also going to change things.

I strongly suggest you do not consider half times when calculating daily limits. I can post an article I wrote covering this but it actually deals with technical not sport exposures. The tenets are the same but we will spend the next 487 posts dealing with bull****... PM me your email address.

 

[Dive-aholic]

I think you misunderstood me. I am not condoning a reliance solely on the computer and I am not sure how you inferred that about my statement anyway. It seems as if you answered your own assumptions about my post and then decided to randomly quote me.

In fact I agree with you 100%.

"Concentrate on HOW you dive and not WHAT you dive with," I believe I said. The "WHAT" in this case is the computer.

Is this more clear? .......If you are going to trick your computer by setting it to air when diving nitrox, then why even use one in the first place. Use your brain instead. It's cheaper and more reliable.

It was the first statement that lead me to interpret it that way. I see your intent, but it's not clear and didn't want anyone to think it's okay to just rely on a computer for diving, which could be done from that statement. No offense meant towards you, just didn't want anyone to take that statement the wrong way.

 

[The Chairman]

As other people have commented on, there is a concern for CNS buildup of o2 in the system from diving NITROX.

From my limited understanding, CNS is from diving too deep (OxTox). This is an overload of the CNS system and causes a veritable "short circuit" in your nervous system.

However, prolonged exposure to Oxygen also poses a different risk which is pulmonary in nature and this is what the OEUs are referring to. Oxygen is an irritant. An irritant in the lungs causes it to secrete a surfactant. The longer/more intense the irritant, the more surfactant is produced. Surfactant can and will inhibit the transfer of both oxygen and CO2.

I have done a number of aggressive dive trips, and I have as yet to exceed my OEUs at any point in the trip. Since I dive relatively long and deep, it occurs to me that simple over exposure doing recreational diving is hard to do.

The SDI or NAUI course will introduce you to all the of these factors and more. You won't be disappointed by taking one of these courses.

 

[spectrum]

If doing multiple, repetative dives (say 4-5 dives a day for a week) to 50-90' is there a danger of too much o2 buildup? just a thought... thanks!

It's a good question and even if you have taken the course (depending on the course) you may been left wondering how likely impact is for the dive week you describe. I wondered the same thing before my first week as a nitrox dive junkie.

As others have said you should not even be close to hitting a limit on EAN 32. However if you get to dive as planned you will probably be seeing the signs of some acceptable buildup in your computer display as soon as day 2 or 3. That will begin to give you a tangible feel for it all. The alternative is to sit down with those tables and model it out for your own perspective. A table analysis will be the most severe since just as with nitrogen your computer will provide credit for time spent at less than max depth.

Pete

 

[scubadobadoo]

It was the first statement that lead me to interpret it that way. I see your intent, but it's not clear and didn't want anyone to think it's okay to just rely on a computer for diving, which could be done from that statement. No offense meant towards you, just didn't want anyone to take that statement the wrong way.

Yeah, that first sentence wasn't very clear. No offense taken and none meant.

 

[ekewaka]

I think there are many situations where the total bottom time over a 24-hour period approaches 180 minutes... the NOAA daily limit for an oxygen partial pressure of 1.5 and 1.4 bar. That's only three hours. Sport dives on live-aboards of one hour plus are the norm in my experience. Doing three in a day is not uncommon. In fact, three dives a day is conservative. Add to this the practice of hanging off the back of the boat sucking surface supplied oxygen for a few minutes after every dive and our punter is getting awfully close.

I thought the OP was talking about no deco dives. 32% and 1.4 ppo2 is 114 feet. The US Navy air tables give a NDL of 15 minutes at that depth. The NDL from the PADI or NAUI tables may be slightly different, but it is definitely not one hour. It would take a lot more than three no deco dives in 24 hours to reach the limit for o2 exposure with 32% nitrox. It might not even be possible.

 

[ianr33]

I played around with this on V-Planner a while ago.

What I found was that if you stay within NDL's and use pO2 0f 1.4 or less than it was not possible to exceed the daily O2 limits,even doing 12 dives a day.

Using 36% it was possible bit rather unlikely.

Also it takes a pretty good SAC rate (around 0.4) to even hit NDL for a first dive on 32% with an Al 80 at 100 feet or less assuming you turn at a sensible rock bottom.

Using high O2 deco mixes or long cave dives on 32% its a different story but using 32%, Al 80's,staying within NDL's at less than 110 feet and stopping to eat once in a while personally I would not worry about it.