miketsp
Contributor
halemano:
At least when I did it a few years ago SSI taught 1.4 also.
What are the many agencies that teach 1.6 as recommended?
Nitrox Fill Question?
- Thread starter HarveyO
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At least when I did it a few years ago SSI taught 1.4 also.
What are the many agencies that teach 1.6 as recommended?
miketsp:
You make a very good point about checking the SPG, as well. Since I dive a 3300psi compact 80, I've gotten into the habit of checking for that extra 10%.
However, I'd be careful about saying a shop deliberately underfills. Many shops (that I know of, anyway), sometimes are too rushed to top off a tank after the fill, and you end up with a hot-fill, short a couple hundred psi. It's not intentional, it's just time-constrained. If I knew a shop was doing that intentionally, for whatever reason, well... 'twouldn't be pretty...
One thing I don't understand is underfilling the Nitrox "to keep the boat on schedule with the air divers." Nitrox doesn't last longer, it just gives you more bottom time in terms of non-deco dives (assuming you're doing multiple deeper, >60ft. dives in a single day).
miketsp
Contributor
BKP:
Precisely.
I don't have time at the moment to work through some examples but I've been repeatedly in situations of 2 (or more) tank dives at 30-40m where doing non-deco (or minimum deco) air dives I've had to return to the boat with a lot of air still in my tank while Nitrox divers with similar or worse SAC than mine were getting much longer useful bottom times.
If you're prepared to accept some longer hang-times this difference can reduce but in quite a few scenarios deco hangs can complicate the overall operation.
That's not what PADI's 1.6 contingency is about... that contingency is that you should stay within the 1.4... but if a situation arises (e.g. massive vertical surge on Molokini's backwall takes you from 75' down to 120'; or you have to go tap your buddy on the shoulder to say "you're deeper than the plan") you *probably* won't have an issue doing it as long as you stay within 1.6 PPO2.halemano:
My Suunto Cobra has a setting that ranges from 1.2-1.6 PPO2, defaulting to 1.4. Oddly, the alarm still goes off if you exceed 1.4, no matter what the setting.
The 1.4-1.6 range is considered by PADI to be a "safety margin" as divers have experienced OxTox at PPO2's approaching (but not necessarily exceeding) 1.6.
If you decide to plan your dive for a PPO2 of 1.6 *and* you exceed that accidentally, PADI's recommendation is to immediately ascend to where your PPO2 is less than 1.4 and end the dive in a controlled, safe ascent from there.
I don't know of any agency that recomends 1.6
I'm ready to eat crow with regards to claiming many other agencies teach mod's based on 1.6 ppO2. The only recreational agency I can verify is IANTD. Looking back on my first post, with much embarressment, it was badly worded except for the last sentence.
If given a second chance, I'd start with the NOAA nitrox tables. After a couple hours e-research, it seems most recreational nitrox tables are versions of these. Apendix F of my IANTD Nitrox Manual reprints NOAA Nitrox II (36%) documents, containing this pertinant sentence. "The normal depth limit for this mixture shall be 110 feet/30.3 M of seawater for dives that do not require decompression".
My image of NOAA diving is that it's not recreational, not hard hat either, but the divers are working divers. IANTD was the first to bring nitrox to recreational diving and maintaining the same tables for non-working divers gives a cushion of additional saftey to recreational divers.
PADI's (and others) use of the NOAA nitrox tables is more conservative in that it relegates the 1.5 and 1.6 ppO2 columns to contingency or emergency. There very well may be wording somewhere similar to Forays and I hope there are instructors that might use similar terms.
My TDI Nitrox; User Friendly Guide to Eanx, page 26, bottom paragraph, states: Maximum scuba diving limit for the partial pressure of oxygen should not exceed 1.6 ATA and should not have a bottom time longer than 45 minutes. Use 1.5 ppO2 (or less) for bottom times between 45 and 120 minutes. A ppO2 of 1.4 (or less) is recomended for particularly strenuous or cold dives (sic). I'd like to assume that 's how TDI/SDI teach recreational nitrox diving.
ANDI was second in the recreational use of nitrox and may also not darken the 1.5 and 1.6 columns of their NOAA based nitrox charts, but I do not have one of their manuals for verification, and I do not know of any ANDI instructors or shops.
As I see it, if you are healthy and in shape enough that recreational diving is not strenuous, cold water diving would be the reason to limit ppO2 to 1.4, although current at depth would cause me to adjust mod accordingly. If you only dive strenuous or cold dives a 1.4 agency is fine. 1.6 might allow you to peak under the house of the USCGC Bibb (Key Largo) without paying more for a custom blend.
If given a second chance, I'd start with the NOAA nitrox tables. After a couple hours e-research, it seems most recreational nitrox tables are versions of these. Apendix F of my IANTD Nitrox Manual reprints NOAA Nitrox II (36%) documents, containing this pertinant sentence. "The normal depth limit for this mixture shall be 110 feet/30.3 M of seawater for dives that do not require decompression".
My image of NOAA diving is that it's not recreational, not hard hat either, but the divers are working divers. IANTD was the first to bring nitrox to recreational diving and maintaining the same tables for non-working divers gives a cushion of additional saftey to recreational divers.
PADI's (and others) use of the NOAA nitrox tables is more conservative in that it relegates the 1.5 and 1.6 ppO2 columns to contingency or emergency. There very well may be wording somewhere similar to Forays and I hope there are instructors that might use similar terms.
My TDI Nitrox; User Friendly Guide to Eanx, page 26, bottom paragraph, states: Maximum scuba diving limit for the partial pressure of oxygen should not exceed 1.6 ATA and should not have a bottom time longer than 45 minutes. Use 1.5 ppO2 (or less) for bottom times between 45 and 120 minutes. A ppO2 of 1.4 (or less) is recomended for particularly strenuous or cold dives (sic). I'd like to assume that 's how TDI/SDI teach recreational nitrox diving.
ANDI was second in the recreational use of nitrox and may also not darken the 1.5 and 1.6 columns of their NOAA based nitrox charts, but I do not have one of their manuals for verification, and I do not know of any ANDI instructors or shops.
As I see it, if you are healthy and in shape enough that recreational diving is not strenuous, cold water diving would be the reason to limit ppO2 to 1.4, although current at depth would cause me to adjust mod accordingly. If you only dive strenuous or cold dives a 1.4 agency is fine. 1.6 might allow you to peak under the house of the USCGC Bibb (Key Largo) without paying more for a custom blend.
For general edification, higher body temp. and activity level, generally lowers the threshold for CNS ox. tox,.
BTW Harvey which shop are you refering to in these posts?
BTW Harvey which shop are you refering to in these posts?
OP
HarveyO
Contributor
I'd just as soon not say what shop it was, since it was the action of one employee, that I was questioning, and I don't know what the shops position is on testing.
After all I took my Nitrox course from them and they stressed testing as part of the training. So I don't want to be bad mouthing an LDS because of one employee. The main thing I was after was getting clear on the liklihood that I could be getting a "questionable fill" and missing it without a test.
I also wanted to provide something for us all to think about in this regard since it sounds like some folks are just picking up tanks and possibly taking it on faith that they're OK.
The responses have made that possibility most clear to me and has resulted in my buying my own analyzer to remove the liklihood of ever having to dive without knowing what I have in the tank. I was shocked at how many people are telling of bad fills.
But when I think about it, in the last year I've already heard about a number of compressor related incidents ocurring in the shops around the island and the idea of less than great air has popped into my mind more than once.
The positive outcomes of this are:
I just today became the owner of my own analyzer.
I made my first Liesure Pro purchase and it worked out just great. So now I now about them
I got to really benefit from the discussion of O2 exposure limits and the way they've evolved. (Just why I like this forum)
As to the LDS maybe people are so slack about testing in the shop because they all have analyzers at home and test them after they pick up their tanks. I don't know. But for sure, I'm not diving without testing first.
Again thanks to all who contributed to this discussion.
After all I took my Nitrox course from them and they stressed testing as part of the training. So I don't want to be bad mouthing an LDS because of one employee. The main thing I was after was getting clear on the liklihood that I could be getting a "questionable fill" and missing it without a test.
I also wanted to provide something for us all to think about in this regard since it sounds like some folks are just picking up tanks and possibly taking it on faith that they're OK.
The responses have made that possibility most clear to me and has resulted in my buying my own analyzer to remove the liklihood of ever having to dive without knowing what I have in the tank. I was shocked at how many people are telling of bad fills.
But when I think about it, in the last year I've already heard about a number of compressor related incidents ocurring in the shops around the island and the idea of less than great air has popped into my mind more than once.
The positive outcomes of this are:
I just today became the owner of my own analyzer.
I made my first Liesure Pro purchase and it worked out just great. So now I now about them
I got to really benefit from the discussion of O2 exposure limits and the way they've evolved. (Just why I like this forum)
As to the LDS maybe people are so slack about testing in the shop because they all have analyzers at home and test them after they pick up their tanks. I don't know. But for sure, I'm not diving without testing first.
Again thanks to all who contributed to this discussion.
I think if you get it from a reputable source, you'll find two things:
1. they will have you analyse it before they release the tanks to you for use.
2. the analysis (using their analyser or yours) will results in the requested mix +/- 0.2% O2. I know that PADI says you don't have to recalculate if it's +/- 1%... but anytime I've gotten Nitrox, it's been within 0.2% from the dive shop I'm thinking of.
1. they will have you analyse it before they release the tanks to you for use.
2. the analysis (using their analyser or yours) will results in the requested mix +/- 0.2% O2. I know that PADI says you don't have to recalculate if it's +/- 1%... but anytime I've gotten Nitrox, it's been within 0.2% from the dive shop I'm thinking of.
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