O2 Question: Why is 40% O2 the threshold for the industry?

[Mcarlson]

Does anyone know why the dive industry uses 40% as the nitrox threashold? It seems that both certification agencies and equipment manufacturers cite this number most often.
Wouldn't the issues with use of enriched oxygen above 40% just be extensions of the issues encountered from 22%-40% (higher PPO2)?

 

[nimoh]

Not sure, but I think it may just come down to the fact that there isn't really a practical value for a recreational nitrox diver to use O2 over 40%. Mixes with over 40% O2 are (to the best of my knowledge) only used for accelerated decompression.

 

[tbone1004]

40% is considered the threshold before regulators/tanks/valves have to be O2 cleaned due to high PPO2's so it translates over to standard nitrox certification

 

[redacted]

I suspect the answer is that the recreational dive industry's experience is that <= 40% is safe. Many references set a limit of 23% for special handling procedures.

http://www.globalgases.com/images/msds/newpdf/GG Australia - Nitrox 11 5 10.pdf

 

[boulderjohn]

I can't speak definitively, but I think some of the following come into play.

1. I is my understanding that NASA's research into the circumstances of the death of astronauts, including Gus Grissom, in the fire of 1967 indicated that in terms of safety, oxygen mixes 50% and above were about as dangerous as 100% oxygen. Once you start going below that level, the danger drops rapidly.

2. As nimoh said, there is little practical value in mixes above 40%. At the shallower depths allowed with those mixes, you get longer NDL times with air or lower nitrox mixes than you are likely to reach on a recreational dive. The only good reason to use such mixes is to accelerate decompression. Accelerated decompression takes skills that are beyond the training of pure recreational divers.

3. As awap said, mixes of 40% and lower are considered safe for recreational diving. We don't say things like the following in a basic nitrox class, but if you have any mix 40% or less in an AL 80 tank on a dive within recreational limits, you are going to have one tough time getting toxed, even if you ignore the MOD for that mix. You don't tox immediately upon violating MOD--it takes a while. You will have to have a pretty good SAC rate to stay beyond the MOD long enough to have a problem. (This is not a guarantee--I am just saying it is not likely.)

 

[descent]

Akimbo very kindly spent an afternoon annotating a section of the Navy Dive Manual to explain some of the thinking behind choosing these semi-arbitrary lines:

&#8239;&#8239;&#8239;&#8239;[URL="http://www.scubaboard.com/forums/diving-medicine/440726-oxygen-toxicity-limits-symptoms.html"]Oxygen Toxicity Limits & Symptoms[/URL].

 

[geoff w]

I read one of the gas blending student manuals for the knowledge, but never did the certification. Industry generally treats up to 40% oxygen the same as air and higher concentrations are treated the same as pure oxygen. OSHA and NOAA follow the same rules. For grins I googled it and found a specific reference from Luxfer at Why does Luxfer require cleaning for oxygen concentrations above 23.5% - Luxfer: Setting The Standard Worldwide

The 40% threshold is cited in a single Federal CFR published by the Occupational Safety and Health Administration (OSHA) of the U.S. Department of Labor: 29CFR910.430, which applies to “Commercial Diving Operations” and states in the section titled “Oxygen safety” on page 854: “(1) Equipment used with oxygen or mixtures containing over forty percent (40%) by volume oxygen shall be designed for oxygen service. (2) Components (except umbilicals) exposed to oxygen or mixtures containing over forty percent (40%) by volume oxygen shall be cleaned of flammable materials before use.”

Like everything else, it probably has as much to do with manufacturer liability and being able to sell equipment to the Government as anything. From a buddy who services regulators for most of the dive shops in the area, regulators used as rental gear need to follow OSHA standards (e.g., annual service, even if the manufacturer has a longer service interval) so would also fall under the 40% rule

---------- Post added June 24th, 2015 at 04:57 PM ----------

The OSHA standard is at https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9988

 

[NAM001]

To add (heard stories ) to what has been said already.... the issue of nitrox was coming to be during the time of the nasa incident. Above 60 reacted like pure O2 and below it reacted like air. With the nitrox issue of noaa using/standardizing 32 and 36% for recreational diving, and giving leeway for a little over mixing the line was drawn at 40 % for recreational scuba. With intent that the 40 was to cover a rich 36% mix. The gap between 40 and 60 was accepted as a conservative measure/buffer borne of consequence, that did not impact the EAN 32 and 36 gas intentions or enter the realm of over 60% hazzards.

---------- Post added June 24th, 2015 at 08:44 PM ----------

Geoff the higher than 40% same as O2 is a statement in reference to scuba and is not a global statement. As far as the regulator thing and 40% you don't hit the min requirement when you dive 32 and 36% standard noaa gasses.

I read one of the gas blending student manuals for the knowledge, but never did the certification. Industry generally treats up to 40% oxygen the same as air and higher concentrations are treated the same as pure oxygen. OSHA and NOAA follow the same rules. For grins I googled it and found a specific reference from Luxfer at Why does Luxfer require cleaning for oxygen concentrations above 23.5% - Luxfer: Setting The Standard Worldwide

The 40% threshold is cited in a single Federal CFR published by the Occupational Safety and Health Administration (OSHA) of the U.S. Department of Labor: 29CFR910.430, which applies to “Commercial Diving Operations” and states in the section titled “Oxygen safety” on page 854: “(1) Equipment used with oxygen or mixtures containing over forty percent (40%) by volume oxygen shall be designed for oxygen service. (2) Components (except umbilicals) exposed to oxygen or mixtures containing over forty percent (40%) by volume oxygen shall be cleaned of flammable materials before use.”

Like everything else, it probably has as much to do with manufacturer liability and being able to sell equipment to the Government as anything. From a buddy who services regulators for most of the dive shops in the area, regulators used as rental gear need to follow OSHA standards (e.g., annual service, even if the manufacturer has a longer service interval) so would also fall under the 40% rule

---------- Post added June 24th, 2015 at 04:57 PM ----------

The OSHA standard is at https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9988

 

[kensuf]

The NASA event was thirty years before nitrox made it's way into recreational diving.

When IAND and ANDI started promoting/teaching nitrox, the majority of the dive industry rejected the concept of using higher fO2's. Heck there was even a big hubbub about this mysterious "voodoo gas" at DEMA. There were fears about Ox-Tox and explosion.

Plain and simply stated, the 40% rule was about risk management and reaching a compromise that everyone could accept and live with. An fO2 of 40% is not high enough to have a large risk of fire, combustion resulting in carbon-monoxide, and can be dove to depths of 99' without exceeding a PO2 of 1.6 (which was the recommended PO2 at the time).

 

[Diver0001]

Does anyone know why the dive industry uses 40% as the nitrox threashold? It seems that both certification agencies and equipment manufacturers cite this number most often.
Wouldn't the issues with use of enriched oxygen above 40% just be extensions of the issues encountered from 22%-40% (higher PPO2)?

Well, you're right in one sense. The actual diving part of it is the same, but that was never really the issue.

I'm just going to venture a guess as to why this distinction is made. I may be way off base but it's the impression that I have, having witnessed the rise of nitrox over a period of 30 years.

First is compressor technology. I'm pretty sure that in the 1980's if you wanted nitrox you needed to blend. Continuous feed systems that delivered at the target mix came much later. It involved buffering pure O2 and mixing into to cylinder. In fact, this is still happening today in technical diving (above the 40% thing) and it *can* be dangerous and blending, especially going back 30 years without the tools we have today, was a bit of a black art.

Secondly, the volatility of Oxygen under high pressure in the presence of certain chemicals, lubricants and metals can make it tricky to work with. For example, titanium will literally spontaneously combust in a pure oxygen environment in the presence of certain catalysts within the parameters of normal regulator pressures (partial pressures of O2). There have been a few accidents over the years involving titanium regulators as a result. At 40% the risk of spontaneous combustion is much lower, so I think this may have been one of the reasons.

Third, getting back to compressors, with the advent of continuous feed systems, the need to buffer nitrox and/or oxygen was eliminated. I assume (although I don't know) that there are limits to this technology but what I know for sure about it is that in many places, getting a license to use a continuous feed compressor in urban areas was possible but only if you were not buffering nitrox or pure O2. In case, in combination of the other risk factors I just mentioned I guess there was a decision to make the distinction at 40%.

None of which has to do with the diving.

I don't suspect that the diving part was really leading in all that but when you consider that 32% is a nice middle of the road gas for recreational "deep" diving and that during the rise of nitrox technical and recreational diving were still two essentially separate disciplines then I can imagine industry experts thinking that nobody in a recreational context would really need (or want) Nitrox mixes much above 32%.... so 40% probably seemed like a wide margin at the time. 50% would obviously have worked out better in the context of modern diving but I don't really think this was foreseen.

R..