Things Scuba Instructors teach that are either bad or just wrong.

[Boarderguy]

I do not understand your point. Why can't I use nitrox to extend dives deeper than 110 feet? Sure, you need a mix with less than 32% O2, but it is still nitrox, and you still get more bottom time.

You're right. I'm used to getting banked nitrox and not blended to a specific dive. I don't bother with it for anything below 90.

 

[Angelo Farina]

Which NDL time limit are you referencing (air or the relevant EAN)? If you mean to end your dive at 75% of the EAN NDL time, the advantage of breathing EAN is a longer dive for the same DCS risk. If you mean to end your dive at 75% of the air NDL time (or if time is limited by the amount of gas carried), the advantage is less risk for the same time.

The NDL depends on the mixture you are breathing. So if I end the dive at 75% of NDL I have more or less the same nitrogen load whatever the mixture.
In reality it is not exactly the same. Using Nitrox the exposure time is longer, at a smaller partial pressure of Nitrogen. So there will be more Nitrogen in the slow tissues and less in the fast ones.
More Nitrogen in the slow tissues is not very appealing, it will take more time for getting rid of it.
Of course staying longer underwater before hitting the NDL limit can be seen as advantageous, particularly if diving for fun in a nice place. But in the last years I did five mostly for work (scientific diving).
And also if it was for fun, this additional bottom time is associated with increased risks from oxygen and from shifting nitrogen load to slower tissues.
So, considering my age and health conditions, I prefer a shorter bottom time with reduced risks, diving in air...
As said, when I was young I acted quite differently. I used pure oxygen CC rebreathers well below maximum safety depth for staying down several hours...

 

[kinoons]

An interesting discussion.

For context, it appears that oxygen toxicity is something that the average recreational diver will never need to consider.

Here is a link to some research into the notion.

Oxygen Toxicity - StatPearls - NCBI Bookshelf

Deep divers (diving below 185 feet) require breathing mixtures that contain less than 21% oxygen to reduce toxicity risk.

At depths of 2-3 ATM during hyperbaric treatments it was noted that…

100% oxygen can be tolerated at sea level for about 24-48 hours without any severe tissue damage. Lengthy exposures produce definite tissue injury. There is moderate carinal irritation on deep inspiration after 3-6 hours of exposure of 2 ATA, extreme carinal irritation with uncontrolled coughing after 10 hours, and finally, chest pain and dyspnea ensue.

So again for recreational divers it’s a non-issue. For deep diving professionals who may stay under for greater durations there is some cause for concern.

I have not found anything about the long term injuries from high partial pressures of oxygen, but my gut $0.50 guess is that for nearly everyone the proportion of time spent breathing high partial pressures of oxygen vs surface time (“normal partial pressure oxygen”) diminishes long term concerns, but I’ll freely admit that’s a guess and I’m open to being proven wrong there.

 

[inquis]

So, considering my age and health conditions, I prefer a shorter bottom time with reduced risks, diving in air...

Given your preference, why not breath EAN but dive to the same time as you would on air? That results in less loading and therefore less risk than when breathing air.

 

[wetb4igetinthewater]

Given your preference, why not breath EAN but dive to the same time as you would on air? That results in less loading and therefore less risk than when breathing air.

You need to program the maximum depth on your dive computer in that scenario.

 

[L13]

Given your preference, why not breath EAN but dive to the same time as you would on air? That results in less loading and therefore less risk than when breathing air.

That would still increase his O2 exposure vs. diving on air. He is trying to reduce both his exposure to O2 (oxygen toxicity, etc.) AND his exposure to N2 (DCS, etc.).

 

[inquis]

That would still increase his O2 exposure vs. diving on air.

A negligible increase of O2 exposure vs. non-negligible decrease in tissue tension.

As an example, NDL at 30 m on air (GF x/80) is 15 mins, CNS of 4%. Adjusting GFs to give 15 mins when on EAN34, one would surface at a GF of 59, CNS of 9%. That's a long way from an O2 exposure of 100%, and even the relevance of "100%" is debated.

 

[boulderjohn]

An interesting discussion.

For context, it appears that oxygen toxicity is something that the average recreational diver will never need to consider.

Here is a link to some research into the notion.

Oxygen Toxicity - StatPearls - NCBI Bookshelf

At depths of 2-3 ATM during hyperbaric treatments it was noted that…



So again for recreational divers it’s a non-issue. For deep diving professionals who may stay under for greater durations there is some cause for concern.

I have not found anything about the long term injuries from high partial pressures of oxygen, but my gut $0.50 guess is that for nearly everyone the proportion of time spent breathing high partial pressures of oxygen vs surface time (“normal partial pressure oxygen”) diminishes long term concerns, but I’ll freely admit that’s a guess and I’m open to being proven wrong there.

You are mixing up two different kinds of toxicity. The long term effect of oxygen on the lungs is called pulmonary oxygen toxicity. It is not talked about so much in recreational diving any more because, as you suggest, it really isn't an issue.

The reason for the MODs of nitrox in both recreational and technical diving is different. We are not concerned as much (hardly at all) with the long term effect on the lungs. We are worried about suddenly going into convulsions, which has an important short term effect, not only on the lungs, but on the whole body. Although it is hard to predict how long it can take, a diver who was breathing 36% nitrox on a dive to a wreck that was between 150-170 feet deep took about 20 minutes before he toxed and died.

 

[boulderjohn]

You're right. I'm used to getting banked nitrox and not blended to a specific dive. I don't bother with it for anything below 90.

Just as an FYI on a dive shop I use for fills when I am in Florida. They have banked nitrox.

They essentially have two "spigots" for gas on the wall: air and 36% nitrox. When you come in, you ask for a mix. They ask you what's in your tank already. They do a little math (often in the head), put in a little of one, add a little of the other, and you end up roughly with what you want. I ask for a little bit lower % that a perfect mix, so if they are a little high, no problem. It helps that I have LP steel tanks. If they come in a little hot, they can touch it up with some air.

 

[Seaweed Doc]

I find the general culture of "you need a certification to fart in a wetsuit" to maybe take things too far, and that is frequently pushed by the dive-agencies.

There are ways to safely pursue independent learning. and yes, those ways may be more frustrating, time-consuming, and have some safety concerns.

I think part of this is liability worries, especially today. Imagine the conversation I have:

"Congratulations! You are a certified diver, to 60' deep."
"May I dive to 70'? PADI says I can with certification or experience. How do I get the experience?"
"Well, you can take my AOW class. Or do something I can't really recommend, though PADI allows (in many ways) that doing this is a "thing" and probably good enough."

Back in the dark ages when I had to swim uphill through the snow to SCUBA school, I really liked my instructor's advice. He basically said try going deeper in small increments. Get comfortable at 60' over several dives. Then try 65-70'. Repeat with slightly increasing depth.

Of course, he also gave us Navy deco tables, taught us how to use them, and told us never, ever to dive deeper than 200' or oxygen toxicity would kill us. So there's that.