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

[Angelo Farina]

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.

Because now I condider the oxygen risk more concerning than the risk of decompression sickness.
Breathing Nitrox for the same time as if breathing air increases the first risk and reduces the second.
If we accept the numbers provided by @inquisit the risk of oxygen toxicity is more than doubled, whilst the risk of DCs is less than halved.
You must also consider the outcome. An oxygen toxicity event has large proabibility of killing you.
Instead diving for a time equal to 75% of NDL could cause a very minor DCS case, possibly just some skin rash. The risk of death is much smaller.
So in the end it is a risk assessment and a risk comparison.
Zero risk is impossible. I prefer a larger DCS risk to an even larger (percentually) oxygen toxicity risk.
Please note that this is a modification of my previous juvanile approach, when, as other suggested, I did not worry too much about oxygen pressure.

 

[BoltSnap]

Because now I condider the oxygen risk more concerning than the risk of decompression sickness.
Breathing Nitrox for the same time as if breathing air increases the first risk and reduces the second.
If we accept the numbers provided by @inquisit the risk of oxygen toxicity is more than doubled, whilst the risk of DCs is less than halved.
You must also consider the outcome. An oxygen toxicity event has large proabibility of killing you.
Instead diving for a time equal to 75% of NDL could cause a very minor DCS case, possibly just some skin rash. The risk of death is much smaller.
So in the end it is a risk assessment and a risk comparison.
Zero risk is impossible. I prefer a larger DCS risk to an even larger (percentually) oxygen toxicity risk.
Please note that this is a modification of my previous juvanile approach, when, as other suggested, I did not worry too much about oxygen pressure.

What risk are you talking about in regards to using Nitrox? Are you talking about CNS or something else? Also, are you talking about risk emanating from exceeding MOD for the particular mix used or something else?

 

[inquis]

I can double my chance of retiring early by buying 2 lottery tickets. I'm still very unlikely to win.

 

[faze]

The quarter turn back when opening the valve on a tank. Still see it taught, drives me nuts. Outdated, irrelevant and dangerous!

 

[Angelo Farina]

What risk at you talking about in regards to using Nitrox? Are you talking about CNS or something else? Also, are you talking about risk emanating from exceeding MOD for the particular mix used or something else?

Oxygen at high pressure makes oxidative processes to run faster, so the cells "age" prematurely. It is not just the risk of getting convulsions (which is a rare event but very dangerous), it is simply that exposing my body to anomalously high oxygen concentration cause general cell damages everywhere.
As said, these oxidative processes can be the cure for some diseases, but it is a strong stimulus to the body, which I would apply only when suggested by a medical doctor, and not just "for fun".
I have witnessed the power of hyperbaric oxygen therapy. So I know first hand how large is the effect of oxygen at high pressure on the whole body.
I leave to medical doctors to evaluate if and when it is appropriate to apply it to my body. It is like assuming powerful medicines. Definitely good and life-saving if used properly, quite dangerous and potentially damaging if used without medical control.
So I am applying a precaution principle. High pressure oxygen can be good, but it can also be dangerous, so tendentially I avoid using it without medical control, exactly as I avoid taking medications without prescription.

 

[Wibble]

The quarter turn back when opening the valve on a tank. Still see it taught, drives me nuts. Outdated, irrelevant and dangerous!

Some people -- me included -- vehemently disagree with you.

I had a manifolded twinset valve stick underwater which I couldn't get enough purchase on it to turn back myself. OK, nobody died, didn't need to do a shutdown, but I wouldn't have been able to if needed.

 

[MarkA]

Some people -- me included -- vehemently disagree with you.

I had a manifolded twinset valve stick underwater which I couldn't get enough purchase on it to turn back myself. OK, nobody died, didn't need to do a shutdown, but I wouldn't have been able to if needed.

Mr Wibble

I come from an engineering background. Was always taught to turn the industrial cylinders on and then back a bit. When first learning to dive was taught to turn the pillar valve fully on and back a 1/4 turn. When the great and good of the training organization changed to say the cylinder valve is either fully on or fully off, I struggled with this and didn’t change from turning the cylinder valve back 1/4 turn.

What changed my mind? Was diving with my buddy when another diver came up to me and showed me his SPG hitting zero every time he breathed. Opened his cylinder valve and everything was OK. He was distracted while assembling his gear and the cylinder valve was only turned on 1/4 turn.

Did you find out what caused your valve to stick? At least with a twinset you have the option of closing the isolator valve.

 

[Wibble]

Mr Wibble

I come from an engineering background. Was always taught to turn the industrial cylinders on and then back a bit. When first learning to dive was taught to turn the pillar valve fully on and back a 1/4 turn. When the great and good of the training organization changed to say the cylinder valve is either fully on or fully off, I struggled with this and didn’t change from turning the cylinder valve back 1/4 turn.

What changed my mind? Was diving with my buddy when another diver came up to me and showed me his SPG hitting zero every time he breathed. Opened his cylinder valve and everything was OK. He was distracted while assembling his gear and the cylinder valve was only turned on 1/4 turn.

Did you find out what caused your valve to stick? At least with a twinset you have the option of closing the isolator valve.

I think it was probably warmer on the top than in the water.

I got another diver to turn the valve and it needed the strength that you do not have with your hand behind your head.

Am very happy that the valve is turned on then backed off a bit; I probably use about 1/8th of a turn, but not hard against the end-stop as metal contracts when it's cold.

 

[MarkA]

I think it was probably warmer on the top than in the water.

I got another diver to turn the valve and it needed the strength that you do not have with your hand behind your head.

Am very happy that the valve is turned on then backed off a bit; I probably use about 1/8th of a turn, but not hard against the end-stop as metal contracts when it's cold.

Thanks for the explanation. As you say it is not good to be hard against the open stop. Gently does it.

 

[boulderjohn]

Like almost everyone, I learned to turn the valve back a quarter turn. When I started technical diving long ago, I was told that was nonsense and our valves should be fully opened. I complied. The problem was that when I was trying to do valve drills, I found it hard to start the valves. It took a good effort to start the turn, after which it went easily. So I don't back it off a quarter turn, but I do back it off.