Diving 32% Nitrox with "Air" Algorithms

[dmaziuk]

I was having this conversation with DAN, in the interests of diving more conservatively, they recommend (as part of other items) diving with two computers running the same algorithm, but one is set to air, whilst the other is set to your actual nitrox mix.

It's probably sad that I find this entirely unsurprising... but I guess that's what happens when you live to be the same age as the old people.

 

[Angelo Farina]

For anyone with a computer that has adjustable GF's, there is no point in using air instead of actual O2. I can see it for computers that have limited conservativness settings, but even then I think there are better ways.

I own the cheapest computer on the market, the Cressi Leonardo, bought new for 99.9 eur on Amazon Prime Day.
Despite being a very basic unit, it allows to specify a Nitrox mixture up to 50% and to set the conservatism factor to three levels.
At max conservatism, diving Nitrox 32% has just slightly larger NDL than diving air at minimum conservatism (which indeed is already more conservative than most other computers).
So even using this supercheap computer I do not see the benefit of using it improperly, setting it to an oxygen percentage different by the real one.
And if one wants even more conservatism, it is possible to set the NDL alarm at 10 min before the NDL instead of 5 minutes.

 

[BoltSnap]

Can't folks set their dive computer with the correct information/gas mix and look at the NDL it gives and come up sooner than the NDL limit given by their dive computer for the added conservatism they want??? Is this so difficult for people to do?

 

[arew+4]

Can't folks set their dive computer with the correct information/gas mix and look at the NDL it gives and come up sooner than the NDL limit given by their dive computer for the added conservatism they want??? Is this so difficult for people to do?

Apparently yes! Even with the best intentions of begining to surface with at least 10 minutes of NDL, it's real easy to see something really cool and get distracted and ascend with only 5 minutes remaining.
Also for people who have had instances of minor DCS while operating well within the limits of their dive computers, I can see where the passive safety margin of diving EAN on Air would be an attractive option. Especially for mostly benign recreational dives to 100' or less.
When you alter the model, you are still working off the basic assumptions that got the person bent, uptake and release of nitrogen vs. differential pressure. When you leave the model alone, and reduce the level of Nitrogen, no matter what the computer model says you will have less nitrogen per "compartment".

 

[Belzelbub]

Apparently yes! Even with the best intentions of begining to surface with at least 10 minutes of NDL, it's real easy to see something really cool and get distracted and ascend with only 5 minutes remaining.

Overall, I agree. It does seem to be an issue for some divers. But, they'd probably have the same problem regardless of gas mix settings, conservatism settings, etc.

Also for people who have had instances of minor DCS while operating well within the limits of their dive computers, I can see where the passive safety margin of diving EAN on Air would be an attractive option. Especially for mostly benign recreational dives to 100' or less.
When you alter the model, you are still working off the basic assumptions that got the person bent, uptake and release of nitrogen vs. differential pressure. When you leave the model alone, and reduce the level of Nitrogen, no matter what the computer model says you will have less nitrogen per "compartment".

The same can be done by utilizing the correct gas mixture. Knowingly putting in a false value can provide a safety buffer, but for people that are going to push things, it may be a false safety buffer. If they see their NDL down to single digits, they may rationalize that they really have a bit more time than they actually do. Net result may be the same.

I think we probably discussed this earlier in this thread (but it might have been another) where sometimes people who are habitually late set their watch ahead a bit to try to not be late. In the end, that's usually not effective, because they know it's 5 minutes fast, so they take those 5 minutes and end up being late just like before.

 

[L13]

Apparently yes! Even with the best intentions of begining to surface with at least 10 minutes of NDL, it's real easy to see something really cool and get distracted and ascend with only 5 minutes remaining.
Also for people who have had instances of minor DCS while operating well within the limits of their dive computers, I can see where the passive safety margin of diving EAN on Air would be an attractive option. Especially for mostly benign recreational dives to 100' or less.
When you alter the model, you are still working off the basic assumptions that got the person bent, uptake and release of nitrogen vs. differential pressure. When you leave the model alone, and reduce the level of Nitrogen, no matter what the computer model says you will have less nitrogen per "compartment".

Setting lower GF's, or setting a more conservative setting on computers without GF's, reduces the level of Nitrogen in a controlled way. The information your computer is giving you is real. Setting your computer to Air while breathing EAN reduces nitrogen in an uncontrolled way. Your computer is not telling you the real info.

Staying away from the 5min or 10min warning on your computer is just as easy as staying away from the 1min or 0 NDL warnings from your computer. You can just as easily get distracted and exceed NDL set to Air as exceeding 10min NDL left on EAN.

If you aren't paying attention to your computer, it doesn't matter how it is set!

 

[L13]

...Also for people who have had instances of minor DCS while operating well within the limits of their dive computers, I can see where the passive safety margin of diving EAN on Air would be an attractive option. Especially for mostly benign recreational dives to 100' or less....

Would you suggest to those same people to set their computers to EAN 14% when diving on Air? That is the air dive equivalent of diving EAN with the computer set to Air. I don't see how one makes sense and not the other. and yet I doubt very many would go for this.

 

[scubadada]

Please see the post I made in a different thread regarding the risk of DCS when diving near the NDL. The examples are for diving 32% nitrox using a computer that runs Buhlmann ZH-L16C. A GF high of 95 would generally be considered liberal. A GF high of 85 would generally be considered middle of the road or moderate. A GF high of 75 would generally be considered conservative.

Easily, the best option here is to set the computer accurately, reflecting the nitrox mix you are diving. Then, choose your level of conservatism to match your personal needs. In addition, adhere to good diving practices such as the ascent profile and attempt to reduce personal risk factors for DCS.

Is there data on how close to NDL undeserved hits occur?

My personal belief is that, absent medical factors which change the DCS probability (like PFO, since we all bubble), it's ascent rate, particularly in the last 30 feet, which plays a much more significant yet underappreciated role. Well this is another factor that we could get data on. I'm...

scubaboard.com

 

[arew+4]

Would you suggest to those same people to set their computers to EAN 14% when diving on Air? That is the air dive equivalent of diving EAN with the computer set to Air. I don't see how one makes sense and not the other. and yet I doubt very many would go for this.

What I am suggesting is that if a person's individual physiology does not fit the model, adjustments to the model may not adequately compensate for the these deviations as they are not well characterized. Do they uptake faster, do they off gas slower, is their deviation depth dependant? All that is known is that the model has failed them, therefore reducing the exposure while tabulating in the same way would reduce the risk of re-occurrence.

 

[tursiops]

There may be some misconception in this thread about what a NDL means, whether on a table or a computer. Yes, there is a lot of individual physiology and specific dive variability, but the way the NDL is chosen is NOT to put a line through the middle of all that variability, but rather use smaller NDLs to allow for most (not all) of that variability so that most (not all) people and dives will be "safe." Adding conservatism means to make that table/computer NDL even smaller, i.e. farther away from all that variability.

I get the impression some think the table/computer NDL is through the middle of the variability, so that adding conservatism means to make the NDL smaller...i.e. move it away from all the individual and dive-specific variability.

Here's a picture to try and explain: