Diving 32% Nitrox with "Air" Algorithms

[John Miller]

With just over 6000 logged dives since 1973 and a Instructor Trainer, all my profession dive career I have hated it when I hear divers and pro's say they dive nitrox with their computers set to air - it is a stupid practice to be telling inexperinced divers taking their first EAN certificatio class. Several people have already warned the dangers of this practice being you are not taking the oxygen partial pressure residuals into consideration. It might be fine if all you are doing is two recreational dives, but on a live-a-board where up to 5 dives a day over a week you are risking your health and possibly your life with that practice. Moreso if they are deep dives of 100+ feet. It is why most good dive computers force you to change the EAN value for each dive. On my own, most live-aboards and shore operations give you 32 so I will tell my computer lock it in until I change it. Diving Truk Lagoon they gave us EAN28 all week except for the San Francisco Mauru which we used EAN26. I will sample every Nitrox tank regardless and if the sample is +/- 2% I will change the computer setting. Most computers have various and multiple conservative settings. Garbage in - garbage out. If you aren't going to set the computer to your nitrox level, don't dive nitrox.

 

[BoltSnap]

On my own, most live-aboards

Do own your own live-aboard?

 

[tursiops]

It is why most good dive computers force you to change the EAN value for each dive.

No, not really. Some do, fewer now than before. A mfg way to force you to make sure your O2 percentage it set correctly, but more often than not it just means you dive on "air" and don't know it.

 

[Belzelbub]

No, not really. Some do, fewer now than before. A mfg way to force you to make sure your O2 percentage it set correctly, but more often than not it just means you dive on "air" and don't know it.

Yeah. The computers I use currently, default to the last used setting. It's really not a problem as the value is prominently displayed and easily changed when starting a dive.

The Oceanic I used to use did default back to air, so it needed to be set. It also had an option to revert to an impossible mix if you didn't manually start the dive. It would assume 21% O2 for N2 calculations and 50% O2 for MOD/O2 calculations. I disabled that on mine as I thought it was a bad idea.

However, there seems to be people that still insist on using the air setting regardless of what mix they are using. This might be a solution for them. Find a computer that runs the 21/50 default, and never manually start the dive. You'll have the safety buffer for N2 in place, and will be fine with MOD as it would assume 50% O2 for that. Just don't dive too deep.

 

[tursiops]

Yeah. The computers I use currently, default to the last used setting. It's really not a problem as the value is prominently displayed and easily changed when starting a dive.

The Oceanic I used to use did default back to air, so it needed to be set. It also had an option to revert to an impossible mix if you didn't manually start the dive. It would assume 21% O2 for N2 calculations and 50% O2 for MOD/O2 calculations. I disabled that on mine as I thought it was a bad idea.

However, there seems to be people that still insist on using the air setting regardless of what mix they are using. This might be a solution for them. Find a computer that runs the 21/50 default, and never manually start the dive. You'll have the safety buffer for N2 in place, and will be fine with MOD as it would assume 50% O2 for that. Just don't dive too deep.

I had one of those computers that reverted to 50% O2...it was a pain in the a$$. More than once I had to ascend, spend 10 minutes at the surface waiting for the computer to think it was a new dive, reset the computer, and then descend again.

 

[Eric802]

It's a little less than 1.6, and 1.6 is considered the contingency depth. If you take a nitrox class, which apparently you have not, you will be taught that although 1.4 is the standard, it is acceptable to go to 1.6 when needed. (That is with almost all agencies.)

When PADI made their EANx tables, they were considered to be nitrox-phobic, so it was a big step for them. As I pointed out before, their tables give you 18 minutes at 130 feet while diving EANx 32. Are you saying that their decisiopn making was flawed? IF so, please identify the data that led to their poor decision making and why their decision was wrong.

How big is that risk? I sadly know of more than a few cases of technical divers dying from oxygen toxicity. I do not know of any cases of recreational, NDL divers dying of oxygen toxicity. If you know of one, please post a link to it. (I have made this challenge several times in the past; so far, nothing.)

As I mentioned in another EAN thread a while back, SDI's Computer Nitrox course still teaches 1.6; 1.4 is mentioned for those wanting more conservatism.

 

[Belzelbub]

I had one of those computers that reverted to 50% O2...it was a pain in the a$$. More than once I had to ascend, spend 10 minutes at the surface waiting for the computer to think it was a new dive, reset the computer, and then descend again.

Yep. I was being mostly facetious. When I first got that computer, I thought for a minute that it might be a good idea, but then I thought harder about it and decided to turn off that option.

 

[BRT]

Yeah. The computers I use currently, default to the last used setting. It's really not a problem as the value is prominently displayed and easily changed when starting a dive.

The Oceanic I used to use did default back to air, so it needed to be set. It also had an option to revert to an impossible mix if you didn't manually start the dive. It would assume 21% O2 for N2 calculations and 50% O2 for MOD/O2 calculations. I disabled that on mine as I thought it was a bad idea.

However, there seems to be people that still insist on using the air setting regardless of what mix they are using. This might be a solution for them. Find a computer that runs the 21/50 default, and never manually start the dive. You'll have the safety buffer for N2 in place, and will be fine with MOD as it would assume 50% O2 for that. Just don't dive too deep.

Our Oceanics allow us to leave the last nitrox setting in or to go the the 21/50 if not set. I choose to leave the last nitrox setting in.

 

[boulderjohn]

The worst were the computers that reverted to air after 24 hours. A diver would check his computer before a dive, go about other preparations, jump in the water, and then discover at some later point that it had changed before the descent.

On the other hand, here is an example of a problem with a computer that keeps its setting. I did a Scuba Refresher for a woman who dived annually on vacation. We started the class by setting up her gear, which included an expensive computer that was air integrated on the HP hose. She set her gear up, opened the valve, and announced that the computer had registered both the pressure and the oxygen percentage of the gas--32%. I had a deuce of a time convincing her that the computer was not reading the oxygen percentage in the gas and that she had set it on 32% at some point in the past.

 

[dmaziuk]

See also: "before going to sleep, the programmer puts a glass of water on his nightstand in case he wakes up thirsty, and an empty glass in case he wakes up not thirsty".

I would argue that a full reset after 24 hours is the less unreasonable default because it's a "well-known" one: no-fly time, missed deco lockout (except when that's 48). But it's one of the ones where you're damned if you do and damned if you don't.