Alternate Gas Backup Computer

[tursiops]

Not to beat a dead horse, but here is some recent "old hearsay" from DAN;
View attachment 693272View attachment 693273

This quoted material is taken from a Q3 Summer 2011 article by Nicholas Bird in ALert Diver, called "Stacking the Deck."

Stacking the Deck

Dive safety is imperative and little precautions and steps you take can collectively make a big difference. Read more on dive safety.

dan.org

Nice to know we've learned nothing in over a decade.

 

[boulderjohn]

The recommendation to use nitrox within air limits is very old school, and some very experienced people repeat that sort of nonsense without giving it any thought. Dive Training magazine, led by a very experienced and knowledgeable diver, used to repeat the same sort of nonsense from time to time. Looking at what it said and giving it a few minutes of thought may be illuminating.

The articles said that divers had two (and only two) choices when it came to nitrox:

1. Dive to the nitrox limits and get longer bottom time
2. Dive to air limits and get better safety

Those articles emphasized that it was not possible to get more safety and more bottom time--it was one or the other. That is exactly the same thing implied by the DAN article. That argument assumes that a diver diving within the nitrox limits must dive fully to those limits, even if that requires using extra tanks to reach those limits.

That is, of course, absurd. The limit for 80 feet on the PADI air tables is 30 minutes. The limit for 80 feet on the PADI EAnx 36 tables is 55 minutes. According to this argument, a diver taking EANx 36 to 80 feet has only two options--1) stay for 30 minutes according to the air table or 2) stay for 55 minutes according to the EANx table. Such a diver is apparently not allowed to dive for 40 minutes at that depth, which would give both better safety and longer bottom time.

 

[inquis]

• Air with GF x/95: 54 minutes NDL
• EAN36 with GF x/58: 54 minutes NDL

@arew+4 The safety margin seems to be your primary interest. Are you aware that when using Air tables, the safety margin changes with your actual nitrox content?

For example, consider using EAN32 for that 60 foot dive for 54 minutes -- i.e., using air tables. That is equivalent (same bottom time) to a gradient factor of x/68. Your safety margin on EAN32 is only 32% instead of the 42% you get on EAN36. If the shop were out of nitrox that day, your safety margin would be 5%.

I would be greatly concerned by that variability. In contrast, I set a consistent safety margin with which I'm comfortable. When the computer knows the actual nitrox content, its guidance is reflective of that desired margin. (This is also true for computers that don't use gradient factors, BTW.)

 

[Wibble]

Look I'm not trying to convince you or anyone else, this just came in the mail the other day. I figured I would post it as a counterpoint to the overwhelming unanimous consensus of the "Board" that seems to indicate that I'm some sort of heretical idiot, or at best a misguided simpleton.

I'm most definitely not implying that. If it came over that way, I unreservedly apologise.

My point is that there are often conflicting pieces of 'advice' which can confuse things. In this case the article you cite is, as you've highlighted, quite clear that setting your computer to "air" when using nitrox is more safe. That's rather old-school thinking.

This is working by limiting the NDL to an air loading without considering the reduced nitrogen loading from using nitrox.

Most of the people here consider that to be unnecessary. Not least because whatever software you use to log and display the dive will be invalid. Also the computer's surface time interval -- off gassing of the nitrogen between dives -- will be incorrect, possibly meaning you'd have to needlessly sit out a subsequent dive.

People here including myself are suggesting that you set up the computer with the correct oxygen percentage and then use the conservatism setting built in to all algorithms.

Edit: @boulderjohn's post explains this very well.

 

[broncobowsher]

This quoted material is taken from a Q3 Summer 2011 article by Nicholas Bird in ALert Diver, called "Stacking the Deck." Nice to know we've learned nothing in over a decade.

I'm glad someone found the source, and confirmed the 'old school' of it.
It's like reading old books that state "The Titanic which has been lost and never found" or "someday man may go to the moon". The statement was relevant when it was published, but times have changed. Add to the ugly that the article was regurgitated a decade later without modern updates applied. Now it looks like the current state of how things work, when it isn't.

 

[NothingClever]

Look I'm not trying to convince you or anyone else, this just came in the mail the other day. I figured I would post it as a counterpoint to the overwhelming unanimous consensus of the "Board" that seems to indicate that I'm some sort of heretical idiot, or at best a misguided simpleton.

Idiot? No.

Simpleton? No.

Good intentions? Absolutely. No doubt about it.

Misguided? I‘m afraid so.

You’re a grown man so you don’t need the internet’s blessing or permission to make your own decisions about how to mitigate risk. However, I would suggest you consider that if you run two computers with different gas settings as a technique to guard against DCS during NDL dives, you will be in a very, very small population. If you were to have a hit, I anticipate the medical provider will want to review your computers and I think your technique will end up causing distraction and confusion.

Learning is winning.

Enjoy the journey.

 

[arew+4]

Old or not, they reprinted it this year, indicating the organization (or at least the editor) still stands behind the advice

 

[tursiops]

Old or not, they reprinted it this year, indicating the organization (or at least the editor) still stands behind the advice

It would be best not to cherry-pick or misinterpret the Alert Diver article, either the original or the reprint. The article does NOT say to set your computer on air if using Nitrox.

It says: "the safety margin enjoyed with nitrox is achieved only by diving air tables or equivalent computer settings." You are not diving air tables, but you CAN dive equivalent computer settings (to air) by using conservatism with a Nitrox setting; then your computer is set on the right gas, your MOD is right, your O2 exposure (although irrelevant) is right, your medics won't be confused when you get bent, and SB can finally get off your back. And both your computers will then agree.

 

[broncobowsher]

Old or not, they reprinted it this year, indicating the organization (or at least the editor) still stands behind the advice

Or they were desperate for filler material.

Editor is looking for material. I've done monthly newsletters for an organization in the past. When you are getting close to deadline and none of your un-paid volunteer staff has submitted enough material, you go and start looking for filler. Old stuff that few have seen or remember that an old buddy wrote and said you could use a year ago. Awesome! Got something to fill the pages. Cut, paste, print. It was a good source, no need to check it.

 

[boulderjohn]

It may interest some people to know that on two different occasions I wrote letters to the editor of Alert Diver challenging something they published. In the first case, they contacted me and asked if I would be interested in writing a responding article. I said I did not feel I was expert enough to write it, so they got a real authority who published an article that pretty much contradicted what they had written originally. In the second case, they printed my letter and wrote a response that pretty much said what they had written had no real validity and could be ignored.