Alternate Gas Backup Computer

[tbone1004]

Say I was to use a Zoop as the Nitrox computer and An oceanic in DSAT as air computer, in Bonaire where one tens to do many shallow dives a day the could the Zoop end up getting violated?

It is highly unlikely, but step back and think about this for a bit.

You want to use that computer to do two things
Keep tabs on MOD-MOD doesn't change with the gas in the tank. Are you incapable of setting a max depth and honoring that max depth or do you need a computer to beep at you? If you are not capable, then you probably shouldn't be diving without a dive master, or arguably diving at all, but in either instance you shouldn't be in areas where you are close to the MOD of that mix. At ppO2 of 1.4, MOD of EAN36 is about 100ft, EAN32 is about 110, and EAN30 is about 120. You have no business going deeper than those depths anyway, but anything less than EAN32 is not going to be an issue within the realm of recreational depths. Just don't go anywhere near those depths if you can't trust yourself to not violate MOD without a computer beeping at you.

Second you want it to keep tabs on your O2 exposure, which I will just go straight out and tell you not to worry about it.
TLDR of the bottom is oxygen exposure is not relevant to NDL diving, so don't worry about it. Understand it, but it's not something you should put much thought into when planning dives.

Go look at the NOAA limits if you believe in them, and you'll see that at 1.4 you are limited to 180mins/24hr period at 1.4. Diving EAN32 at 60ft is a ppO2 of ~0.9 where NOAA gives you 360mins per 24hr period. You will be into serious decompression long before you hit those limits, but more importantly you won't be that deep as an average depth anyway.
5 dives/day, average depth of 33ft, average time of 1.5 hr, on EAN40 which is a worst case scenario is 7.5 hours of bottom time, at a ppO2 of 0.8 which allows for 450mins/7.5 hours. That amount of diving is insane, frowned upon by every agency in the world, and is completely unrealistic but that just barely has you touching the oxygen exposure limits for a 24hr period.
Slightly less insane but plausible is 4 dives/day, 33ft average depth, 2 hours/dive on EAN32 is 8hrs @0.64 where you are allowed over 10 hours of exposure so you're nowhere close.
If you go deep, for 4 dives/day avg depth of 66ft on EAN32 with a 1hr dive is still 4hrs @0.96 when you're allowed 6 hours.


So, we circle back to the original point which is you don't need a computer to track your actual oxygen exposure, if you need a computer to beep at you for MOD then that is an equipment solution to a skills problem and you need to fix your skills. If you're going to follow outdated advice of lying to your computer *and no I really don't care that DAN published it this year, it doesn't mean it is considered best practice*, then you do you, but run both computers similarly so they are both doing something useful, and if you run two computers then they should have similar algorithms that you understand and are in control of. I do not think you should lie to your computer about the mix that you're using, but if you want to dive more conservatively we have given you the tools to figure out how to do it.

 

[tbone1004]

An article just published in a special safety issue of Alert Diver, the DAN puplication.

who wrote the article? That also weighs much more heavily into the validity of it. Being published in that magazine does not mean a whole lot to most of us, it all depends on who wrote it.

 

[arew+4]

I want to run back up computers in nitrox mode at for actual gas values to track O2 exposure, and MOD etc.

Question is how important would it be to match deco algorithms?

TIA,

Because in the event of an accident/medical emergency it would be prudent to have actual data that is representative of reality.

Not because I cannot track depth and or time. My first 100 dives or so were done with tables and a watch and a depth gauge.

 

[tbone1004]

Because in the event of an accident/medical emergency it would be prudent to have actual data that is representative of reality.

Not because I cannot track depth and or time. My first 100 dives or so were done with tables and a watch and a depth gauge.

if you want an accurate representation of reality then don't lie to your computer and the investigators will have all of the data on a single device instead of trying to figure out why you had two computers diving two completely different profiles and control your conservatism within the algorithm instead of lying to it.

 

[inquis]

... or equivalent computer settings.
Which is what everyone has recommended.

 

[arew+4]

Not expecting any catastrophic

... or equivalent computer settings.
Which is what everyone has recommended.

Re read that.

 

[Belzelbub]

if you want an accurate representation of reality then don't lie to your computer and the investigators will have all of the data on a single device instead of trying to figure out why you had two computers diving two completely different profiles and control your conservatism within the algorithm instead of lying to it.

Exactly. In the event of an actual emergency, having redundant sources of information is good, but only if those two sources are saying the same thing. If there is an actual emergency, the diver may not be able to answer questions about which computer has the right infornation.

@arew+4 Set the two computers to the same gas. Adjust conservatism on one to what you want to dive. If you want to avoid having a computer lock you out, follow the plan of the more conservative computer. If it wants you to do a deco stop, do it, unless gas supplies don’t permit. If that is the case, you’ve messed up the dive, and might be wise to stay out of the water for a bit.

 

[broncobowsher]

who wrote the article? That also weighs much more heavily into the validity of it. Being published in that magazine does not mean a whole lot to most of us, it all depends on who wrote it.

To that point, I have written articles that have been published in magazines. Nobody ever asked for my reference material. Nobody ever checked my credentials. I wrote it off the top of my head as I felt it should be. 15+ years later I still see people referencing that article.

What I read is 'old school'. Someone who was taught that way 20 years ago. Has not modernized.
"safety margin with Nitrox is achieved only by diving air tables or equivalent computer settings" is wrong. The word "only" is what makes that statement false. That is one way of adding a safety margin, but far from the 'only' way of doing it. A lot of computers let you adjust your conservatism. Some will give you a couple options, others will let you really adjust things a lot. Remember the line "garbage in, garbage out" True with dive computers. You put lies into the computer, you will get manipulated lies back at you. Much smarter to just use a computer correctly, feed it good accurate data. One of those data points is that you want a conservative profile. What it will give you in return, accurate data with the conservative profile you asked for. For example, have a read around page 25 https://www.shearwater.com/wp-content/uploads/2021/09/Peregrine_Manual-metric_2021.pdf

 

[scubadada]

There are 2 methods to track oxygen exposure. The NOAA exposure limits are for a 24 hour period. The Seiko Epson method employs a 90 minute elimination half life Theory behind the half-life of CNS toxicity?

Diving from my home in Florida, I frequently exceed the NOAA exposure limits when I have five 70 minute dives, with an average depth of about 60 feet, using 36% nitrox, in a 24 hour window. This exposure is 350 minutes, where the limit at a PO2 of 1.0 is 300 minutes. My trusty old Oceanic VT3 computer uses the NOAA limits and warns me of my transgression.

Many/most computers use the Seiko Epson method with the 90 minute oxygen elimination half life. This includes my Shearwater Teric. Using this method, you never come close to reaching oxygen exposure limits in recreational diving. The highest oxygen exposure I have had on my Teric is just 44%

 

[inquis]

... or equivalent computer settings.
Which is what everyone has recommended.

Re read that.

?? Lots of computers use gradient factors for conservatism, and a GF of x/95 is very close to the DSAT algorithm used by the PADI tables. Consider a dive to 60 ft on EAN36 with computer set for one of the following:

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

54 minutes vs. 54 minutes. Equivalent. Computer. Settings.