Is my approach to diving with Nitrox logical?

[MaxBottomtime]

The incident rate of DCS cases is already low, less than 1/10th of 1% per number of dives. Diving 40% will increase safety, but not enough to make any real difference. It would be like driving 64mph on the highway rather than 65mph.

 

[NAUI Wowie]

very good point, I have been thinking about running two computers. one air one Nitrox for this reason

except if your nitrox computer malfunctions then what? best to have both computers set for the gas you use and just cut some time off your dive. Same thing but more accuracy if you lose a computer on a dive

 

[NAUI Wowie]

Let’s say that you are a very safe diver and you are worried about decompression sickness, so you want to optimize your Nitrox mix. Then I am assuming that you are also worried about running out of gas for you and your partner, so you have calculated the minimum amount of gas you and your partner will need to share off each other to get to the surface safely for the depth that you plan to dive.
Let’s say that depth is 80 feet, your calculated minimum amount of gas before ascending is 30 cubic feet at a SAC rate of 1 cubic feet/minute at the surface for each diver. You dive AL80s, each holding 77 cubic feet. This means that you should both start ascending at a gauge reading of 1200 psi. That gives you 47 cubic feet to use at 80 feet (3.4 atm). That gas will last you 14 minutes before you reach rock bottom pressure and you have to go up, well before the 25 minutes NDL on the PADI air tables and nowhere close to the 36 minutes on the 32% nitrox PADI table.

To even match the PADI air NDL limit at 80 feet, you would need to bring 115 cubic feet of air. Assuming that you aren’t each diving HP120s, the limit of you diving “safely” is not the NDL, air or nitrox, but the gas in your tank.

THIS is why god invented dive computers!

 

[NAUI Wowie]

glitch?

 

[BoltSnap]

If you are pushing your NDL with Nitrox or Air it doesn't matter as you are still absorbing Nitrogen. You can cheat it some by running your computer in Air but you need to know your MOD for the gas your breathing.

very true, and yes running two computers would be ideal set up for this

Why on God's blue ocean would you lie to your dive computer(s)? If you are using a second computer as your backup in case your main goes down, then it should be set exactly as your main not set with false information. If your main goes down, or the backup, then you are Shiit out of luck. Also, trying to mange using two different dive computer with two different set of "settings" and two sets of dive limits underwater with increased narcosis effects is just too much and confusing especially when there is too much going on around you. This is asking for trouble.

There is a very simple way to handle what you are concerned about, whatever your O2% is, configure your dive computer according to this % and your backup dive computer just the same. If you want a physiological advantage of using Nitrox, then set your "conservatism" factor in your dive computer(s) to a higher conservatism factor and you get more conservative NDL and, at the same time, correct MOD and CNS, etc. from your dive computer without any confusion or additional task loading managing two dive computers with different settings and erroneous limits displayed.


If custom mix is available for me without too much headache, I will pick a best mix for my anticipated dive's max depth - 2%. 2% less than the best mix % gives me a slight margin for when the boat has to go to another dive site that maybe a little deeper than planned. The boat isn't going to go to a much more deeper alternate site most likely anyways.

Don't lie to your dive computer(s) so it won't lie back to you!! TELL the computer to be more conservative with the correct settings and the computer will do its job properly and accurately. Do NOT lie, otherwise God will send you to hell!

 

[tursiops]

The threshold is a moving target. I believe up until the last few years, 1.6 or 1.5 was considered "safe". 1.4 is certainly better and a safer standard.

1.4 has been the recreational standard for at least a quarter century.

 

[kelemvor]

The incident rate of DCS cases is already low, less than 1/10th of 1% per number of dives. Diving 40% will increase safety, but not enough to make any real difference. It would be like driving 64mph on the highway rather than 65mph.

I thought I'd seen somewhere that there was a recent push to de-emphasize DCS in training and emphasize things that actually are happening more frequently. Can't quite put my finger on where I got that idea... Might have been some of the nsscds presentations I watched recently.

 

[BoltSnap]

1.4 has been the recreational standard for at least a quarter century.

Not really, 1.5 was for a while.

 

[inquis]

So pulmonary toe would be related more to many repetitive dives on a rich mix?

Yes, pulmonary toxicity involves repetitive dives. However, the dependency is on ppO2 (which combines the effect of "richness" and depth). The NOAA Diving Manual discusses this (PADI, and perhaps other agencies, teaches the same material).

However, you would be very hard pressed to exceed the recommended limits as a recreational diver. At ppO2 of 1.4, your exposure accumulates at 1.63 OTUs/min. Limiting daily exposure to 300 OTUs (the lowest daily limit NOAA discusses), that's more than 3 hours at that partial pressure every day. Even backing off of that, to say 80%, is going to be very difficult to achieve. You'd probably hit the CNS toxicity limits first, but they're also very difficult for a recreational diver to reach.

 

[inquis]

Thought about a second computer to run on air while one is on Nitrox for this reason

That gives a variable cushion depending on the mix you actually breath. I think a better approach is to use a computer using the ZHL-16GF algorithm (Buhlmann+Gradient Factors) and tell it the truth about what you're breathing. In return, it will give you a consistent cushion for the actual gas you're using. Another option is to regard an NDL of 5 mins (or 10 or whatever) as the "do not exceed" point, and again, don't lie to your computer.