40% Nitrox

[boulderjohn]

Using 32% in 100 feet or less takes all the guess work out and gives you a very good understanding of how decompression works and able to do in your head your no deco limits if computer fails.

I don't know what this means.

1. What guesswork are you talking about?
2. How does it teach you how decompression works?
3. If your computer fails and you have no backup, you are supposed to end the dive, which requires no calculations.

Adding more oxygen really saves nothing from decompression standpoint and increases chance of cns tox event with the higher PO2.

Adding more oxygen decreases the amount of nitrogen and does indeed affect decompression. That's why you do it. As you yourself noted elsewhere, there is very little chance of oxygen toxicity in a recreational dive. I agree with the part where you said there may never have been such a case in history. In past years I have challenged people to find an example. The best anyone can do is find a case or two where divers who were found dead after dying with no witnesses had sometimes been breathing nitrox, and people suggest that there is a possibility of oxygen toxicity.

Oxygen is arguably more narcotic than nitrogen by looking at it scientifically all though no practical way to prove if it is, or is not.

Recent research suggests the opposite. A study published a couple years ago indicated that nitrox is a little less narcotic than air.

 

[boulderjohn]

Adding more oxygen really saves nothing from decompression standpoint

I am still puzzling over this. Here are a few quick numbers using the PADI tables for comparison. This will tell us roughly what a computer running the DSAT program would read at those depths.

• If you are diving air at 80 feet, your maximum dive time is 30 minutes.
• If you are diving 30/30 at 80 feet, your maximum dive time is 40 minutes (assuming helium is the same as nitrogen, as recent research suggests).
  
• If you are diving 32% at 80 feet, you maximum dive time is 45 minutes.
• If you are diving 36% at 80 feet, your maximum dive time is 55 minutes.
• If you are diving 40% at 80 feet, it is complicated with the tables because of a HUGE rounding issue as you convert equivalent air depth. It is technically also 55 minutes, but it is within a whisker of being 80 minutes. A computer running DSAT (no rounding) would probably have it at about 75 minutes.

Thus, running 40% on a dive in the 75 foot range will give you double the bottom time of a 30/30 dive. Of course, you have to have enough gas to make that happen, but it is possible. If you don't have the gas for it and shorten the bottom time, you will have that much less decompression stress on the body.

 

[tursiops]

Why would you want to use 40% Nitrox?

The max depth for diving in the Maldives is 30m, so 40% would give a PPO2 of 1.6 at max depth.
That is higher than many/most people would like to have for a recreational dive, but it would allow you to maximize your bottom time.
And at 25m and shallower, it is a perfectly comfortable PPO2, i.e. 1.4 or less.

 

[boulderjohn]

Decisions on this sort of thing depend upon local logistics. What mixes are available to you? How much does it cost? I thought the situation I was in for a couple of months might illustrate some of this.

I did a number of recreational dives that maxed out at around 60 feet, done off a boat that gave about a 45 minute surface interval and a maximum time allowed at depth of one hour per dive. The dive shop where I got my fills offered a maximum of 36% because of their banking procedure, with no difference in price for any nitrox mix, so that is generally what I used. With a fill card, the price was not much more than air. I also got plenty of gas in each cylinder, so I surfaced at one hour exactly on each dive.

I could have done those dives on 32% and been fine, although the second dive would have been pushing up against the NDL at the end of 60 minutes. With 36%, when I reached safety stop depth, my Surf GF was usually about 50, often less. (For those who don't know, the Surf GF is a computer feature that tells you roughly how close you would be on a percentage basis to the limit if you surfaced at that point.) One time it was close to 40. That means I was not remotely close to NDLs. My buddies (also on EANx 36) had computers that did 3 minute safety stops, so I did those stops with them, even though they were obviously not needed. So, I didn't gain any time, but the decompression stress on my body was probably about the same as if I had done the dives on air at 30 feet. With no difference in price, why not?

 

[tridacna]

OP never returned. He is an instructor so I assume that he understands the vagaries of Nitrox.

 

[boulderjohn]

OP never returned. He is an instructor so I assume that he understands the vagaries of Nitrox.

As is true in all such cases, the real audience is not the OP but the other readers who have tuned in and are trying to learn something. My responses in this thread have nothing to do with the OP; they were inspired by other posts.

 

[tridacna]

As is true in all such cases, the real audience is not the OP but the other readers who have tuned in and are trying to learn something. My responses in this thread have nothing to do with the OP; they were inspired by other posts.

Not referring to you. Just wondering why he never returned...and John, you're always inspiring.

(Could never figure out his intention anyhoo).

 

[EFX]

If you are diving 40% at 80 feet, it is complicated with the tables because of a HUGE rounding issue as you convert equivalent air depth. It is technically also 55 minutes, but it is within a whisker of being 80 minutes. A computer running DSAT (no rounding) would probably have it at about 75 minutes.

Using Dive Xcel (my spreadsheet) set to DSAT I get 77 minutes with a GF of 95/95. Arriving at the safety stop gave a SurfGF of 47. At a GF of 100/100 I get 84 minutes and arriving at the SS gave a SurfGF of 50.

 

[boulderjohn]

Using Dive Xcel (my spreadsheet) set to DSAT I get 77 minutes with a GF of 95/95. At a GF of 100/100 I get 84 minutes.

Thanks! I should have asked you!

 

[EFX]

Thanks. You don't need to ask. You can run it yourself for all kinds of dive scenarios. You can download it from here. The hosting site TinyUpload.com is down now but try again later if you're interested. If you want it right now PM me and I'll upload it to you. You can also get it from this forum on page 3. Look for "Dive Calculation Spreadsheet".