Nitrox 32 first dive, air second dive?

[DandyDon]

I notice a fairly substantial difference. Diving on air, I'm usually in bed asleep by 10 or earlier. On Nitrox I can go get wild.

The placebo effect is marvelous.

No, you are virtually clean after your slowest tissue has had 6 half-times to offgas. 90 minutes would only clear the 15-minute compartment, which is one of the fastest. PADI uses the 60-minute compartment to track satus; the US Navy uses the 120-minute compartment. So, 6h (PADI) would be a safe wash-out time, not 90 minutes. Either way, of course, you are in pretty good shape by the next day's diving.

Did you read by my statement of "virtually clean after 90 minutes according to tables," or just ignore it? To that, I also included "but your computer will show time to fly which is more realistic. There is still some buildup left, but too small to calculate."

I personally find going deeper detracts from my experience

I enjoy the buzz.

There is no shortage of divers who got bent who never came close to their NDL

Well, those are actually rare. If you go to a Coz chamber with a DAN card and any complaint at all, you're likely to get cotton PJs and a ride; better safe than sorry and mostly they can use the income. I think that real hits have more to do with fast ascents, skipped safety stops, skipping float time (Dr Deco calls that the "hidden safety stop" but whatever. Always good to float effortlessly for a minute after a very slow ascent from 15 ft.) and exertions after diving.

 

[Ron Lee]

As first dives are often on walls, and/or deeper, using air gives you more max depth in the case of an emergency. As many/most of the second dives are more shallow, and are usually over a hard floor, you don't run into the MOD issue.

This is why I prefer air on the first dive. Should someone start sinking and need assistance, using air allows a greater depth for a given ppO2 limit.

 

[tursiops]

This is why I prefer air on the first dive. Should someone start sinking and need assistance, using air allows a greater depth for a given ppO2 limit.

Doesn't this argument mean you prefer air on ALL dives?

 

[tursiops]

Did you read by my statement of "virtually clean after 90 minutes according to tables," or just ignore it?

I'm sorry you do not understand my response, which is based on tables. I can only surmise you do not understand tables.

 

[nodakdive]

Doesn't this argument mean you prefer air on ALL dives?

He may be referring to the specifics of Cozumel 2nd dives? For instance, many have heard the joke that, "...if you want to go deeper than 25 feet at Columbia Shallows, you'll need a shovel...". You could make the same joke, substituting different depths, for many of the common 2nd dives there. There are often hard floors well above the MOD for EAN36. You won't dive Paradise and suddenly find yourself at 110 feet because of a down current. You might find yourself face to face with a cruise ship propeller, but that's a different consideration.

 

[MMM]

He may be referring to the specifics of Cozumel 2nd dives? For instance, many have heard the joke that, "...if you want to go deeper than 25 feet at Columbia Shallows, you'll need a shovel...". You could make the same joke, substituting different depths, for many of the common 2nd dives there. There are often hard floors well above the MOD for EAN36. You won't dive Paradise and suddenly find yourself at 110 feet because of a down current. You might find yourself face to face with a cruise ship propeller, but that's a different consideration.

I understood him to mean "wouldn't you want to save everyone on a dive" if diving air would enable you to do so. But perhaps he will elaborate.

 

[mstevens]

If I use a pO2 of 1.4 as a limit, Nitrox 32 has an MOD of 111 fsw. For a pO2 of 1.6, MOD would be 132 fsw. Based on my own reading of the available literature, I prefer to limit my exposure to a pO2 of 1.4.

Everyone has their own preferences, but nearly 15% of my Cozumel dives are deeper than 111 fsw. I have quite a few below 140 fsw, for which at a pO2 of 1.4 best mix would be 26%. For a 160+ fsw Maracaibo dive or a Barracuda dive if currents get flaky, I wouldn't want anything more than 23% oxygen.

Except for a couple of shops that do their own blending, you can order Nitrox on the island in nominal 32% and 36%, and most shops can't supply anything else*. I've ended up with tanks that differ markedly from their labeled content, so I personally assume that I could end up with "Nitrox 36" on the boat even if I'd ordered "Nitrox 32" or vice-versa. I'd also want to assume that a cylinder that was blended to be "36%" could analyze at 34-38%, which has happened to me often. That means I could end up with MOD determined by a cylinder that tests out at 38%.

Thus I dive air for my first dive unless I absolutely know I'll be at a site with a bottom of 90 fsw or less (yeah, MOD at a pO2 of 1.4 for 38% is 89 fsw, but I'm reckless). The only way I can be certain of this is if I know every diver on the boat or have chartered it. With the shop with which I dive and the DM with whom I dive, it's a safe assumption that 36%(-ish) will be appropriate for my second dive.

I never assume there will be a spare cylinder on board. Maybe there will be, but I do not want anyone swapping me their air for my mislabeled nitrox on a deep dive. I know many DM's would do that to keep divers happy, even at risk to themselves. If I'm planning to dive nitrox, it's MY responsibility to know what I'm doing and to be aware of likely contingencies.

I do understand that people are on a budget. I don't understand why anyone would quibble over ten or so bucks for nitrox given what diving costs in general unless they simply don't like, agree with the value of, or "believe in" nitrox. In those instances, though, cost is not the real objection. Otherwise, I can save up more before a trip or skip a few margaritas.

And, seriously: has anyone's nitrox training ever failed to emphasize the necessity of personally verifying ones gas? Every boat I've ever been on in Cozumel has an analyzer if there's nitrox on board. If not, you can use mine.

*No, "EAN21" isn't a thing, since there's nothing "enriched" about plain air.

The placebo effect is marvelous.

It certainly is.

Placebo (and nocebo) effects are puzzling and difficult to understand. They're not, however, imaginary or fake. They can be pretty significant, which is why study designs must account for them.

 

[deepsea21]

I guess my point is that it's sort of arbitrary to say ahead of time that this or that depth is good enough, and that anything lower than the random depth that I am currently at isn't worth the descent. There is all sorts of stuff to see everywhere. All nitrox does is give you less N2 loading above the MOD. Unless you know that your target depth is below the MOD of your available mix, what's the downside? $70 extra? If you are flying to the Caribbean for a dive vacation, I just can't imagine that being the thing that makes the decision.

Unless you are diving very shallow, you will either have a safer dive or a longer dive. Why wouldn't you want the option of staying longer?

With the general shallower and more conservative profiles we dive on air we already experience bottom times of 60+ minutes and are usually the last or among the last to surface (which is why we always dive with a SMB). Given our profiles and Tres Pelicano's guidelines that all are free to dive their own tank, DC or 70 minutes (whichever comes first) we've no need for Nitrox as if we pushed it further we would hold up the group from their SI at the beach.

As far as cost is concerned, $10 a tank doesn't seem like much but 2 people, 2 dives a day for 12-13 dive days that's around $500. Heck, we eat great dinners with drinks at great places every night for a lot less than $500 over a 2 week trip. Nitrox we don't need or great food every night? Easy decision.

 

[doctormike]

With the general shallower and more conservative profiles we dive on air we already experience bottom times of 60+ minutes and are usually the last or among the last to surface (which is why we always dive with a SMB). Given our profiles and Tres Pelicano's guidelines that all are free to dive their own tank, DC or 70 minutes (whichever comes first) we've no need for Nitrox as if we pushed it further we would hold up the group from their SI at the beach.

As far as cost is concerned, $10 a tank doesn't seem like much but 2 people, 2 dives a day for 12-13 dive days that's around $500. Heck, we eat great dinners with drinks at great places every night for a lot less than $500 over a 2 week trip. Nitrox we don't need or great food every night? Easy decision.

You are right. Dive air.

 

[halocline]

Interesting. The opposite approach of what I was thinking. Let me mull that one over. I like the idea of the added depth safety for dive 1. I have almost run into NDL's on the first dive on air though, and that's using an Oceanic Computer.

I have been diving in cozumel with an aeris computer for many years and have never violated NDL on air. I go as deep as I want to (sometimes 110-115 feet and I've done the obligatory devil's throat a few times which is deeper) and I don't understand how you could really have a deco obligation, unless you are staying a lot deeper than I generally would. I personally don't pay too much attention to NDL anyway; I'm much more interested in the N2 loading bar graph towards the end of the dive. With the right profile, you can nudge up close to NDL near the beginning of the dive and easily surface with your N2 load in the 'green' zone. I do extended stops at 10 ft, especially after the 2nd dive, and that has helped quite a bit with post dive fatigue.

I have never paid for nitrox in Cozumel while on 2 dive days. To me it's not worth it, I just do my long stops and I feel fine on surfacing. If you really feel that you are being limited in your profile by air, then it might be worth it for you, but you might be able to enjoy your long deep dives just as much on air as long as you are willing to extend your safety stop.