Two cylinders-1 Nitrox, 1 Air

[CC1020]

Although I seem to remember something about this topic, I as unable to find anything through a search, so if I'm being redundant, allow me to apologize in advance. I have two cylinders, one Nitrox, one air. If I'm doing a deep profile first, say in the 80-100fsw range, and the second dive to 60-70fsw, which cylinder would you recommend using first, which one second, and why?

 

[The Kraken]

Depends upon the percentage of O2.

But, I'd recommend using the nitrox on the first dive to reduce the EAD and giving you a longer bottom time. Or you could dive nitrox but use the air tables and reduce your surface interval time for the next dive. That is, unless, the second dive would be more interesting and you'd prefer to have more bottom time on it.

Just keep in mind, if you're diving EAN36 at 80', your EAD is 60', which would make your first dive effectively shallower, with respect to nitrogen loading, than your second dive.

Like "they" always say, "Just depends . . ."

the K

 

[MrConclusion]

Well, as long as you have a good nitrox mix for the depth of the first dive, I would go nitrox first and air second. The way, you have less of a nitrogen load for the second dive.

If you're using a computer, some of them won't let you out of "nitrox" or "EAN" mode until your residual load is gone, sometimes for up to 12 hours. If this is the case, just reduce the oxygen setting for the second dive to 21% - the computer will still calculate correctly because AIR (basically equals) EAN21

If you're using tables, make sure you have the correct residual nitrogen carryover from your nitrox dive to your air dive. If you have ANY DOUBT WHATSOEVER about the tables, calculate them both as AIR dives. This will give you an overly conservative max profile, but better safe than sorry.

 

[cummings66]

If you calculate Nitrox dives as air dives be careful because of the O2 clock. Pay attention to that.

 

[The Kraken]

Good point, Route 66 . . .

But given he's only doing 2 dives, I think he'd be well within his max allowable O2 percentages.

But we don't know "Rest of the story . . .", do we?

the K

 

[Rainer]

Well, as long as you have a good nitrox mix for the depth of the first dive, I would go nitrox first and air second. The way, you have less of a nitrogen load for the second dive.

If you're using a computer, some of them won't let you out of "nitrox" or "EAN" mode until your residual load is gone, sometimes for up to 12 hours. If this is the case, just reduce the oxygen setting for the second dive to 21% - the computer will still calculate correctly because AIR (basically equals) EAN21

If you're using tables, make sure you have the correct residual nitrogen carryover from your nitrox dive to your air dive. If you have ANY DOUBT WHATSOEVER about the tables, calculate them both as AIR dives. This will give you an overly conservative max profile, but better safe than sorry.

What's this "basically equals"? Why would you ever change from "nitrox mode" to air mode?

 

[The Kraken]

What's this "basically equals"? Why would you ever change from "nitrox mode" to air mode?

I think, but don't quote me, it's because in a given period of time, the algarithms of the computers have to work from a constant.

I know in the Oceanic computers you can dive nitrox and air within the same period, but one must set the "air" tank to an O2 percentage of 21%.

It has to do with the maximum daily allowance of O2 absorption.

the K

 

[ronbeau]

Depends upon the percentage of O2.

But, I'd recommend using the Nitrox on the first dive to reduce the EAD and giving you a longer bottom time. Or you could dive Nitrox but use the air tables and reduce your surface interval time for the next dive. That is, unless, the second dive would be more interesting and you'd prefer to have more bottom time on it.

Just keep in mind, if you're diving EAN36 at 80', your EAD is 60', which would make your first dive effectively shallower, with respect to nitrogen loading, than your second dive.

Like "they" always say, "Just depends . . ."

the K

The NAUI Nitrox course I took indicated that you could go from Nitrox to Nitrox with the same or higher O₂ level but not to a mix with a lower O₂ percentage on the second dive.

The explanation I got when I asked why was that it was effectively the same as a reverse profile dive.

 

[The Kraken]

The NAUI Nitrox course I took indicated that you could go from Nitrox to Nitrox with the same or higher O₂ level but not to a mix with a lower O₂ percentage on the second dive.

The explanation I got when I asked why was that it was effectively the same as a reverse profile dive.

Yes, within reason.

If one were to make two 100' foot dives, the first on EAN36 and the second on EAN32, the EAD of the first dive would be 75' and the EAD of the second dive would be 81'.

Conversely, if one dove the nitrox mixtures in reverse order from that above, the EAD would be 81' and 75' respectively,thus generating a deep to shallow profile.

Do the numbers.

the K

 

[CC1020]

Thanks very much for the input everybody. SB rocks