02 on non deco dives

[BigJetDriver]

Besides, there is a long history of the boundary between tech and rec diving being blurred. Nitrox started out as a purely technical diving gas and was regarded as some sort of devils brew by the rec community - until the boundary got blurred and then eliminated completely.

I remember Karl Shreeves of PADI, in particular, standing up in front of a seminar full of diving professionals and telling everyone that Nitrox was a "commercial" gas and that PADI would never be involved in teaching its use. (Of course, I heard him say much the same thing about decompression procedures at yet another seminar.)

This is not to mock PADI. (In the interests of full-disclosure, I am a PADI IDC Staff Instructor.) This is simply to re-inforce what DA has said. In essence, "Never say never!" Right, Karl? :11:

 

[msandler]

De-mystifing tech diving does tend to put a crimp that sort of ego massage.

Its not me with the huge "CUSTOM MIX" sticker on my tanks. Whatever guys, sorry I offended you. Perhaps my reply put a crimp on some egos. Its always funny to see novice divers swim circles around the self proclaimed tech divers as the tech diver struggles with the manifestations of his knowledge as it does not apply to practical application. Rec does benefit from Tech. This does not imply that everything tech is suitable to any circumstance. Don't dive to use tech, use tech to dive!

 

[BigJetDriver]

Don't dive to use tech, use tech to dive!

Nicely put!!

 

[ScubaDadMiami]

Everything in degrees. Would I use oxygen after a 30 foot dive when I am already on an EANx mix on the main portion of the dive (or even on air)? Nope.

When you say "recreational," that could also include a 130 foot dive for 15 minutes. Would I use something then? Probably if the circumstances are right. On the other hand, I would not really sweat it if I didn't have deco gas but had a reasonable profile and did a nice, slow ascent with enough stops on the way up to the surface.

It is possible to do dives on air and even decompress on air if you do it for long enough. You have to balance the benefit and the hassle and risk in making the decision. There will be more equipment, more task loading but there will also be potential benefits. It all depends on everything.

So, to simply say "Is it better to do a recreational dive within the limits and then use oxygen after?" isn't enough to make the decision. It is only part of the equation. You can't solve the equation without knowing all or at least most of the factors.

 

[mempilot]

All true. But if divers plan to decompress/safety stop on gas mixes other than their BG, then they should run the dive as a decompression dive. While using higher O2 mixes within the NDL accelerates offgassing, the diver is already within acceptable limits. Without the predive planning using a deco algorithm, how do you plan for the next dive, and the one after that? You start spending money on gas and equip you don't need, create extra drag in you profile, and you have no idea where your are with RN and CNS percentage. I'm all about being safe, but I don't see the need to drag O2 on NDL dives unless you plan on utilizing the lower RN for the next dive. So in short, plan it as deco, or plan it as NDL.

Everything in degrees. Would I use oxygen after a 30 foot dive when I am already on an EANx mix on the main portion of the dive (or even on air)? Nope.

When you say "recreational," that could also include a 130 foot dive for 15 minutes. Would I use something then? Probably if the circumstances are right. On the other hand, I would not really sweat it if I didn't have deco gas but had a reasonable profile and did a nice, slow ascent with enough stops on the way up to the surface.

It is possible to do dives on air and even decompress on air if you do it for long enough. You have to balance the benefit and the hassle and risk in making the decision. There will be more equipment, more task loading but there will also be potential benefits. It all depends on everything.

So, to simply say "Is it better to do a recreational dive within the limits and then use oxygen after?" isn't enough to make the decision. It is only part of the equation. You can't solve the equation without knowing all or at least most of the factors.

 

[DepartureDiver]

But if divers plan to decompress/safety stop on gas mixes other than their BG, then they should run the dive as a decompression dive.

Why? They are just being safer.

Without the predive planning using a deco algorithm, how do you plan for the next dive, and the one after that? . . . and you have no idea where your are with RN and CNS percentage.

You don't need to use a deco program or make any changes in your planning. The procedure results in less nitrogen in your body. So using a great RN as if there was no gas switch is only a safety pad - which is the point. So, trying to calculate the lower RN is not necessary. As far as calculating CNS, the divers using nitrox already know how to calculate it ... so a few minutes on an enriched gas at some safety stops is an easy calculation ... which in reality is not going to even affect the CNS level much to begin with if the max ppO2 is 1.3 or less. In essence, this one extra calculation is all that is different and can easily be done when planning the CNS amount for the bottom portion of the dive. I can respect you may not want to use this procedure and that it may not be beneficial for the shallow dives,, but I don't see the advocation against it for divers that have the equipment and the training.

 

[mempilot]

Why? They are just being safer.

I'll give you an example of how a an Advanced Nitrox diver that just uses the NDL table to dive, but uses O2 on the safety stops can get hurt.

Diver goes on vacation and decides to hunt lobster during the mini-season. They want to get in at least 2 dives a day for the next three days. The depths, for sake of discussion will be 90 and 60 fsw seperated by 1.5 hour SI. They plan on using ean36 for their backgas and O2 on the 3 minute safety stops. After looking at it on the surface, they should be safer. They decide since they are just using the O2 on the safety stops it shouldn't be a problem.

Day 1

Dive 1:
ean36 @ 90' for 25min and a 3 min O2 stop, CNS% 23

1.5 hour SI

Dive 2:
ean36 @ 60' for 55min and a 3 min O2 stop, CNS% 49

21 hour SI

Day 2
Dive 1:
ean36 @ 90' for 25min and a 3 min O2 stop, CNS% 72

1.5 hour SI

Dive 2:
ean36 @ 60' for 55min and a 3 min O2 stop, CNS% 99

Day 3

Dive 1:
ean36 @ 90' for 25min and a 3 min O2 stop, CNS% 122

Seemed harmless enough. You can get hurt. This kind of profile is not uncommon down here during lobster season, and they can get even more aggressive. The same dive schedule without the O2 allows for the CNS% to be right at max. Using the nitrox manual for a one gas dive, you can figure this out. I'm not saying, that everyone will go out and do this. But I'm an advocate of planning your dive and diving your plan. Using 2 gas mixes for a dive without planning for it is not good SOP.

 

[Jason B]

what about the 6 hour wash out period that DAN and others claims?

 

[DepartureDiver]

. . . But I'm an advocate of planning your dive and diving your plan. Using 2 gas mixes for a dive without planning for it is not good SOP.

I completely agree with you. Okay, I see where you are coming from regarding the CNS issue - which I agree always needs to be taken into consideration. However, the 3 minutes of O2 at 20’ only adds 7% to each dive and you are not giving yourself credit for the CNS being reduced between dives. There are several ways to view this. One is by using a 90 min. half-time decay (as most tech divers do) and another on the conservative side is a linear 24 hour digression ... among others. My numbers will be slightly different than yours. The way you did your calculations, the problem was not the stop, but the actual CNS buildup from the dive.

90 minute half-life method:
Dive 1
90/25 on EAN36 with 20/3 on O2: 21% CNS

1.5 hour SI now gives 11%
(the CNS was reduced by one-half due to the 90 min SI)

Dive 2
60/55 on EAN36 with 20/3 on O2: 32% CNS

21 hours SI: 0% CNS
(14 half times)

So you can see that the second day of diving would not be a problem.

Using NOAA’s 24 hour window (conservative), the following results would be obtained

Dive 1
90/25 on EAN36 with 20/3 on O2: 21% CNS

1.5 hour SI now gives 20%
(6% reduction of the 21%)

Dive 2
60/55 on EAN36 with 20/3 on O2: 49% CNS

21 hours SI: 6% CNS
(87% reduction of the 49%)

Day 2
Dive 1
90/25 on EAN36 with 20/3 on O2: 27% CNS

1.5 hour SI now gives 26%
(6% reduction of the 27%)

Dive 2
60/55 on EAN36 with 20/3 on O2: 55% CNS

and likewise Day 3 is not a problem. No matter how it is calculated, the 21 hour surface interval is significant and will reduce the CNS to virtually nothing. But you are absolutely correct that CNS needs to be accounted for.

The simple solution if one is desired is to not count the 1.5 hour surface interval (so calculations need not be done) and then treat yourself as clean after the 21 hour SI - which is acceptable.

 

[mempilot]

My point was just that as you stated, that there are things to be accounted for that the normal diver isn't thinking about when dragging that O2 bottle along.

I just plugged in the profiles to Deco Planner really quick, so I'm not sure why it didn't credit the decay. Interesting. I agree with your profile.

See, we need to plan in detail when doing this stuff. Not just grabbing a tank and going because it seems like it would be safer.