Ean30?

[beanojones]

Whether people plan on tables or a computer, the theory comes from the same place. Generally, I plan my dives using a computer, and if I have one mix leaner than the other, I very definitely look at which one to dive first. I don't see why it's bad advice, you have to pick one or the other, what's the problem with using some sort of reasoning to arrive at the answer?

The point is you are using the wrong reasoning. 'Always limit exposure to nitrogen' is the only reason to dive nitrox at all in the first place. Air is cheaper, easier, has less oxygen risk, no labeling andf handling issues, etc, etc.

So purposely exposing yourself to more nitrogen sooner than you have to is getting confused by the details of the tables and missing the entire point of nitrox in the fist place.

And badly misunderstanding decompression theory to boot.

 

[kotik]

The point is you are using the wrong reasoning. 'Always limit exposure to nitrogen' is the only reason to dive nitrox at all in the first place. Air is cheaper, easier, has less oxygen risk, no labeling andf handling issues, etc, etc.

No. You are using the wrong reasoning. If our whole goal was to limit exposure to nitrogen, we'd just stay out of the water. We don't, though, do we? We very deliberately expose ourselves to elevated pressures of nitrogen, and we plan ahead and then follow a dive profile accordingly to deal with that.

This thread started because the OP didn't have the option to limit his exposure to the degree he wanted. He wanted 36%, but 30% is what was available. It happens to me sometimes that I have one richer and one leaner mix. We don't usually have complete control over everything-- sometimes you have two different mixes. Sometimes, the dive has a certain minimum depth. Sometimes you're on a boat and aren't the one in charge of the surface interval, if you want to do a second dive that is.

So purposely exposing yourself to more nitrogen sooner than you have to is getting confused by the details of the tables and missing the entire point of nitrox in the fist place.

You seem really hung up against the concept of using tables as a quick and portable way to sketch out a general strategy for the day. For your benefit, when I have a little more time I'll run some profiles on a dive planning software and present those results. I'm confident it's going to point to the same choice of which sequence to use different mixes, because the same theory applies whether it's a table or a computer. It's just that tables are handy when used this way, so that's what I used.

Your real aversion though seems to be to the concept of planning any farther ahead than your next dive. We'll have to just agree to disagree on that one, and really I'd say if there's been a single piece of bad advice given in this thread, that'd be it.

And badly misunderstanding decompression theory to boot.

If you want to have a conversation to explore our respective understanding of decompression theory, I think I could hold my own.

 

[beanojones]

Sometimes you're on a boat and aren't the one in charge of the surface interval, if you want to do a second dive that is.
.

So given a shorter interval you should dive the leaner mix one the first dive?

 

[rlynch]

Beano, we (i) plan and use "tables" just custom ones cuz were doing deco for offshore. The profiles are pretty much square. The computer if used as one is backup.

I don't use the "p" tables but keep a copy of the "n" tables in my pocket for bailout if needed.

All this on my petrel BT

Anyway... If you can the mental math is much easier using 32% at less than 100 fsw. For single or respective dives and breaks down rapidly with mixes over and less so under.

It gets easier the deeper you go (and adding He)

We get people toxing just about every year on hot mixes, like Collim is eluding to. 30% gives a margin of safety for the person and the supplier.

The thing that gets me is how rare you see people checking their mix prior (At the dive site) to diving it.
Always know what your diving

 

[kotik]

So given a shorter interval you should dive the leaner mix one the first dive?

You should dive as you choose, and plan your dives however much or however little you choose. There's no need to concern yourself with how I dive and plan if you don't feel like following the reasoning I use. My only intent was to point out to the OP that when planning repetitive dives, there's often more bottom time allowed by saving the richer mixes for later dives.

You should not expect to get away with telling me I'm wrong or giving bad advice just because I do my planning with considerations for repetitive dives that I expect to conduct that day, however convinced you are that every dive should be planned as though another won't happen before your tissues clear.

If you're truly interested in how I plan my own dives, or even if you're just looking for things to pick apart, here it is: I take stock of what I have that I can't change, what I can change, and what I have available.

Sometimes it's easy-- I'm someplace with a shallow enough depth, and enough time, that I can do the diving I feel like doing without putting much thought into planning around deco.

Sometimes it's more complicated-- the site is deeper, with leaner O2 acceptable, longer surface intervals become a concern and limits the time available more noticeably... it has even happened, as mentioned, that I've had more than one FO2 to think about.

In these cases I work with the things I can't change, and try to come up with the best options I can among those things I can play with. Best as far as deco is concerned means longest available NDL time if the dives are recreational, or minimize deco time if decompression is part of the plan.

 

[macado]

Like others have said, some places bank 30% depending depth of the dives. Pretty much all diving operators in North Carolina bank Nitrox 30% because the majority of the wrecks are in 100fsw or more. I can't speak for Florida or other places though.

Makes sense to have everyone on the same mix so if plans need to be changed everyone on the boat can still dive the site (E.G. everyone on boat has 30% but one diver has 36%)

 

[Superlyte27]

Here's a good read about why some shops bank 30% instead of 32%

Cave Country Diving - Dive Articles written by friends and staff - Why does Cave Country Dive Shop NOT bank EAN32?

 

[DiveNav]

....... when I have a little more time I'll run some profiles on a dive planning software and present those results..... .

I just did run your 2 examples in divePAL (using moderate conservatism) and with sequence 1 (80->60) I get a final NL of 57%. But, if I run sequence 2 (60->80) I ended up with a final NL of 96% ...

But, If I look closely at the nitrogen loading distribution by compartment, it is clear that the end of sequence 2 is dominated by the fast compartments while for sequence 1 the mid/slow compartments were driving it. As you might know, the fast compartmens are the ones that will most benefit from a surface interval.

For your info, you might want to check the article I wrote a while ago: Reverse Profiles: An Urban Legend?

Of couse is just mathematics

Alberto (aka eDiver)

 

[XS-NRG]

I've been to 2 different shops in different countries where they had banked EANx29. The reasoning was based on MODs set to 1.6 MAX to do a direct drop onto ship wrecks in the area and then immediately start your ascent up the boat. It wasn't meant to stay at that depth but to get the best drop and then slowly work your way up the boat. I thought rather than setting comp to 1.6 given that blend I'd just go for an air dive but the time limits were so short and they consistently do these dives every day and have had a good track record. It's obviously up to the individual diver if they want to "push it" so to speak.

 

[kotik]

I just did run your 2 examples in divePAL (using moderate conservatism) and with sequence 1 (80->60) I get a final NL of 57%. But, if I run sequence 2 (60->80) I ended up with a final NL of 96% ...

But, If I look closely at the nitrogen loading distribution by compartment, it is clear that the end of sequence 2 is dominated by the fast compartments while for sequence 1 the mid/slow compartments were driving it. As you might know, the fast compartmens are the ones that will most benefit from a surface interval.

For your info, you might want to check the article I wrote a while ago: Reverse Profiles: An Urban Legend?

Of couse is just mathematics

Alberto (aka eDiver)

Thanks, it looks like your software is a little more convenient than my planning software for looking at NDL's...

Just to be completely clear-- I haven't made any attempt above to tell people how they should dive, or made any blanket statement that you're gonna die if you first do a shallow dive and then a deeper one. Now that I've poked around the archives on this subject, it seems this has been a fear in certain circles... I had no idea there was some taboo built up around it, I never learned that fear. As I was taught, the only issue was that available bottom times are shortened otherwise. This is something that's easily verified, provided you consider the mainstream decompression models to have some validity.

For my part, I'm perfectly willing to follow a shallower dive with a deeper one, if there's a particular reason to do so. I did just this two days ago, for the sake of working up someone's comfort level who had been out of the environment for a while. If there's no other extenuating circumstances in the decision-making process though, my preference would usually be to do the deeper dive first.