Dive planning? Nitrox?

[Chazzjoh]

I have a few questions about dive planning and 1 about nitrox. I only have a few "logged" (meaning they weren't shallow, go get food with speargun or clean/fix boat) dives and was wondering how you plan a dive to a spot you've never been to before. All of my logged dives have been with a divemaster that guided us around, gave us most of the info we needed and knew the area like the back of his hand. My friend is coming to visit in a few weeks and while he is a divemaster has never dived here before and we'd like to "explore." I'm sure he knows how to plan a dive, and I'm sure he could teach me, but I'd like to have a general idea of what to do before he gets here. I know tables and the wheel and I've heard the "plan your dive and dive your plan" mantra, but all the dives we plan in OW classes were mainly max depth/ bottom time and such. I'd like to start out with "easy" dives (listed as beginner) and kinda work my way up from there.

How do you account for current?
Does seeing something interesting "over there" and a little deeper than you were planning on mean you can't go take a look?
Does it totally scrap your plan to go to 65 ft when the plan only called for 55ft? (you didn't just dive your plan!)
Do you plan the dive for max depth if it's fairly shallow? (IE bottom is 50ft, plan whole dive for 50ft knowing that you'll most likely NOT spend all your time there?)
Is it acceptable to plan the dive as "go down, come up at XXXX psi? (provided of course that you are within NDL)
How complicated does the dive plan have to be?
What do I really need to do to plan a dive?

My nitrox question is maybe less complicated. I plan on doing AOW and Nitrox in 2 weeks. (maybe more info on dive planning from my instructor!) Does it make sense to just leave my dive computer on EAN 21% for all air dives after I have my nitrox cert? I have an Oceanic Versa Pro and it won't switch back to air after a Nitrox dive until lockout timer has passed. It will allow me to switch to EAN 21%... just leave it in EAN mode permanently?

Any help is appreciated.

 

[The Chairman]

Current? Start your dive INTO the current. When you are done, you get a free ride back.

Deeper? Plan not only for what you think the depth is, but sork out a contingency for AT LEAST 10 fsw deeper. This way you establish your limits AHEAD of time and not trying to do it when your head might be addled.This is made simple by the use of a computer.

Nitrox? Yes, my Cobra is set to default to 21% NitrOx. If it was set to air then I have to wait 24 hours to program in a NitrOx mix.

Did I miss any?

 

[Damselfish]

My nitrox question is maybe less complicated. I plan on doing AOW and Nitrox in 2 weeks. (maybe more info on dive planning from my instructor!) Does it make sense to just leave my dive computer on EAN 21% for all air dives after I have my nitrox cert? I have an Oceanic Versa Pro and it won't switch back to air after a Nitrox dive until lockout timer has passed. It will allow me to switch to EAN 21%... just leave it in EAN mode permanently?

Yes, it makes sense to leave it in EAN mode and change the percentage as needed. Not being able to switch from Nitrox back to air wouldn't be a big deal, you'd just change the percentage anyway. The more annoying situation is to be stuck in air mode when you really want to take advantage of Nitrox on some dives.

 

[MikeFerrara]

Nitrox? Yes, my Cobra is set to default to 21% NitrOx. If it was set to air then I have to wait 24 hours to program in a NitrOx mix.

Did I miss any?

Is that a typo? It won't let you do a nitrox dive after an air dive?

 

[The Chairman]

No, it's not a typo. That's what I was told when I bought it, and I have never tested it otherwise.

 

[tangfish]

Seems like you've got some good answers on the nitrox. About diving and currents, the best thing you can do is link up with some experienced local divers and get a hang of what sites are current intensive. Also, pay attention to the tide tables when planning what day/site/time to dive. Different locations around the world are affected more or less by tides than others. Big exchanges between high and low tide result in swifter currents, in addition to any prevailing currents that might exist for other reasons around your local waters.

You'll become more comfortable with planning, diving and making adjustments as you go. The fact that you asked these questions at all tells me that you're going to be a cautious diver who respects nature and has a fun and safe time diving as a result.

Cheers!

 

[MikeFerrara]

A dive plan can be as simple or as complex as you need it to be for the dive at hand.

Some things that you might consider planning for are depth, deirection, distance, air, decompression and who leads or is responsible for what tasks. What's the purpose of the plan? First of all, to make sure you get back right? Second, it might be to accomplish a set objective...like seeing something specific maybe.

You need enough breathing gas reserved to make an ascent sharing gas or, depending on the dive, to make the return trip sharing gas in the case where you don't want to surface away from the entry point. Depending on the site it could be as simple as "lets go down and swim around for a while and turn back/surface at or before x pressure". If you have to navigate to somthing and back back you might want to be more specific about directions and who is doing the navigating. If you have enough bottom time left and gas what's the difference if you go to 55 ft or 65 ft? you mentioned a weel so I'm assuming you're using DSAT tables. What's the NDL for 65 ft? How much gas do you need to keep in reserve at 65 ft? Do you have enough gas and bootom time? Your dive plan can be very flexible.

 

[Damselfish]

Is that a typo? It won't let you do a nitrox dive after an air dive?

I think it's pretty common, once you dive you're stuck with the mode until it clears. Certainly true of the Suuntos. I imagine switching modes would complicate the software a little, like if you were in air mode and it wasn't tracking O2 exposure, what would it do if you switched? I'd think they could get around that one easily by tracking O2 no matter what and just not showing it, but there's probably other gotchas.

 

[TSandM]

You were taught tables, and tables are perfect for boat dives on specific sites -- eg. wrecks, or some other structure at a known depth. Tables become more and more onerous if you are doing terrain-based shore diving. You won't be doing direct descents or ascents, and if you plan your dive based on your maximum depth, unless the dive in question is very shallow, your bottom time will be ridiculous. This is actually where a computer comes in very handy for a novice diver.

You should find some reference material on the dives you are contemplating, whether that's a local shore diving book or a website or just information from other, local divers. You should have some idea of the topography of the site before you go in -- how deep it goes, what the interesting things are to see and where they are located. I don't know about your area, but in ours, divemasters have done mapping projects and we have underwater maps you can find of the most popular sites.

A shore diving book will also give you information about whether the site is current intensive, and any hazards which are present and about which you should know.

A good dive plan includes who's leading and navigating, and apportions any tasks which will be done (I'm assuming you guys won't be sending up an SMB on these dives!) You talk about how much gas each of you has, and what kind of dive it is -- if it's a shallow shore dive, you can surface at any time and swim in on the surface, so you only need to reserve enough gas to get you and your buddy to the surface from any point of the dive. If you'd prefer NOT to do a surface swim, you can plan to use half of your available gas (the amount above the emergency reserve) and turn when you're at that point. For a dive where you HAVE to get back to a certain point, you need to keep more reserve.

You need to discuss your planned trajectory, because either one of you should be capable of navigating back from wherever you get to. Otherwise, what do you do if the "navigator" gets confused?

You should agree on a "hard deck" for the dive -- the deepest you plan to go. It doesn't have to be carved in stone, if you get to 60 feet and there's something fascinating a few feet further down, but this is where people can get into trouble. If you are diving tables, make sure you know the limits for ten feet deeper than you plan, so you can adjust if need be. If you are diving a computer, you should still have some idea of how your available no decompression time will change as you go deeper.

It's good to have some idea of the proposed duration of the dive. In talking about this, you may find out that one of you tends to get cold after 45 minutes, so even if your air lasts longer than that, being out 30 minutes from shore when somebody starts shivering is not a good thing.

And you should have a plan for the ascent -- will it be swimming up slope, or are you going to do a direct ascent, and if so, where, using what reference, and using what profile?

This sounds like a lot, and it seems that way at first, but after a while, you realize you do the basic planning (tides, currents, topography) at home, and run through those things and the immediate logistics quickly at the site, followed by your equipment checks. With enough repetition, it gets pretty easy.

 

[MikeFerrara]

I think it's pretty common, once you dive you're stuck with the mode until it clears. Certainly true of the Suuntos. I imagine switching modes would complicate the software a little, like if you were in air mode and it wasn't tracking O2 exposure, what would it do if you switched? I'd think they could get around that one easily by tracking O2 no matter what and just not showing it, but there's probably other gotchas.

The nitrox computers I've had track O2 exposure all the time regardless of the mixture that they're set for. There isn't anything unusual about using different mixes for different dives and even having one of those mixes be EAN21.

If I'm understanding NetDoc, once he dives EAN21 he can't dive an enriched mix for 24hours? That's crazy and doesn't make any sense to me. Or does the computer treat "air" different than EAN21? That wouldn't make any sense to me either.