Intro to recreational deep diving

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If you have been to 100ft, you have experienced narcosis. Didn't your AOW instructor have you do a timed exercise at depth and in the shallows to demonstrate this?

That, from my experience, is one of the LEAST effective ways of demonstrating narcosis.

 

[scubadiver888]

ScubaSloan,

On first pass I didn't notice it but as I read your message I started to get the hint that maybe you were not wording things clearly. Sounds like you have a pretty good handle on things for someone with less than 25 dives. There are a lot of posts to external links on gas management, SAC rate, etc. Search the board for them. Some can be a little technical but I get the feeling you'll like them.

Regardless of what the depth is, just get into the habit of making small adjustments all the time. I just notice when I inhale I rise a little, when I exhale I drop a little. If this is not true then I adjust my air accordingly (i.e. if inhale and I'm not rising, add some air. if I exhale and don't descend, dump a little air).

 

[scubadiver888]

That, from my experience, is one of the LEAST effective ways of demonstrating narcosis.

What are alternate ways of demonstrating nitrogen narcosis?

 

[Teamcasa]

What are alternate ways of demonstrating nitrogen narcosis?

A simple manual dexterity/coordination test usually brings it out.

 

[Reg Braithwaite]

I just completed my AOW and have several dives to 90-100 feet and am planning several with max depth of 120 in the near future. I will be diving a single HP 120, no pony.... Anything else to think about?

I think one or two other people questioned the combination of a steel tank and a wet suit. I am not as experienced as many of the other folks on the board, but I can share the folk wisdom from up here in Ontario: steelies are almost always combined with redundant buoyancy, which is typically a combination of dry suit and wing.

You can also get wings with redundant bladders. I am not going to say anything about that as there is some controversy around whether this is a good idea or not. I have read mention on this board of people using a lift bag as redundant buoyancy. I have no experience with that and wouldn't care to fool around trying to manage a slow ascent using a lift bag should my wing fail while I'm diving wet.

But... I would encourage you to look into whether an AL80 would be sufficient for your needs. I'm personally thinking "yes." You're new to this, as am I. Go down, take a quick look around, come on back up and take your time ascending.

Dive well under your NDL, and return to the boat with tons of air. Why sweat it? Dive safe, you can always come back later and see more.

 

[TSandM]

But... I would encourage you to look into whether an AL80 would be sufficient for your needs. I'm personally thinking "yes."

Okay -- It's late at night, and I worked a crazy shift, and I'm not up for doing the math right now. But rock bottom from 100 is 40 cu ft, which is 1500 psi in an Al 80. If you assume the diver is going to do the allowed 10 minutes at 120, which is 4.6 ATA, at a SAC rate of .75 (because it makes the math easy, and probably isn't farfetched for a fairly new diver), you'll be using 35 cu ft during your bottom time, and need 40 cu ft for rock bottom; that's 75 cu ft, and just about everything in your tank, assuming your ascent is almost instantaneous.

The rule of thumb is never to dive deeper than the cu ft in your tank, and it's a pretty good one, and a 120 foot dive on an 80 cu ft tank violates this.

Having a single steel tank with a wetsuit in very cold water is not a good thing, but I think it's better than planning a dive that runs your gas to the absolute minimum.

Scubasloan, you are looking at some of the right issues. Also consider that narcosis is variable and sometimes difficult for the individual to diagnose in himself. It manifests as perceptual narrowing (bad for understanding what's going on with the dive, or with your buddy) or as euphoria (bad for monitoring important things like gas) or anxiety (bad for making good decisions and maintaining composure under any kind of stress). NDLs at the depths you are talking about are extremely short -- no more than 10 minutes on air -- and gas consumption is frighteningly more rapid.

I would highly recommend getting more experience with dives under a variety of conditions in the under 100 foot range, before extending your dives to the lower part of the recreational depths. Those dives, pushing NDLs, require good buoyancy control and an understanding of decompression theory and decompression profiles, I think, to execute safely. My first square profiles in that depth range resulted in what I suspect may have been skin bends. I have been extremely careful and conservative since then, and I have gotten a LOT of training before going back to do those dives again.

 

[Reg Braithwaite]

Okay -- It's late at night, and I worked a crazy shift, and I'm not up for doing the math right now. But rock bottom from 100 is 40 cu ft, which is 1500 psi in an Al 80. If you assume the diver is going to do the allowed 10 minutes at 120, which is 4.6 ATA, at a SAC rate of .75 (because it makes the math easy, and probably isn't farfetched for a fairly new diver), you'll be using 35 cu ft during your bottom time, and need 40 cu ft for rock bottom; that's 75 cu ft, and just about everything in your tank, assuming your ascent is almost instantaneous.

The rule of thumb is never to dive deeper than the cu ft in your tank, and it's a pretty good one, and a 120 foot dive on an 80 cu ft tank violates this.

Well, I would say three things:

1. I never suggested staying for the full NDL. In fact, I suggested a quick look around and then leaving. There is no reason to try to push to a new depth and push the profile envelope at the same time even with the perfect gear.
2. I thinkk if you leave the bottom with rock bottom in your tank you're ok as I calculate rock bottom. Unless we are talking about different things, rock bottom is the amount of air needed for two stressed divers sharing air to take a minute at depth and then safely ascend to the surface. Therefore, if you leave the bottom when you hit RB, you are ok. The more padding the merrier, of course, so if yu are not right on the ascent line or whatever you need t work rule of thirds on top of that and so forth.
3. The limit of 80' on an AL80 sounds very sensible, leaving me to suggest that perhaps given all of the compromises involved, the best course of action is to skip the 120' dive until (a) guided by a very experienced deep diver with appropriate redundant gear like doubles, or (b) the appropriate gear with redundant buoyancy is in place.

When I took my AOW, I did dives in the 80-90 foot range on an AL80 with a wet suit. And this was in the presence of an instructor and a dive master, each of which had long hoses and doubles. Both stressed the importance of gaining experience and gear before doing dives like the Forest City (135'-150').

Not everyone up here is into tech, but most suggest that 120' no-deco dives are serious affairs where dry suits and redundant air are strongly suggested.

 

[Aardvark]

A simple manual dexterity/coordination test usually brings it out.

The first time I did a deep dive as part of training I was asked to write my name backwards on a slate at depth and at the surface(!) and time both. I came away not too worried about nitrogen narcosis.

The next time I did a deep dive as part of training I was asked to:

swap masks with my buddy
change to breathing from each other's alternate reg
swap masks back
recover and resume breathing from primary reg

These instructions were given to me on a slate at depth. They were written as simple instructions (1. remove your mask, 2. hand it to your buddy, etc...) I remember having to concentrate hard to comprehend what was written in front of me and execute the steps I was asked to take. I came away from the dive with a much healthier respect for narcosis.

I wear a 5mm farmer john style wetsuit with 5mm jacket and am neutral with about 50bar in my tank at 5m. At the start of a dive if I don't add air gradually to my wing I drop like a rock once I get below 15 m or so. You need to be very comfortable with your changes in bouyancy and controlling your ascent and descent rates.

120cuft is a lot of gas if you're sticking within NDLs for the dive. I personally would take a pony at that depth regardless of how much I trusted my buddy. There are plenty of (heated) discussions on the board about the pros and cons, search is your friend.

Speaking of NDLs, I'd stay well clear of them I don't have tables handy, but if you've got a NDL of, say, 15mins then I personally would be planing a max bottom time of 12 mins and add some deco time anyway.

 

[scubadiver888]

The first time I did a deep dive as part of training I was asked to write my name backwards on a slate at depth and at the surface(!) and time both. I came away not too worried about nitrogen narcosis.

I have read articles about how sentence comprehension is a better test of nitrogen narcosis but all the testing was done dry using compression chambers. I wonder if this is a viable option to do at 100'+.

The next time I did a deep dive as part of training I was asked to:

swap masks with my buddy
change to breathing from each other's alternate reg
swap masks back
recover and resume breathing from primary reg

These instructions were given to me on a slate at depth. They were written as simple instructions (1. remove your mask, 2. hand it to your buddy, etc...) I remember having to concentrate hard to comprehend what was written in front of me and execute the steps I was asked to take. I came away from the dive with a much healthier respect for narcosis.

I guess the test depends on your environment and the diver's skill level. I wear 5mm gloves. I don't think I could put my mask on with my gloves on even on the boat.

 

[rstofer]

Having a single steel tank with a wetsuit in very cold water is not a good thing, but I think it's better than planning a dive that runs your gas to the absolute minimum.
.

I have read a couple of hysterical posts on this subject but found the rhetoric obscuring the issues.

Would you care to expand on this a little? I have been playing with the buoyancy calculator spreadsheet over in the Equipment -> Buoyancy Compensators forum and I think I have a pretty good idea what my buoyancy will be at depth (wetsuit, steel HP 100) but I have to believe there is more to the issue than just ballast. And I'm not sure why adding 6# of weight to make an Al 80 sink like an HP 100 is a great solution whether the weight is on the diver or strapped to the tank.

I really would appreciate a reasoned explanation and your replies are most insightful.

Thanks!