Would like to get my mind around a tank/buoyancy concept

[Hepcat62]

I disagree with "ALWAYS".

If you don't need additional weight to stay down after your tank is at/near empty, they you don't have to do anything else additional. I can dive my steel 130s with absolutely no weight if I am not wearing exposure protection in salt water, and never in fresh water (even in a dry suit).

I think you're both right. If you do a weight check and are neutral at 10fsw with an empty wing and full tanks (assuming you are breathing as you would be during the dive), then in that situation you should always need to compensate for the gas lost with lead. If you are negative and require air in the wing to be neutral, then you should need to compensate for (weight of gas - buoyancy of wing as filled to be neutral) with lead. In the latter situation, you are in or approaching what some divers/organizations refer to as an "unbalanced rig", and it's up to you to decide whether or not that exposure-suit + tank combination is manageable with a total wing failure given your dive plan, physical capability, and experience...

Honestly though... This is why I think it's easier to just always do weight checks with empty tanks. That's the number that really matters (although weight of gas can affect wing selection).

 

[lowviz]

........................Honestly though... This is why I think it's easier to just always do weight checks with empty tanks. That's the number that really matters (although weight of gas can affect wing selection).

Just read through the whole thread and my first post (http://www.scubaboard.com/forums/5669252-post10.html). I never got to the "punchline" probably because things just got too complex and I got lost in it. I started out by showing how a tank can go negative to buoyant in water (big deal, most people know that) but left off:

When properly trimmed out you should never know when your tank goes buoyant, it just doesn't matter. (http://www.scubaboard.com/forums/5669118-post2.html). You already compensated for the uneven weighting. The weight of gas decreases evenly throughout your dive -without altering your balance.

All (my) tanks are butt-light, full or empty. Just checked with a piece of angle iron.

***I think*** that the problem revolves around raising the BCD and tank off of the diver's body when the tank starts to float. This will alter your balance point.

To the OP: Try tightening your rig and see if the problem continues.

 

[The Chairman]

Honestly though... This is why I think it's easier to just always do weight checks with empty tanks. That's the number that really matters (although weight of gas can affect wing selection).

You really don't need empty tanks to do this. Most of the empty tank weight checks want your eyes to be level with the water. Well, each inch out of the water is about a pound of head. So your eyes are 5 pounds or so.

So, with full tanks. Empty your BC bladder. Fold your arms and legs (no propulsion) and see how much you stick out of the water. Add a pound for every inch out of the water. Recheck until the top of your head is just awash on intake. If you exhale, you should start to slip beneath the waves. When you have that 7 pound swing, you'll be just fine at fifteen feet.

 

[Blue Sparkle]

When properly trimmed out you should never know when your tank goes buoyant, it just doesn't matter. (http://www.scubaboard.com/forums/5669118-post2.html). You already compensated for the uneven weighting. The weight of gas decreases evenly throughout your dive -without altering your balance.

All (my) tanks are butt-light, full or empty. Just checked with a piece of angle iron.

***I think*** that the problem revolves around raising the BCD and tank off of the diver's body when the tank starts to float. This will alter your balance point.

To the OP: Try tightening your rig and see if the problem continues.

Hi lowviz,

I think I see what you are saying. And it may be that the AL80, for example, trims me out somewhat head heavy all through the gas cycle, not just when empty. I can't say that I was well balanced even with a relatively full tank, now that I think about it. I have compensated by putting some of my lead on the tank base and with that I can trim out fine with either an AL63 or AL80 (although it is a pain when changing tanks for the second dive).

When I used the LP95 I was trimmed out nicely with no extra "aft" weight - just weight on my weight belt (less of course since the tank was more negative).

With the various tanks and exposure protection I've used so far, I've experimented and taken notes so that now I'm able to weight myself to just able to hold my stop with a nearly-empty tank, and to trim reasonably well. But I still wanted to understand it conceptually, and I kept hearing about aluminum tanks being buoyant and therefore different

I can say with some certainty that my tank is not lifting; between the waist belt and crotch strap it is quite solid on my back (which is great).

Blue Sparkle

 

[The Chairman]

Interesting thought... what BC do you use, Blue?

 

[lowviz]

............And it may be that the AL80, for example, trims me out somewhat head heavy all through the gas cycle, not just when empty. I can't say that I was well balanced even with a relatively full tank, now that I think about it. ................Blue Sparkle

I think that I'm a little more clear on this also. How about:

When your tank goes buoyant there is a trim shift so you rotate in the water. Maybe you sensed this as a buoyancy change and were compensating by adjusting your buoyancy. (Your buoyancy didn't need any adjustment.)

-fun thread, lots to think about...

 

[TSandM]

Blue Sparkle, if you are using Al80s or 63s a lot, why not get a set of negative fins instead? I hated Al80s with my original split fins, because they DID get butt-light, but with Jets, I don't even notice the effect.

 

[Eric Sedletzky]

Hi lowviz,

I think I see what you are saying. And it may be that the AL80, for example, trims me out somewhat head heavy all through the gas cycle, not just when empty. I can't say that I was well balanced even with a relatively full tank, now that I think about it. I have compensated by putting some of my lead on the tank base and with that I can trim out fine with either an AL63 or AL80 (although it is a pain when changing tanks for the second dive).

When I used the LP95 I was trimmed out nicely with no extra "aft" weight - just weight on my weight belt (less of course since the tank was more negative).

With the various tanks and exposure protection I've used so far, I've experimented and taken notes so that now I'm able to weight myself to just able to hold my stop with a nearly-empty tank, and to trim reasonably well. But I still wanted to understand it conceptually, and I kept hearing about aluminum tanks being buoyant and therefore different

I can say with some certainty that my tank is not lifting; between the waist belt and crotch strap it is quite solid on my back (which is great).

Blue Sparkle

By your statements here I'd say you understand it perfectly. Try not to overthink it, it's actually very simple.

BTW, you should have your package tomorrow or Tuesday.

 

[Blue Sparkle]

For a BC, I'm using a DSS backplate (steel, weighs 3.8#), an Agir -Brock aluminum STA, Oxycheq 18# wing, and a basic webbing harness. It fits really nicely, and the STA allows me to get the tank just a scoch lower than I could without it (has three sets of slots so there is one lower top set). I'm already using Jet Fins. My original snorkeling gear came from a garage sale ($2!) and included a set of Rocket Fins. I liked them but they were kind of.... non-resilient feeling? (hard to quantify). I read that the Jets were a bit less "boardish," so I got a pair from eBay to experiment with. I'm really happy with them and don't anticipate using the Rockets again.

I have a feeling the "head heavy" effect is something to do with a combination of being short waisted (so the tank's effect is transferred through the mounting rather high on me, comparatively), and having some female-hip buoyancy. So maybe I'm not head heavy, but "aft" light as compared to a long skinny male type physique What I maybe should have made clear in my initial post is that I have myself trimmed out just fine now in either the AL63, AL67, AL80, or LP95 (which is all I've used so far), so the real life application is okay. But it was nagging at me *theoretically* as I like to understand things. I kept reading things about aluminum tanks needing extra buoyancy compensation because they went positive, and I always mentally wondered about it, even though through experimentation I had my weighting set in a practical sense. So, I was asking more to understand the physics of it; rather than to figure out how to get myself neutral. I just hate it when I can't get my mind around something!

Now I understand that either way you just compensate for the weight of used gas. You account for the statistics of the tanks (full weight vs. empty weight in the water), but there is nothing "voo-doo-ish" about the aluminum tanks beyond the facts, in terms of pure weight. If you already need lots of weight (for other reasons), then of course you can "subtract" some of the steel's negativity, i.e. use it for part of your "other" weight.

However if you are already neutral -- for example, on my first weight check with my own gear I was just at eye level at the surface with a normal breath and full tank; started to slowly sink if I exhaled. For that setup I added 4 lbs. just to compensate for the gas I would breathe, but not "for" the aluminum factor specifically. Maybe that confused the issue because most of the time people do need weight (exposure suit, etc.). It just so happened that my first dives with my own BC/gear were in 86º water (which was 68º last month.... not quite the same). I should perhaps have made it more clear that I was asking theoretically and not "practically."

Interestingly, I did not need any special "tail weight" with the LP95. That was nice.

Also, I dived for the first time in fresh water last month, and I was surprised at how different it felt. I mean, I knew I would need less weight, but it also changed my trim so that with an AL63 or AL67 I did not need any "tail weight." I was a bit of a mess when I first got in, with my usual weight strapped around the tank base. I was flailing around thinking "but no, I could hold still before, really!" (as the instructor looked on :blush I shifted it to just weight-belt-weight and all was fine again, so I was wondering if I had changed or something in the interim. But no, when I got back to the Keys I was back to nosediving without the extra weight low/back; so I guess it is just the difference in buoyancy characteristics on me between salt/fresh. It does not seem to occur with the LP95 in salt water though, even with the same amount of total weight , so maybe it is not all in my own buoyancy characteristics and there is something to the aluminum tank buoyancy, I'm not sure (although it is not just at the end of the dive).

At any rate, I am able to dive trimmed out; I just have to do different things with weight placement between aluminum tanks and steel ones (for the ones I have dived to date).

Thanks all,
Blue Sparkle

PS: ZKY, your post came in while I was typing. Thanks!

 

[The Chairman]

Interestingly, I did not need any special "tail weight" with the LP95. That was nice.

After all is said and done: what works: works. I imagine that the lp95 is LONGER than the other tank. You might want to try loosening your shoulder straps and tightening you crotch strap. That made a hug difference for me.