Is negative Buoyancy of steel tanks ever a problem?

[UKmarcus]

[FONT=arial, sans-serif]If I carry an S80 aluminum, then I will have weights to ditch in an emergency, since i'll be carrying at least 5lb in compensation weight. Since with steel I may not be carrying weights, especially if I'm not wearing a wetsuit and am in fresh water, how would I conduct an emergency ascent, if I have no weights to ditch, no air in my tank to inflate my BC and my tank is still negatively buoyant? If I was wearing a wetsuit, I could ditch the BC and the wetsuit would [/FONT][FONT=arial, sans-serif]probably[/FONT][FONT=arial, sans-serif] take me to the surface if I'm not too deep, but otherwise what's the deal? I only need 4-6lb in freshwater with a suit, and probably none without, so I'm negative all the way to the bottom and no way to ascend if my BC fails? Just a thought that's been in my head all morning! Thanks.[/FONT]

 

[TravisD]

If your rig is negatively buoyant to the point where you can't swim it up, even after you've ditched weights, then you should probaly consider some sort of redundant buoyancy, such as a lift bag.

 

[nimoh]

the first question is what are you doing on the bottom with no air and no redundant supply of air (pony or buddy)?

However, in answer to your question, if you aren't using ditchable weight, then you need to be able to swim you and your gear up with no air in your BC and have some redundancy. Diving steel tanks in a wetsuit commonly makes this too difficult, so wetsuit divers commonly use aluminum...but not always. For redundant lift (BC failure) you could use a SMB, drysuit, or other diver. Drysuit doesn't work as well, since you can't inflate it orally, or off of a pony bottle (unless of course you keep an LP inflator hose dangling off your pony bottle).

For me, I don't generally dive with ditchable weight, and I can swim my gear up from depth with no gas in my BC. In an emergency, I could ditch my canister light (about 4lbs), but that would have to be a real emergency since it is expensive -- lost dive team, lost buoyancy in BC plus something like a leg cramp or two or lost fins.

The point is that you generally plan for one or two catastrophic events and not for all of them to happen at once. If they all happen at once, ditch the gear, do a CESA and hope for the best.

 

[danvolker]

If I carry an S80 aluminum, then I will have weights to ditch in an emergency, since i'll be carrying at least 5lb in compensation weight. Since with steel I may not be carrying weights, especially if I'm not wearing a wetsuit and am in fresh water, how would I conduct an emergency ascent, if I have no weights to ditch, no air in my tank to inflate my BC and my tank is still negatively buoyant? If I was wearing a wetsuit, I could ditch the BC and the wetsuit would probably take me to the surface if I'm not too deep, but otherwise what's the deal? I only need 4-6lb in freshwater with a suit, and probably none without, so I'm negative all the way to the bottom and no way to ascend if my BC fails? Just a thought that's been in my head all morning! Thanks.

You are supposed to be swimming up anyway.....and if you can't "swim" up 5 or 6 pounds of negative weight, there is a major problem with your fins or your kicking technique.
You do know many freedivers can easily breath-hold down 20-30 feet or more, and then pull up a 20 pound anchor.

 

[mala]

you are more likely to have an uncontrolled ascent due to dropped weights than a bcd failure.


can I ask where you dive fresh water with no wetsuit?

 

[UKmarcus]

Some great answers, so thank you. Emergency ascents aside, there is also the point that some large steels are so ridiculously negative at the beginning of a dive that if you weigh under 150lb you're going to be seriously overweight even with a wetsuit (and like a brick without) and need significant air in your BC to stay off the bottom, which will make you less streamline and also require constant BC air adjustment throughout a multi-level dive. I guess my point is that steel tanks have been sold to me verbally for some time for their ''buoyancy characteristics'', and I'm not seeing the big advantage. Having made that point, I have to now say that it's just not true that steel tanks are necessarily negative throughout the dive. A quick look at the Faber website shows that while most of the HP tanks remain somewhat negative, at least 50% of the LP tanks go positive, just like aluminium, some of them by several pounds. I didn't know all of this until my recent purchase of a LP steel tank for it's "buoyancy characteristics" only to find I would still need to carry weight. In terms of it staying negative, it's worse than an neutral aluminum 80 and barely better than an S80. So before you buy your steel tank, check out the Faber (Blue steel) tables, because all steel tanks are by no means equal in terms of their buoyancy characteristic. They vary drastically and the main advantage of the larger steels would appear to be simply extra air volume, rather than any assistance with weighting.

---------- Post added February 1st, 2014 at 12:29 AM ----------

It's rare - but as an example, a swimming pool during training exercises.

 

[kwinter]

If I carry an S80 aluminum, then I will have weights to ditch in an emergency, since i'll be carrying at least 5lb in compensation weight. Since with steel I may not be carrying weights.......

I'm not sure what you mean by "compensation weight." If you mean to compensate for the difference in buoyancy between a full tank and an empty one, that difference is exactly the same whether the tank is aluminum or steel. It doesn't matter whether the empty tank is positive or negative when it comes to "compensation."

 

[halocline]

I think he means the fact that AL80s are 4.4lbs positive when empty, and divers need weight to sink the tank even before considering exposure protection.

Back to the OP, sounds like you're a fairly new diver who's learning about tank characteristics and already de-bunking some of the generalizations about steel vs AL tanks. As you've figured out, the key figures are the empty buoyancy of whatever tank you're using and the buoyancy of your wetsuit. Many cold water divers like negative steel tanks because they're wearing more buoyant wetsuits, and AL tanks (or less negative steels) are popular in warmer water.

A couple of things to consider: Be careful about the so called 'neutral' AL tanks. They simply have some extra metal around the bottom to weigh more, which means you're carrying that weight around all the time. You can accomplish exactly the same thing by strapping on a weight on a regular AL tank, then take it off when you're carrying the tank on land. Another thing is that the ditching of your own weights at depth is serious no-no if you are concerned about a runaway ascent, which any diver should be, and it should also never be necessary. Weight ditching is something to be done on the surface if you're having trouble staying positive for whatever reason. If you weight yourself correctly you will have no problem swimming to the surface without the assistance of your BC, even with no wetsuit.

The answer to your original question of "are negative steel tanks ever a problem" is yes they are, if a diver is overweighted with one even without any lead. But that's a little unusual if the diver is wearing any sort of compressible suit. The older faber tanks (3AA 3500 PSI) are extremely negative and likely only useful in fairly cold water or with divers that need a lot of lead. The new FX series (3442) are about 2lbs negative empty so they work great in most situations.

 

[Stoo]

In these parts, most serious divers (i.e. those that want a season that's more than 8 - 10 weeks) wear drysuits, and many of us wear double tanks. I huge ADVANTAGE of steel tanks is their negative buoyancy since they can either reduce or even eliminate the need for a weightbelt.... which of course means no ditchable weight.

In my case, I have a wing, (which frankly mostly gets used at the surface to float my tanks while I climb back into my boat), my drysuit, (which is my primary buoyancy control method) and also carry a small 50# liftbag/SMB. Two of these pieces of gear require air in my tanks to use. The suit and wing inflators feed from different cylinders, but I also have a spare inflator hose on my stage bottle reg. (Not that I've ever used it, but I use that same reg mucking about in a pool, so it's just there...)

Of course, Nimoh kinda asked the important question... What the heck are you doing at the bottom of the lake without any gas to breath? I dive solo "a lot" so keeping a little go-go gas in my bottles tends to be something of a priority for me. I assume that your question falls into the realm of the hypothetical, so the short answer, as others have mentioned, is a redundant floaty thing... liftbag, SMB... but all of these are tough to use if you truly have to air. So basically, you're pooched!

 

[daz88]

someone that is under 150lb should not be wearing a large steel tank that is so ridiculously negative. It's like anything.....you should have right tool for the right job. and the right tool is different for every person. And as you learned....not all tanks are created equal. Different tanks are for different things....different people. It's not a matter or what everyone else likes or what tanks are they using......what tank works for you? What are your needs are, not anyone else's.
That said.....I own HP steel tanks and don't dive AL tanks unless I'm traveling someplace. I'm a big guy and I could dive with one of the crazy big tanks that is ridiculously negative, but just have some 100s. I got them at a great price or I would have 120s. I've used a friends 130s a few times just to try them out before I bought my tanks, but didn't like the way my balance was with those.
and if you're stuck on the bottom with no air in your tank you have a few more issues than being a couple of pounds negatively buoyant.