Question on tank bouyancy and weights

[RonFrank]

Some divers would suggest that diving Steel tanks without a backup buoyancy device is a nono.

You need to do a weight check, and dive with what you need, not some amount of ditchable weight that does nothing but make you less negative.

You need to be able to swim up your rig, and it's not a foolish idea to be able to stay at the surface without adding air to the BC.

How you go about all this is up to you. However it *might* involve getting a different tank, and you may not be open to that idea.

Carrying a sausage or lift bag is likely your best alternative if you choose to use your current setup.

 

[halocline]

That's why the Al 80 is perfect for warm water. It is sufficiently buoyant that a diver will almost always require some weight. T shirt diving in south east Asia, I would wear about 6# of weight with an Al 80 and a somewhat floaty jacket BC. With an empty tank and no weights I would be at least 10# positive on the surface - more than enough to keep my head above water even if the BC had failed.

It would make no sense to put that 6# on a tank band and not have any ditchable weight.

Doubles are a different problem. Here the double bladder wings make sense.

I picked a few statements out of your post to respond to. There are few people that would describe AL80s as "perfect" for warm water. More accurately, they're cheap, and that's why they're used so much. Adding weight to counteract a buoyant tank does nothing to improve diving.

The value of ditchable weight in warm water is debatable. Accidental dropping of quick release weights is fairly common, more so than warm water divers not being able to stay afloat. Guess what happens when you accidentally lose your weights with a buoyant tank and wetsuit? If you're trying to determine which is safer you have to balance those issues.

Double bladder wings are controversial at best; redundant buoyancy is almost certainly better served with a lift bag (if using a wetsuit) or even a decent sized SMB. In case you don't know why this is, it's because A) the extra inflator hose adds task loading and failure points; sticking inflators are pretty common. How would like to be heading for the surface in an overhead or deco situation (common use for doubles) while trying to figure out which inflator hose is sticking? B) since the two bladders are in the same shell, whatever might puncture one could very well puncture both, while a lift bag or SMB is safely stowed.

 

[rstofer]

There are few people that would describe AL80s as "perfect" for warm water. More accurately, they're cheap, and that's why they're used so much. Adding weight to counteract a buoyant tank does nothing to improve diving.

Having a tank that is only -2# full and +4# empty implies that the diver needs to carry very little weight as long as they are diving in a T shirt. As a result of the lack of a wetsuit, buoyancy control is trivial and essentially unvarying except for air consumption. More important, the BC doesn't need a lot of air so it doesn't change lift as much with changes in depth.

Adding unneeded negative ballast just means that more adjustments have to be made in the BC as a function of changing depth.

The degenerate case of this type of diving wouldn't even require a BC. Just a couple of pounds of weight to overcome the tank buoyancy at the end of the dive and that's it. Contrast this with a -10.5# tank (start of dive) where the diver couldn't possibly be neutral at depth without wearing a BC.

The value of ditchable weight in warm water is debatable. Accidental dropping of quick release weights is fairly common, more so than warm water divers not being able to stay afloat. Guess what happens when you accidentally lose your weights with a buoyant tank and wetsuit? If you're trying to determine which is safer you have to balance those issues.

Losing weights is a potential disaster for cold water divers wearing wetsuits. If they can get off the bottom, they will have a heck of a time controlling the ascent.

Warm water (T shirt) divers only have a few pounds of weight and losing it is much less of a problem. Still, there is no reason on earth to lose a weight system (other than integrated weights held with Velcro) regardless of the type of diving. Get rid of plastic buckles, or better, ditch the belt in favor of a harness and all these problems (or potential problems) go away.

Even integrated weights (as long as they are held with something other than Velcro) make sense for warm water diving. The diver can float without the BC and the BC can clearly float without the diver.

Integrated (web belt attached) weights do not make sense for cold water diving. A 30# lift wing will not support a -10.5# tank, -20# lead, -6# backplate and -2# regulator (-38.5#). Yet a 40# wing isn't really necessary. The diver can wear the 20# of lead and still have plenty of buoyancy with a 7mm wetsuit and the 30# wing can easily support the tank, backplate and regulator (-18.5#). Added backplate weights are also a possibility for moving 8# (give or take) from the harness to the backplate. Now, however, the BP/W with tank weighs 60#. That's a lot to lift off the tailgate.

Warm or cold, getting the diver and rig balanced is the goal.

Double bladder wings are controversial at best; redundant buoyancy is almost certainly better served with a lift bag (if using a wetsuit) or even a decent sized SMB. In case you don't know why this is, it's because A) the extra inflator hose adds task loading and failure points; sticking inflators are pretty common. How would like to be heading for the surface in an overhead or deco situation (common use for doubles) while trying to figure out which inflator hose is sticking? B) since the two bladders are in the same shell, whatever might puncture one could very well puncture both, while a lift bag or SMB is safely stowed.

I don't dive doubles so none of these issues are of any concern to me. I'm only slightly worried about a stuck inflator valve. In any event, I have a little plastic hat attachment to make disconnecting the hose a lot faster with gloves. I would be more concerned about an elbow failure.

Total redundancy for lift is the best solution. But for a single tank diver, there is no need for this approach if the setup is properly balanced.

What do you tell your buddy? "Hey, I'm so negative at depth with a full tank that I know you can't rescue me with your BC alone. I don't have any ditchable weight so, if my BC fails, I sure hope you know how to use a lift bag." What if they don't know how to use a lift bag?

I don't think you can make a good case for no ditchable weight. By extension, I don't think you can make a case for a highly negative tank removing the requirement for ditchable weight.

Richard

 

[halocline]

Having a tank that is only -2# full and +4# empty implies that the diver needs to carry very little weight as long as they are diving in a T shirt....(snip)....I don't think you can make a good case for no ditchable weight. By extension, I don't think you can make a case for a highly negative tank removing the requirement for ditchable weight.

Richard

Actually, I already did make a good case for diving without ditchable weight in warm water. Namely, there is zero risk of accidental weight loss at depth, and the weight requirements are such that it's typically easy to swim to the surface with a non-functioning BC. That's based on lots of experience. Just because it doesn't agree with what you seem to have read here.....

Regarding the AL80 and weight you need to carry, one thing is for certain; you need to add 4 lbs to compensate for the tank. How that makes it "perfect" for warm water I just can't see. Compare that with something you personally already have, a LP72. That tank is smaller, weighs 5 lbs less, and is neutral when empty. (Make that 9 lbs less when you figure the added 4 lbs you're carrying to sink the AL80.) My idea of a "perfect" or "ideal" warm water tank would be a LP72 filled to 2700 PSI, in which case it holds a little over 77cf, just like an AL80. Or, if you're considering currently manufactured tanks, LP85 is the same weight but more compact than an AL80, neutral empty, and holds more gas. But there are lots of current tanks that are close to neutral when empty, any one of them is going to have better buoyancy characteristics for almost all warm water divers.

Everyone knows the only reason AL80s are so common is because they're cheap and require no maintenance.

 

[rstofer]

Regarding the AL80 and weight you need to carry, one thing is for certain; you need to add 4 lbs to compensate for the tank. How that makes it "perfect" for warm water I just can't see. Compare that with something you personally already have, a LP72.

Yes, the LP 72 has great merit (if little capacity) but you can't walk into a store and buy one. You can easily get an Al 80 or any other steel tank. Too bad they aren't made anymore. I might be in the market for another LP 72 if the opportunity arises. But, for my purposes, they are only useful in the pool. There's no point in using them in cold water.

One reason I like the Al 80: in Singapore, we couldn't take steel tanks into Malaysia. Period. In fact, I don't think I ever saw a steel scuba tank in Malaysia. It wasn't a matter of costs. Steel tanks just weren't allowed.

Both of my Al 80's are Luxfer S80's which means they are only 2.26# positive when empty. They are not as buoyant as the newer 80's which are 4.4# positive. Of course, they weigh 4# more on land.

The positive buoyancy is WHY I think the Al 80 is ideal for warm water. The diver, in warm water with no wetsuit, is essentially neutral. With a slightly positive tank, the diver will have to wear some ditchable weight. This makes rescue, at the end of the dive, quite simple. Ditch the weights and the diver is positive whether the BC works or not. In fact, they might be positive at the beginning of the dive without weight. Or, at least, they are very close to neutral.

I certainly don't see using highly negative steel tanks in warm water. Other than capacity, and that is a valid consideration, being pinned to the bottom with a blown BC is something I would rather avoid.

As I said, I don't really see cost as the only justification for Al 80's. My HP 100's are worthless for pool sessions.

Richard

 

[halocline]

You know, there's a guy selling steel 72s on ebay from Stockton CA, I believe. Check it out, he has lots of them. You might be able to pick out the best ones and you'd have no shipping costs.

Those S80s are not very common, I don't think I've ever seen one. The new AL72s have some potential, they're also 6.9" and neutral empty. The drawback is that they're 72 cf at 3000PSI, which means you're unlikely to get more than 72 cf out of them. 2700 PSI in a steel 72 is not difficult to get and then you have 77 cf.