Buoyancy question

[ljpm]

Hi All,

I have a question regarding buoyancy differences of Al and steel tanks. First, I know that differences in buoyancy characteristics will effect trim so this question ignores trim. From what I have been reading, most people consider the buoyancy characteristics of steel to be better than Al. The reason I have most often seen stated is because an Al tank is positively buoyant when empty. My question is why is this even a factor? A steel 80 has full/empty buoyancy of -8/-1.75 or a difference of 6.25. An Al80 tank has full/empty buoyancy -1.4/+3.4 or a difference of 4.8. If I was to set my buoyancy for both Al and steel in the same way at the beginning of a dive then shouldn't the greater difference in buoyancy of the steel tank result in a greater positive buoyancy over all. Again, this is ignoring trim.

Jim

 

[Insta-Gator]

If you're going to ignore trim, then my guess would be that folk prefer carrying the weight (neg tank buoyancy) on their back rather than lead around their waist. But I can't see having a realistic tank discussion on buoyancy without considering trim. YMMV

 

[tbone1004]

because you have to carry extra lead for the al80.

Luxfer AL80 is 4.4lbs positive empty, weighs 32lbs and holds 77.4cf of gas
Faber FX80 is about 2.5lbs negative *the bluesteel specs don't include the valve*, weighs 28.6lbs and holds 85cf of gas.

So on land, I have to carry an extra 4lbs for the tank itself, plus an extra 8lbs of lead to offset the positive buoyancy of the tank, and I get less usable gas from it. So if you want to carry an extra 12lbs when you're walking on land, then go for it, but because the steel tanks sink, you can cut a lot of weight off of your belt that is there for no reason than to offset the positive buoyancy of the aluminum tank.

Most steel tanks are much less negative than the HP80's, usually around 1.5lbs negative for the 3442 tanks, and around neutral for the low pressures, so the normal recommendation is to remove between 4-6lbs of lead when going from aluminum to steel. For a better comparison, the AL80 that weighs 32lbs plus an extra 4lbs to make it sink, plus about 6lbs of gas is a total weight of 42lbs, for just under 2cf/lb of weight. In contract, a tank like the FX100 weighs the same, but holds 108cf, for 2.5f/lb. Much better ratio. I used the FX100 because it's about the same size tank as an AL80, so you have a better gas to weight ratio, holds almost 50% more gas, and you are carrying the same amount of weight on dry land. Win win situation

 

[ljpm]

Thanks guys, that's pretty much what I had come up with.

It's just I keep seeing comments regarding the positive buoyancy of an AL tank at the end of a dive causing you to have trouble maintaining depth for safety stop. That makes no sense to me because it isn't the buoyancy of the tank per say, but the change (dalta) buoyancy which would be the cause and from the numbers I have seen Al and steel are similar with steel having a slightly larger delta buoyancy.

 

[PaloPinto]

only adding to confusion is that fact that most AL80's are not a true 80, the Catalina for example is only 77.4 cubic feet, compare that to a Worthington HP80, the true capacity is 81.00 cubic feet.

 

[doctormike]

It's just I keep seeing comments regarding the positive buoyancy of an AL tank at the end of a dive causing you to have trouble maintaining depth for safety stop.

Anyone who makes that comment doesn't understand the basics of weighting for scuba diving.

 

[Bob DBF]

It's just I keep seeing comments regarding the positive buoyancy of an AL tank at the end of a dive causing you to have trouble maintaining depth for safety stop.

If your weighting is correct you should have no trouble maintaining the stop however, if you do not have proper trim (weight in the right place) the AL tanks buoyancy will make you butt light and tend to rise, if you don't correct for the neoprene and air in the back of your wing/jacket getting more buoyant as it gets higher in the water column, this will drag you up. With experience you can control the issue and it will have you thinking about how important trim can be.

That makes no sense to me because it isn't the buoyancy of the tank per say, but the change (dalta) buoyancy which would be the cause and from the numbers I have seen Al and steel are similar with steel having a slightly larger delta buoyancy.

The change in buoyancy of a tank is caused by the change in weight of the air in the tank as you use it. The issue with an AL80 is that as you use air, the base of the tank is more buoyant than top and screws with your trim as you use your air.


tbone1004 gives an excellent reason why steel tanks are more popular with divers that own tanks.

There are reasons to use AL tanks, my big reason to start with AL was cost. When diving without thermal protection, the weight is not an issue and, as I said before, one can easily control the buoyancy issues at a safety stop with practice.



Bob
-----------------------------------
There is no problem that can't be solved with a liberal application of sex, tequila, money, duct tape, or high explosives, not necessarily in that order.

 

[Dive Right In Scuba.]

Thanks guys, that's pretty much what I had come up with.

It's just I keep seeing comments regarding the positive buoyancy of an AL tank at the end of a dive causing you to have trouble maintaining depth for safety stop. That makes no sense to me because it isn't the buoyancy of the tank per say, but the change (dalta) buoyancy which would be the cause and from the numbers I have seen Al and steel are similar with steel having a slightly larger delta buoyancy.

Anyone who makes that comment doesn't understand the basics of weighting for scuba diving.

I, too, have seen those posts. They are made, as doctormike notes, by people "who don't understand the basics of weighting for scuba diving." What is surprising is that many of these people are otherwise pretty knowledgeable about diving in general. For some reason their minds shut off when different kinds of tanks are involved.

I recently dived for several days with a very well known and popular dive center in Florida. On some of the days, the dive locations made nitrox a good choice, and on other days the dives were to be very shallow, so there was no point in paying for nitrox. I discovered while getting the tanks that the nitrox tanks were all different from the air tanks--the nitrox were Luxfer 3300 PSI tanks, and the air tanks were Luxfer 3000 PSI tanks. I thought the 3300 tanks were different in terms of buoyancy, and I asked the dive shop employee who handled the tanks about it. He said "no difference," but I was not convinced. I had my smart phone with me, and I went to the Luxfer site and showed him the very different buoyancy characteristics (about 4 pounds) of the two tanks. He said, "What difference does that make? They're both aluminum." He asked another dive shop employee, who said the same thing--one tank being 4 pounds more negative than another would make no difference in terms of a diver's weighting needs if both were aluminum.

 

[DA Aquamaster]

As others have noted, it's a little more complicated than "steel good, aluminum bad".

I'll do side mount cave dives in MX with a 22 pound wing, in a 5mm wet suit (albeit one crushed to about 3mm by too many deep dives) with a pair of AL80s, an AL 80 stage (231 cu ft total), a can light and some spools - with the only weight being a pair of 2 pound weights to the keep the tank tails down on the primary tanks.

I love diving that same basic configuration with steel 72s, as the 4 pounds of weight isn't required, (some) steel 72s don't go light in the tails to the same extent as an AL 80, and a steel 72 at 3000 psi holds 86 cu ft (249 cu ft total).

The same 22 pound wing in that same configuration will also float a pair of X7-100s and a stage (277 cu ft)

And, I can (barely) float a pair of painted Faber LP95 without a stage (259 cu ft).

Anything more negative than that however isn't going to work unless I go with a bigger wing, or use a dry suit.

On the other hand, on a cold water dry suit dive with thick underwear, I'll have to add a significant amount of weight to ensure I am neutrally buoyant at the end of the dive, if I am diving AL80's. However, with a steel tank like a PST 95, I can make the same dive with no added weight at all.

-----

In short, you have to look at the tank(s), exposure suit and wing as equal partners in a buoyancy system, and ensure that the combination as a whole:

1) allows you to obtain neutral buoyancy at all points in the dive, and
2) allows you to either dive a balanced rig that you can swim to the surface in the event of a wing failure, or
3) provides for redundant buoyancy to get you safely to the surface, without dropping weight - so that you can still achieve neutral buoyancy and complete a safety or decompression stop.

 

[seeker242]

Thanks guys, that's pretty much what I had come up with.

It's just I keep seeing comments regarding the positive buoyancy of an AL tank at the end of a dive causing you to have trouble maintaining depth for safety stop.

I underweighted myself once to try it out to see it was like, just practicing in the shallows. It was not that easy!

Sometimes I think people say that so they can sell the noobies steel tanks instead of aluminum tanks, because steel tanks are more expensive... The salespeople I have met are always trying to sell me the most expensive gear...because they think I don't know any better... "Here, buy this expensive one, I'll will make diving easy!" seems to be a common theme...