Messed up and ascended like a missile

[Eric Sedletzky]

It is surprisingly hard for many people to understand. I bet I have read 100 posts on ScubaBoard over the years telling people not to use aluminum tanks because, unlike steel, at the end of the dive the tanks float and will be pulling them to the surface.

Only if you don’t add a little extra weight somewhere to make up for it.

I once had a guy try to tell me that aluminum tanks had a larger weight swing than steel tanks because they were made out of aluminum. In other words, a steel tank holding 80 CF would lose 6 lbs. and he insisted that an aluminum 80 would lose 8-9 lbs. lol!
BTW, this was a kid that worked at my LDS years ago.

 

[MichaelMc]

But a diver does need some extra detachable weight with AL cylinders, compared to a steel cylinder of the same capacity, right ?

Detachable weight, no. Ballast yes. You + near empty tank should be neutral at SS. You + AL80 + 4 lb. extra ballast or You + HP80 - 2 lb. ballast. Where that extra ballast may be in a plate (non-ditchable) or some in a belt (ditchable). All in addition to any other ballast you needed, before considering your tank.

 

[MichaelMc]

I once had a guy try to tell me that aluminum tanks had a larger weight swing than steel tanks because they were made out of aluminum. In other words, a steel tank holding 80 CF would lose 6 lbs. and he insisted that an aluminum 80 would lose 8-9 lbs. lol!
BTW, this was a kid that worked at my LDS years ago.

I buy that the intended methods and standards of current instruction are better than the old days. But the frequent confusion on buoyancy physics does not always point to it being successful.

 

[Soloist]

The decision on what kinds of tanks to purchase depends upon a number of factors. For me the biggest factor is the amount of gas it will hold.

Agreed. I have two HP120 steels sitting in the corner that were to be used for diving wrecks off the coast of North Carolina a few weeks ago. They are filled to 4000 psi. Don’t try that with an aluminum tank. The dives were cancelled due to the weather......again!

 

[Soloist]

I once had a student for OW dives in cool, fresh water who had had another instructor from our shop for the pool sessions. Knowing he would do his OW dives in a 7mm suit, he had insisted on using a 7mm suit for the pool sessions so he would start the OW dives with his weighting needs dialed in. He told me that because of that, he knew 22 pounds was exactly what he needed. I looked at his slight build and said, "No way." By the time we finished his OW dives, we was down to 10 pounds, and he was much, much happier with his control of buoyancy and trim. I have no idea how his instructor-supervised weight check had gone, but it resulted in him thinking he needed more than twice the weight he actually needed.

I am often baffled when divers tell the crew how much weight they require for a single aluminum tank recreational dive. Huge numbers! I know, I know, everybody and every “body” is different. I also understand the impact poodle jackets, BPW, exposure protection and skill set can have on weight requirements, but I still can’t rationalize the numbers.

What’s worse is when they have no clue how much weight they need. Some ask the crew to tell them how much they should carry. This ALWAYS devolves into “it’s better to have too much weight than not enough”. Although true, you have to understand the repercussions.

This reminds me of my own naïveté as an oblivious new diver eight years ago. My OW instructor discussed proper weighting as part of the curriculum, but it obviously had no impact on me. Fortunately we took AOW shortly thereafter. When I told my new instructor how much lead I required he very politely informed me that I was an imbecile. He immediately removed 6 pounds and continued to remove more and more weight as the week progressed. The difference was no less than amazing.

 

[boulderjohn]

I buy that the intended methods and standards of current instruction are better than the old days. But the frequent confusion on buoyancy physics does not always point to it being successful.

One of the first times I heard the bogus claim on the difference between steel and aluminum tanks was from a Course Director who regularly taught accurately the details of Archimedes' principle. He thoroughly understood the physics when you presented it to him as physics, but he was merely repeating something he had heard without giving it enough thought. You see that in all walks of life. People believe ridiculous things when they are presented out of the context of scientific principles they know well.

 

[Roger Hobden]

As you have pointed out, there are tons of contradictory information on ScubaBoard and elsewhere about the buoyancy of steel vs aluminum cylinders, and it is frankly quite confusing for beginner divers to detect what is really accurate.

Just to be perfectly clear on the buoyancy issue: If an aluminum tank contains 500 psi / 32 bar of air or less, will it float on the surface of the ocean, lake, etc. ?

 

[boulderjohn]

As you have pointed out, there are tons of contradictory information on ScubaBoard and elsewhere about the buoyancy of steel vs aluminum cylinders, and it is frankly quite confusing for beginner divers to detect what is really accurate.

Just to be perfectly clear on the buoyancy issue: If an aluminum tank contains 500 psi / 32 bar of air or less, will it float on the surface of the ocean, lake, etc. ?

It will float nose down.

 

[Roger Hobden]

Detachable weight, no. Ballast yes. You + near empty tank should be neutral at SS. You + AL80 + 4 lb. extra ballast or You + HP80 - 2 lb. ballast. Where that extra ballast may be in a plate (non-ditchable) or some in a belt (ditchable). All in addition to any other ballast you needed, before considering your tank.

So a BP/W BC with a steel plate is more then enough to compensate for the very slight positive buoyancy of a nearly empty aluminum 80 cylinder ?

 

[boulderjohn]

So a BP/W BC with a steel plate is more then enough to compensate for the very slight positive buoyancy of a nearly empty aluminum 80 cylinder ?

Sure--but that is not the only part of your total diving package, and it is the total diving package that counts. You have your own body composition, your thermal protection, etc. as well. All of that combines to create your total buoyancy and the resulting need to include some amount of ballast for your dives. In some cases, you may not need any. In others, you may need a lot.