Overweighted at beginning of dive but underweighted at end in shallows

[crofrog]

Strange, I have one 120 Steel and one 100 Al. With both empty the Al floats, the Steel sinks.

Your tanks when they’re steel contribute ballast to you but if you meed say 15lb to stay down without a scuba tank. If you dive an al80 than ends 2lb positive you’ll need 17lb of lead . If you dive an hp80 than ends 2lb negative you’ll need 13lb of additional lead.

But either way you need 15lb of negative buoyancy to stay down and you’ll be 6lb heavier in the beginning than the end

 

[johndiver999]

Your tanks when they’re steel contribute ballast to you but if you meed say 15lb to stay down without a scuba tank. If you dive an al80 than ends 2lb positive you’ll need 17lb of lead . If you dive an hp80 than ends 2lb negative you’ll need 13lb of additional lead.

But either way you need 15lb of negative buoyancy to stay down and you’ll be 6lb heavier in the beginning than the end

No. air weighs less in an aluminum tank.. It is all in the chart. Or is it more?

Whatever, I KNOW it is different depending on what container it is in.

 

[Bento.Box]

No. air weighs less in an aluminum tank.. It is all in the chart. Or is it more?

Air weighs the same. It depends on tank size and pressure.
Steel tanks are denser than Aluminium tanks.

 

[Belzelbub]

You can always read this since you give no credence to my post.

Steel vs Aluminum Scuba Tanks: Which One is Best? - Scuba.com

Before you buy your own air cylinders, take a look at tips you'll want to consider when choosing steel vs aluminum scuba tanks.

www.scuba.com

Nice Little chart here:

Actually, the chart you posted proved @tursiops post more than your own.

Take the AL100 and steel 100. Find the difference between buoyancy full and buoyancy empty. I get 7.4 lbs for the AL and 7.3 lbs for the steel. The 0.1 lb difference is likely due to rounding.

The swing is still the same. Steels tend to stay negative, so you can carry a bit less weight. You still need to account for the swing though, so you’ll be negative at the start of the dive if weighted for neutral at the end.

 

[Derbdiver]

Actually, the chart you posted proved @tursiops post more than your own.

Take the AL100 and steel 100. Find the difference between buoyancy full and buoyancy empty. I get 7.4 lbs for the AL and 7.3 lbs for the steel. The 0.1 lb difference is likely due to rounding.

The swing is still the same. Steels tend to stay negative, so you can carry a bit less weight. You still need to account for the swing though, so you’ll be negative at the start of the dive if weighted for neutral at the end.

In the end it does make a huge difference if you are diving cold water with 7mm wet suit or dry suit how much weight you will need to strap on. If you can remove some of that weight by just carrying what is necessary (tank) then overall its better.

 

[Derbdiver]

In the end it does make a huge difference if you are diving cold water with 7mm wet suit or dry suit how much weight you will need to strap on. If you can remove some of that weight by just carrying what is necessary (tank) then overall its better.

The tank is also a much more balanced weight.

 

[The Chairman]

In the end it does make a huge difference if you are diving cold water

In diving dry, weight = warmth.

 

[Adr1an]

Just to share something that left me totally puzzled this August in Sharm: diving in a 2mm shorty with a 15L Alu tank (Coltri, big diameter "classic" 15L Alu) I was comfortably at the end of the dive with just 4Kg of lead. Could have dived with 3 Kg no problem. One of the dive centers had these 15L Chinese tanks which are much longer then the Coltri but the same diameter as the 12L regular tanks. With one of these I barely managed to dive at the beginning of the dive (emptied the lungs almost completely to go 3m down etc) and had to ask the guide for another kilo do be able to do the safety stop comfortably. I'd be very curious to know the empty weight of these 15L tanks but I'm wondering how they will perform safety wise on the long run...

 

[Derbdiver]

Just to share something that left me totally puzzled this August in Sharm: diving in a 2mm shorty with a 15L Alu tank (Coltri, big diameter "classic" 15L Alu) I was comfortably at the end of the dive with just 4Kg of lead. Could have dived with 3 Kg no problem. One of the dive centers had these 15L Chinese tanks which are much longer then the Coltri but the same diameter as the 12L regular tanks. With one of these I barely managed to dive at the beginning of the dive (emptied the lungs almost completely to go 3m down etc) and had to ask the guide for another kilo do be able to do the safety stop comfortably. I'd be very curious to know the empty weight of these 15L tanks but I'm wondering how they will perform safety wise on the long run...

I am betting the standard for high pressure tanks is different than the US. The DOT requirements are specifically designed for safe transport and the hydro requirement every 5yrs insures that the cylinder doesn't expand abnormally during pressure testing. If the tanks were exactly the same (AL or Steel) they should weigh the same and have a similar buoyancy. I spent some time doing training in Shenzhen and while they are way ahead of us in Electric Vehicles and a lot of high tech products, they don't have the government safety systems in place for industrial product safety.

 

[Juliet of the Sea]

Aluminum 80s are notorious for getting light as they empty, and will float up at the tail end, as well. For my own tanks I prefer steel 80s which stay negative. If renting, an AL63 will stay relatively neutral. Over the years, I have found that I get as much bottom time (or more) from an AL63 as an AL80, since I am not having to account for and fight the positive bouyancy. of the 80