Steel vs. Aluminum

[JustinW]

I dropped 6 pounds, just because it was convienent, when I dive one of the little HP steel tanks.

The weight isn't just transferred to the tank, a HP steel tank is much denser with a smaller volume (i mean volume as in water displacement, not interior volume).

The faber 71.4 that i was referring to weighs 29 pounds (I assume empty?) and is 5.9 pounds negative when empty, 11.3 pounds negative when filled.

a Catalina AL 77.4 (AL80) with a 3300 psi working pressure is 35 pounds (assumed empty?), is 5.8 pounds negative when full and neutral when empty.

So on land you will need to carry 6 pounds extra lead to compensate for the tank, plus the AL80 will be 6 pounds heavier to start with. So going with a HP steel 72 vs an AL80 will mean that when going from your car to the boat or beach or whatever that you will have and extra 12 pounds of weight to carry.

Does it make sense now?

ctopus: Pasley, I don't believe I was double counting anything, I will go through it again using my above numbers. And I used that tank, because thats the tank i usually use instead of an AL80, so I thought it would be interesting to myself to see how much lead I am saving.

I have 6 pounds difference between the two tanks on land and 5.9 pounds difference between weight (in the water) for an empty tank. Therefore to compensate for the bouyancy of the AL80 and my tank, I need to add 5.9 pounds of lead to my belt/pockets. Additionally, since the AL80 weighs 6 mounds more on land, I have to carry around an additional 12 pounds (11.9 actually). Look again, I don't think i double counted anything, but hey, point it out and i'm happy to admit a mistake.

 

[Stirling]

ctopus: Pasley, I don't believe I was double counting anything, I will go through it again using my above numbers. And I used that tank, because thats the tank i usually use instead of an AL80, so I thought it would be interesting to myself to see how much lead I am saving.

I have 6 pounds difference between the two tanks on land and 5.9 pounds difference between weight (in the water) for an empty tank. Therefore to compensate for the bouyancy of the AL80 and my tank, I need to add 5.9 pounds of lead to my belt/pockets. Additionally, since the AL80 weighs 6 mounds more on land, I have to carry around an additional 12 pounds (11.9 actually). Look again, I don't think i double counted anything, but hey, point it out and i'm happy to admit a mistake.

Just for clarity - you may be correct about the end result, but to better explain how you get there - the AL80 you're talking about is a HP tank that is neutral when empty, correct? So strictly speaking you're not adding weight to compensate for the buoyancy of that tank, because it is never positively buoyant.

The steel tank is negatively buoyant when empty, so some of the weight required to compensate for your wet suit (more likely) is in the steel tank. When you switch from steel to aluminum, 6 pounds of the weight you need to compensate for the wet suit has to go from the steel tank to somewhere else - the weight belt or other weights.

So I think you're right in the end result - you have to carry 6 pounds more of weight, and the aluminum tank is heavier in air than the steel tank by another (did you say 5 or 6?) pounds, so your increased burden out of the water is the total of those two numbers.

 

[DA Aquamaster]

Let's not over think it here, as it is not rocket science.

If you have satisfactory bouyancy with your normal every day AL 80 and you go on a live aboard and use an LP 95 that is 7 pounds more negatively bouyant, you need to remove 7 lbs of lead to achieve the same bouyancy. It really is that simple and that was essentially the question.

Where it gets more complicated is where you also change exposure suits, etc as the end result may be that you need to spend your first dive ensuring your bouyancy is well adjusted as suit bouyancy has many more variables involved.

Knowing the swing weight of the tank is useful as it allows you to adjust for correct neutral bouyancy at the beginning of that initial dive with a full tank and then add the appropriate amount of weight so that you will be neutral at your safety stop at the end of the dive. This approach ensures that you enjoy the first dive and can get everything right on one dive instead of messing with your bouyancy during the firt few to several dives of the trip. The safety stop at the end of the first dive serves as a check to make sure you did it right and if so, there is no need for further adjustment.

For example if you adjust your bouyancy so that you are neutral at the surface with a full LP 95, you will want to add about 7 lbs of weight as the 96 cu ft of air in the tank weighs just a bit over 7 lbs. If you are sure you will be back on the boat with 500 psi, then you can figure the swing weight based on 78 cu ft and add only 6 lbs of weight.

Comparing the gross weights of the tank and lead weight combinations out of water to determine how much one configuration weighs compared to another is a simple matter of adding the weight of the tanks(full or empty) to the lead weight required for each configuration and comparing the two. But it is the least useful information to have as it has no bearing on how things work in the water and is realyl only relevant if you are planning on buying tanks and are interested in having to haul the least amount of weight to and from the dive site.

In general since aluminum tanks have thicker walls than steel tanks, they tend to actually weigh more that steel tanks of similar capacity and they tend to be larger and displace more water than a steel tank of similar capacity. So with a AL tank you end up with a tank that is a few pounds heavier out of the water and you end up carrying a few more pounds of lead to acheive neutral bouyancy in the water compared to what you would need with a steel tank. So with few exceptions, you will remove weight when switching to a steel tank and the whole configuration will also weigh less out of the water.

 

[DA Aquamaster]

A good site to see the bouyancy of different tanks full and empty is:
http://www.sportdiverhq.com/tkcht.htm
Will give you a good idea regardless, hope it helps.

Thanks for that site Lorien, I managed to lose the link several months ago and could not find it again.

Here is another useful site for tank specifications:

http://www.diverlink.com/gear/tankspecifications.htm

 

[DiveMaven]

Wow, you guys sure answer a question thoroughly!! I love this board.

 

[JustinW]

Oh boy, I was looking back at those two charts, I looked at a few different tanks, and tried to crossreference them with each other between tables and found very different results. For example, I just picked up a pair of PST 102s and I wanted to see how I should adjust my weighting compared to the smaller Faber/SP HP72 I was diving. Both sites gave me different numbers. Hmm which one is right? Or are they both wrong?

 

[DA Aquamaster]

I am not seeing any significant difference:

Sport diver site:

3000 psi Faber 71.4 = 29 lbs, -11.3 bouyancy full, -5.9 bouyancy when empty

3500 psi PST 100 = 33 lbs, -7.5 bouyancy when full, 0 when empty

Diver Link Site:

3000 psi Faber 71.4 = 29 lbs, -11.3 bouyancy full, -5.9 bouyancy when empty (under the "Scuba Cylinder Chart" section and "Steel Cylinder" sections as well as -6 listed in the "Scubapro" section)

3500 psi PST 1000= 33 lbs, -7.6 bouyancy when full, 0 when empty

So there are a two differences of .1 lb. but .1lb is nothing to get one's underwear in a bunch over.

There may be some confusion however as the diver link site also lists 2 Faber 71.4's under the "steel" section. The second one is the "slim" 71.4 offerred for a short while by Scubapro but it is not very commonly encountered and was not included on the sport diver site.