Steel vs. Aluminum

[DiveMaven]

I know that Steel tanks are negatively bouyant and that a diver carries less weight than with an Aluminum tank. Can anyone tell me how much weight a diver might lose if they switch from an Aluminum 80 to a Steel 90???

We're doing a Nekton cruise later this year and they have Steel 90s. We've only dove with Aluminum 80s, so are interested to know how much weight difference we might have. Thanks!

 

[lorien]

I know that Steel tanks are negatively bouyant and that a diver carries less weight than with an Aluminum tank. Can anyone tell me how much weight a diver might lose if they switch from an Aluminum 80 to a Steel 90???

We're doing a Nekton cruise later this year and they have Steel 90s. We've only dove with Aluminum 80s, so are interested to know how much weight difference we might have. Thanks!

I love my HP steel 80's even if everyone makes fun of my little tanks When I switched to the steels I took around 8 lbs off my weight belt which also made me pretty happy! The total amount depends on the negative bouyancy of the tank empty and different tanks are different. You will have to see how those particular tanks affect you but you can get a good idea of where to start before doing a bouyancy check.

 

[DA Aquamaster]

I think what they are referring to as "steel 90's" are PST LP 95's that hold about 96 cu ft at 2640 psi and 88 cu ft at 2400 psi.

If so, they would be about 7 lbs more negatively bouyant than an AL 80 when empty.

 

[lorien]

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.

 

[DiveMaven]

Thanks guys, this really helps!!

 

[pasley]

As some have already said you will take maybe 8 pounds off your weight belt. But that does not tell the whole story. How much more does the steel tank weigh than the AL80? For that weight is not really gone, just moved from ditchable weight to tank weight. In the end the weight of you on the scales with full gear on will probably change about 4 - 5 pounds total.

 

[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?

 

[pasley]

....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?

Justin699 You have introduced some variables I had not fully considered. This is a complex quesiton. I dive AL80 so I must rely here on the charts for the steel information.

First lets talk apples to apples. I would rather not compare a 77.8 Cu Ft. Tank to a 72 Cu Ft. tank when we can compare Steel 80 (Faber 80 with 80 CuFt) with AL80s that are a lot closer in volume of air (2.8 CuFt difference).
http://www.huronscuba.com/equipment/scubaCylinderSpecification.html
Tank______________Weight empty______Buoyancy Empty_____Buoyancy Full
Cat AL80 (77.8 CUft)_____32#____________+4.0#_____________-1.8#
Faber 80 (80Cuft)_______30#_____________-1.7______________-7.5#

77-80 cubic feet of air will weigh about 6 pounds (the shift in weight between empty and full regardless of tank composition)

I think your calculations are double counting some values here.

So lets take a look. Right off the top we see the difference in terms of raw metal weight the AL tank is only 2 pounds heavier than the steel.

Buoyancy: when empty the AL is positive 4 pounds or 5.7 pounds more buoyant than the empty steel (-1.7).

So what do you have to do to your weight belt? Ah well that was the original question wasn’t it? Well the raw metal weight difference between steel and AL in terms of weight is 2 pounds. By switching to Steel from AL you loose 2 pounds of weight. If all other factors are equal, that is 2 pounds you will have to add to the weight belt to compensate for neoprene and body fat.

The buoyancy factor is probably more important as the AL tank swings from negative to positive with a shift in weight of 5.8 pounds. Since the AL is positive by 4 pounds when empty and is 5.7 pounds more buoyant that steel you will have to add 4-6 pounds to the weight belt to compensate for the buoyancy difference caused by the displacement of the tank. So with AL you would wear only 4-6 pounds more than you would with steel.

Man, this is a hole lot more complex that one would first think.

 

[Charlie99]

So what do you have to do to your weight belt? Ah well that was the original question wasn’t it? Well the raw metal weight difference between steel and AL in terms of weight is 2 pounds. By switching to Steel from AL you loose 2 pounds of weight. If all other factors are equal, that is 2 pounds you will have to add to the weight belt to compensate for neoprene and body fat. .

Man, this is a hole lot more complex that one would first think.

You're making it more complex than it needs to be. The raw metal weight isn't relevant to how much lead you need on your weightbelt.

The one and only number that affects that is the empty buoyancy.

Using your numbers, the steel is -1.7#, the aluminum is +4.0. That's the 5.7 pounds difference in buoyancy. So you can carry 6 pounds less lead with the steel tank. (If you DO want to make it complicated, you will note that to get 5.7 pounds change in buoyancy,you need to add or remove about 6.2 pounds of lead, but that's another subject.).

Since the steel tank is also 2 pounds lighter in air, the total weight of gear you have to haul down to the shore or up and down the boat ladder is 8 pounds lighter with the steel tank.

Charlie Allen

p.s. For the orginal poster: I'm sure the Nekton has answered your question many, many times before. When I get a strange tank on a boat, they may not always know what the buoyancy empty is, but they always seem to know how much difference it is from a standard AL80.

 

[pasley]

Since the steel tank is also 2 pounds lighter in air, the total weight of gear you have to haul down to the shore or up and down the boat ladder is 8 pounds lighter with the steel tank.

You are quite correct on all points.