If weight is the problem, why not go for carbon fibre? They can make carbon fibre cylinders that hold 2,400 psi, but they are so light, you could easily wear double 80s. But (as mentioned above), you better carry a lot of extra lead...
Titanium scuba tank
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Hi Rhone Man,
Low pressure carbon fiber is not the answer. With the weight needed to make them neutral, it would weigh more than aluminum or steel. When weighted for neutral, a 2215psi Luxfer carbon fiber tank weighs almost 1 pound more than a Luxfer AL50.
And without weight, it might blow away in the wind.
I looked thru the specs required for DOT-3AL aluminum tanks and calculated the pressure if the only thing changed was the tensile strength of the aluminum from 6061-T6 to 7075-T6. It would allow for aluminum tanks to be made with the same dimensions, wall thickness and buoyancy characteristics with 80% higher service pressure and almost 80% more capacity. That means that a 40 sized tank could be filled to 5400psi and hold 72 CF. At 4500 psi, it would still hold about 60CF (50% higher capacity). A pair of them would hold the same air as a X8-119 but be almost 10 pounds lighter with the same buoyancy characteristics and a lower profile.
If the walls were thinned to provide the same safety factor of a 3AA steel or 3AL aluminum tank at 4500psi rather than 5400psi, a 7075-T6 tank the size of the present AL 63 or HP 80 tanks would hold about 105 CF (67% higher capacity). It would weigh under 25 pounds empty and would only weigh about 29 pounds when weighted to neutral.
My conclusion is with the availability of 4500psi compressors, a 7000 series aluminum alloy would be the optimum material for a SCUBA tank. In general, size for size, a 7075-T6 tank would hold about 30% more air than a HP steel tank and once weighted to the same buoyancy of the smaller capacity steel tank, the total weight would be the same. Pressures quite a bit over 4500 psi would be needed to give Titanium an advantage.
At 4500psi, it's kind of a toss up between carbon fiber and 7075-T6 (for SCUBA) but the 7075-T6 would be cheaper and less effected by water.
Increasing the wall thickness of an HP tank to allow for 4500psi service pressure would make for a very negative and heavy tank.
If the walls were thinned to provide the same safety factor of a 3AA steel or 3AL aluminum tank at 4500psi rather than 5400psi, a 7075-T6 tank the size of the present AL 63 or HP 80 tanks would hold about 105 CF (67% higher capacity). It would weigh under 25 pounds empty and would only weigh about 29 pounds when weighted to neutral.
My conclusion is with the availability of 4500psi compressors, a 7000 series aluminum alloy would be the optimum material for a SCUBA tank. In general, size for size, a 7075-T6 tank would hold about 30% more air than a HP steel tank and once weighted to the same buoyancy of the smaller capacity steel tank, the total weight would be the same. Pressures quite a bit over 4500 psi would be needed to give Titanium an advantage.
At 4500psi, it's kind of a toss up between carbon fiber and 7075-T6 (for SCUBA) but the 7075-T6 would be cheaper and less effected by water.
Increasing the wall thickness of an HP tank to allow for 4500psi service pressure would make for a very negative and heavy tank.
pescador775
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7075 aluminum (aircraft aluminum) is subject to cost, quality and process problems when drawing large cylinders. It might be possible to make these by spinning pipe. It has high strength in compression loads but tension (expansion) is only fair. It is common practice to use a 4X safety factor (vice 2.5X1) when making high pressure cylinders from 7075 pipe. This is due to the low ductility and other characteristics such as notch sensitivity which can lead to sudden failure under pressure. Even so, a SCUBA tank with 4X factor would be slightly lighter than a 6061 aluminum tank. However, I don't know if it could be hydro tested reliably. Sometimes, a large vessel made with 4X factor is exempt but those with which I am familiar are more commonly made of steel. I suspect that a hydro test would not mean anything. A 7075 AL made to 4X would probably always pass and a 7075 AL manufactured to 2.5 X could pass a hydro and explode the next day.
Hi Pescador,
Actually, the 7000 series alloy used in the DOT cylinders is 7032. I am not sure of the exact differances between 7075 and 7032, but 7032 is allowed twice the wall stress of 6061-T6 alloy. The minimum elongation of 7032 is 12% which is 2% higher than 6061-T6.
The 7000 series tanks are still required to handle 100,000 cycles to service pressure and 10,000 cycles to test pressure.
Actually, the 7000 series alloy used in the DOT cylinders is 7032. I am not sure of the exact differances between 7075 and 7032, but 7032 is allowed twice the wall stress of 6061-T6 alloy. The minimum elongation of 7032 is 12% which is 2% higher than 6061-T6.
The 7000 series tanks are still required to handle 100,000 cycles to service pressure and 10,000 cycles to test pressure.
Flightlead
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by volume. we s/b talking about the strength of material?
Ti density 4.506 g·cm−3
Al density 2.70 g·cm−3 (of course alloys of each would vary somewhat). So, for a given volume Al is 60% of the weight of Ti.
I'm not this kind of engineer, but I suspect Youngs modulus is the best measurement of required strength for this application.
Ti modulus = 116 GPa
Al modulus = 70 GPa
Therefore Ti is 1.657 times as stiff as Al, or would require 60% of the amount of material that Al would require for the same stiffness erasing much of Al's gain.
Diamond has a Youngs moludus of avg. 1,100 GPa and a density of about 3.5 g·cm−3 .
Personally I think this would be the best option for your application. It would also satisfy the "bling" requirement.
I'm not sure how you'd cut the threads at the tank neck though.
Youngs modulus doesn't matter much for a tank. It only effects the REE, not the strength. Youngs Modulus is important for things that flex, like bicycle frames.
Ultimate Tensile strength and yield strength are the main factors that are used to design a tank. Typically, tanks are designed to not exceed 67% of the ultimate strength or 80% of the yield strength of the material at the Test pressure. For referance, DOT allows ultimate tensile strengths of 38KSI for 6061-T6, 76.2KSI for 7032 and about 104KSI for 3AA steel tanks. Ti alloys vary a lot. Pure Ti is fairly weak but there are Ti alloys over 200KSI.
By the way, since Ti alloys can be 5 times as strong as aluminum but only about 60% stiffer, Ti tanks stretch more when filled. My Titanium tanks would stretch stainless steel bands when full and would become loose as the tank was emptied.
Ultimate Tensile strength and yield strength are the main factors that are used to design a tank. Typically, tanks are designed to not exceed 67% of the ultimate strength or 80% of the yield strength of the material at the Test pressure. For referance, DOT allows ultimate tensile strengths of 38KSI for 6061-T6, 76.2KSI for 7032 and about 104KSI for 3AA steel tanks. Ti alloys vary a lot. Pure Ti is fairly weak but there are Ti alloys over 200KSI.
By the way, since Ti alloys can be 5 times as strong as aluminum but only about 60% stiffer, Ti tanks stretch more when filled. My Titanium tanks would stretch stainless steel bands when full and would become loose as the tank was emptied.
Flightlead
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doesn't have as much bling as diamond though.
did someone mention a tank exploding? http://www.scubaboard.com/forums/tanks-valves-bands/50551-i-would-love-see-tank-explode.html
pescador775
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Yes, Luxfer makes the light weight "L7X" alloy extruded cylinder from 7032 or 7060, I believe. These are high copper/zinc alloys which are annealed, extruded, solution heat treated and aged in such a manner as to reduce the ultimate strength by about 20% and thereby increase toughness required to prevent fatigue cracking such as may be associated with 7075 when used in similar applications.
I took a look at their sales literature and Luxfer offers a 3000 psi oxygen tank for medical use which is made of that alloy.
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