Steel 72s v. Aluminum 80s

[jiveturkey]

Just how old are these tanks? Something to consider is how many more hydros you can get out of them. If the tanks are 20 years old you may not get a lot more time out of them. Yes, I know some tanks will last longer but after 4+ hydros you're getting to the end of the line for many tanks.

 

[muddiver]

I think it is really funny that no one looks at the change in buoyancy charateristics between steel tank vs aluminum. If you read the specification table for your favorite brand you might notice that the change in weight of a steel tank of compairable size to an aluminum is quite a wider range. The change in buoyancy of a steel tank can be as much as 7 to 10 lbs. where an aluminum 80 only changes by 5 lbs. You really are not reducing the amount of weight needed at the end of a dive if you are trying to maintain neutral at 15' to 20' on an empty tank. Yes the steel tank is always negitive as opposed to an aluminum, but with the larger change in overall buoyancy you loos the advantage to the overall change in buoyance due to depth and wet suit expainsion and you still end up positive.

 

[Rick Murchison]

Just how old are these tanks? Something to consider is how many more hydros you can get out of them. If the tanks are 20 years old you may not get a lot more time out of them. Yes, I know some tanks will last longer but after 4+ hydros you're getting to the end of the line for many tanks.

My '72's are from '65 and '72, still passing hydro every five years and I see no reason they shouldn't continue to do so. If steel is cared for and not overstressed, it has excellent elasticity & should outlive you. Aluminum's another matter.
Rick

 

[Rick Murchison]

I think it is really funny that no one looks at the change in buoyancy charateristics between steel tank vs aluminum. If you read the specification table for your favorite brand you might notice that the change in weight of a steel tank of compairable size to an aluminum is quite a wider range. The change in buoyancy of a steel tank can be as much as 7 to 10 lbs. where an aluminum 80 only changes by 5 lbs. You really are not reducing the amount of weight needed at the end of a dive if you are trying to maintain neutral at 15' to 20' on an empty tank. Yes the steel tank is always negitive as opposed to an aluminum, but with the larger change in overall buoyancy you loos the advantage to the overall change in buoyance due to depth and wet suit expainsion and you still end up positive.

This is an old wive's tale, and tables that show such changes are innacurate. So are some Scuba "textbooks" from some agencies that say so. (Scuba textbooks usually claim AL tanks have a larger buoyancy swing than steel, however, which is the opposite of what you are saying here!) The swing in buoyancy between a full and empty tank is the weight of the air used, which is independent of the tank material. (Yes, there is some tank expansion involved, but not enough to make any measurable - whole pound - difference).
Rick

 

[jiveturkey]

If steel is cared for and not overstressed, it has excellent elasticity & should outlive you. Aluminum's another matter.
Rick

True but with older tanks your risk increases. Does the OP know the history of the tanks? How about the shops where they were filled throughout their life: do these shops fill quickly or overfill? It's just another thing to consider.

 

[sarita75]

Does the OP know the history of the tanks? How about the shops where they were filled throughout their life: do these shops fill quickly or overfill? It's just another thing to consider.

I have actually decided against the tanks for a number of reason - most importantly the hydros and vi's were out of date or just about to on all of the tanks. Call me a ninny, but the total cost for them (and then adding on the testing) plus the fact that I am a relative beginner and a bit worried about safety, I decided that I will wait and buy new tanks when the time is right.

 

[Sideband]

I think it is really funny that no one looks at the change in buoyancy charateristics between steel tank vs aluminum. If you read the specification table for your favorite brand you might notice that the change in weight of a steel tank of compairable size to an aluminum is quite a wider range. The change in buoyancy of a steel tank can be as much as 7 to 10 lbs. where an aluminum 80 only changes by 5 lbs. You really are not reducing the amount of weight needed at the end of a dive if you are trying to maintain neutral at 15' to 20' on an empty tank. Yes the steel tank is always negitive as opposed to an aluminum, but with the larger change in overall buoyancy you loos the advantage to the overall change in buoyance due to depth and wet suit expainsion and you still end up positive.

The change in weight of any tank is going to depend on the amount of air in it at the beginning of the dive and the amount of air in it at the end of the dive. It doesn't matter if it is a steel or aluminum tank, if you use the same volume of air from both tanks they will both be lighter by the same amount of weight. Air weighs .0745 lbs/cft.. It doesn't matter what the container is. The only way one tank can have a bigger swing is if it has a larger capacity. What is important to those choosing one material over the other is wether the tank is positive, negative or neutral after the swing. A tank that ends positively buoyant when near empty requires the addition of lead to counter that buoyancy. A tank that ends neutral requires no lead adjustment and a tank that ends negatively buoyant allows lead to be removed from the belt.

 

[garyfotodiver]

I have actually decided against the tanks for a number of reason - most importantly the hydros and vi's were out of date or just about to on all of the tanks. Call me a ninny, but the total cost for them (and then adding on the testing) plus the fact that I am a relative beginner and a bit worried about safety, I decided that I will wait and buy new tanks when the time is right.

IMHO, you made a wise and considered decision...

 

[sarita75]

IMHO, you made a wise and considered decision...

Thanks Gary. I had a hard time saying no to them ... but there you go.

 

[DA Aquamaster]

True but with older tanks your risk increases. Does the OP know the history of the tanks? How about the shops where they were filled throughout their life: do these shops fill quickly or overfill? It's just another thing to consider.

I have seen welding tanks made prior to WWI come through the local shop for hydro and these tanks are still passing hydro tests with flying colors.

Steel tanks do not have the fatuigue and sustained load cracking issues that aluminun tanks have nor are they likely to be dinged to the extent that they need to be condemned as is tha case with an aluminum tank where it is comparatively easy to ding the tank to a depth that would render it unsafe.

Steel tanks virtually never fail a hydro. What causes them to be condemned is rusting that progresseses to pitting and with proper care (VIP's and perhaps tumbling every 10 years or so to remove accumulated flash rust) they will easily last 50 plus years and a 100 year lifespan would not be unexpected. I have tanks made in the mid 60's that are still going strong and can be expected to last another 40 years with proper maintenence. If a steel tank is in current hydro and passes a current VIP, would not worry about where it has been or how it has been maintained as there will be no hidden damage that may bite you later.

The same cannot be said about Aluminum tanks. The general consensus is that they will not have nearly the same life expectancy as a steel tank.

Aluminum is sensistive to heat and temps in the 300-325 degree range are enough to cause issues with the T-6 heat treatment used on both 6351 and 6061 alloy scuba tanks. Even with that "low" temp, It would take some serious intentional effort to get them that hot during a fill. Some fire departments have a bad habit of slam filling their aluminum SCBA tanks in under a minute and that does not even create that amount of heat. (The tanks so abused due however tend to fail hydro after 5 or 10 years and or develop neck cracks.)

Steel tanks are not heat treated and you would pretty much have to heat them to a dull red to damage them. I don't care how fast you fill them, they are not going to ever get THAT hot.