Air cylinders

[Bob DBF]

Diving Dubai said: ↑
steel tanks do not have flat bases like AL

SOME steel tanks. Not all.

The steel scuba tanks I've seen have round bottoms, but most of the steel industrial gas cylenders such as O2, Acetylene, CO2, and HP air bank cylenders are concave on the bottom. All are 3AA tanks.


Bob

 

[Rred]

Perhaps that is because the industrial tanks are expected to be chained or fixed in a vertical position during use, while the scuba industry teaches us to always lay tanks down--so they can't fall down. If making a concave bottom would cost a buck more, that's nothing to the industrial market, but the scuba market would opt not to have them made that way.

?

Speaking of which, why can't the scuba industry in the US manage to adopt and accept any one of the multiple color coding standards for a tank containing a nitrogen-oxygen blend?

 

[cool_hardware52]

Steel makes better ballast than aluminum. That may or may not be an advantage, depending on the application.

Tobin

 

[Nemrod]

Based on what I'm reading, there is no real up side to aluminum.

Well, you would be wrong. They are useful for warm water diving, use as stage and pony bottles and are low maintenance and cheap relatively.

N

 

[XS-NRG]

I think the upside is just that it's less expensive.

The other upside is that they don't rust and you don't have to tumble nearly as long if at all during annual viz inspections. You will get aluminum oxide but if filled from a good reputable source you likely can just tap the tank when the valve is off upside down and the tiny amount of ALOxide will come out and then you can finish the viz quicker and easier which can lead to a lower cost or less time if you are a viz inspector yourself like I am. The only thing that would counter that is the old 6351 aluminum alloy cylinder requiring an eddy current test manufactured up until 1989.

 

[Diving Dubai]

The other upside is that they don't rust and you don't have to tumble nearly as long if at all during annual viz inspections. You will get aluminum oxide but if filled from a good reputable source you likely can just tap the tank when the valve is off upside down and the tiny amount of ALOxide will come out and then you can finish the viz quicker and easier which can lead to a lower cost or less time if you are a viz inspector yourself like I am.

There is the constant confusion and misinpretation about this rusting on Steel

Firstly Steel and Ali both corrode with similar mechanisms:

Atmospheric:
Galvanic (dissimilar metal)
Stress corrosion
Fatigue corrosion
Wormhole or filiform

There are others but we don't need to consider those in this discussion

Just because steel exhibits its corrosion as a red coloured oxide (rust) and Al as a white powdery oxide doesn't make them any different. Indeed it worries me as a materials specialist that people believe the white powder oxide is less of an issue than rust scale.

Just because you can remove the white powder with ease doesn't mean you have removed the corrosion. Think of it as a fungus. you can remove the fungus but the spores are still there and it will grow again unless treated. All you have done is removed the oxide. Would you just knock off the rust scale and say the tank is okay?

Wormhole is the biggest killer. It starts with a pinprick defect in the paint and metallic coating, thus is harder to see.

All wire brushing does it to burr the surface material over the hole. As does sanding. The hole is still there.

Remember the hole is a loss of thickness of the parent material.

The only true way to examine cylinders to establish corrosion damage would be to carry out an ultrasonic inspection to measure material loss over what area.

Eddy current around the necks of tanks will only find surface defects (the depth of its penetration is so small to be considered surface) Especially given the thickness of the material. Eddy current is only suitable for Aluminum. Steel would be easier to inspect for cracking with magnetic particle but all of these are outside the scope of an LDS, both with the cost of equipment and operator training

My Steel tanks are 5 years old. I live in a climate with high temps, high salinity and high humidity yet all of mine are in near perfect condition. The y get washed after each diver and properly washed after every trip The insides are perfectly clean (never been tumbled) and each year I re paint the outsides (ensuring I don't break through the original primer) The top coat ensures salt water doesn't come into contact with the steel and is just the final protective coating.

Aluminum is prevalent because it's cheap not because its better Steel is lighter for an equivalent volume but is negative - hence you don't want a bunch of steel deco cylinders.

I've been to quite a few dive ops that use all steel or have AL 80 and steel for anything bigger.

Buoyancy characteristics, and cost are the main deciding factors Both materials have their pro's and con's.

This constant line of Al is better than steel because it doesn't rust is disingenuous at best and a concern that people can be certified inspectors without being taught or understanding basic principles of corrosion

 

[NAM001]

ignore this

 

[Remy B.]

Techduikshop.nl

This is what I wish to own.

 

[scubadiver888]

First, congratulations on owning tanks. The more equipment you own the more you tend to go diving.

Second, use the search box on here and search for "tank markings". In general, searching for answers on here will yield a wealth of information. Just be warned that the information is free and you often get what you paid for.

Once you learn how to read the markings on the tank you'll know a bit more about the tanks. They should be hydrostatically tested every 5 years. When they test them and they pass the facility will stamp the year and the month in the shoulder of the tank. For example, on my tanks there is a stamp for 03*11 and 01*16. The first stamp is the stamp the manufacturer put on the tank when they were created (March 2011). I could have waited until March 2016 to have them tested again but I figured I'd do it now before dive season started. So the second stamp is January 2016 (4 years and 10 months later).

If I get them stamped every 5 years, they will have more and more stamps on them. The same will be true for the tanks you have. The metal the tanks are made out of and the size of the tanks are stamped on there as well. Sometimes it isn't as straight forward as you might expect. The 3AL3000-S63 means 3AL is aluminum, 3000 is the service pressure (MP or medium pressure), S63 means at service pressure they hold 63 cu.ft. of air. An Aluminum 80 might have something like 3AL3000-S80 however, an S80 actually only has 77.4 cu.ft. when filled to 3000 PSI.

The tank without air could have got moisture in it. If that was the case, it could have been corroded on the inside to the point it was unusable. So you are lucky there. There is still a chance the AL80 might have moisture in it and it will be unusable. Sometimes bad fill stations pump moisture into the tanks. I've heard of tanks with a few cups of water in them even though they are full of air.

A new AL80 would have cost around $150 to $200. A used tank would probably need a hydro and vis ($30 to $50). Might need to have the valve rebuilt ($15 to $20). Might need to have the tank tumbled to get rid of corrosion ($15 to $30). Let's say you are lucky and don't need a tumble but the valve needs to be rebuilt. You could be looking at $70 plus $50 for the tank for a total of $120. Even if you need a tumble, rebuilt valve and hydro you might spend $100 plus cost of the tank. Looks like worst case you might break even. Unless worst case the tank fails hydro.

If everything works out well for you, you don't need to worry about this. But if you find yourself in the market for a tank again, some tips. When you go to buy used tanks, make the sale conditional on the tank passing hydro. If it doesn't pass hydro, you don't pay for it. If it does pass hydro then you have a good chance you got a new scuba tank. If you can get it for $50 or less than you saved a little money.

One last snag, if the tank is aluminum and made before 1990, it could be made of a different aluminum alloy than they use today. Search for "Sustained Load Cracking" on here. The tanks which suffer from SLC need to have a visual inspection AND a eddy current test. This means testing every year is a little more expensive. Additionally, some shops won't touch these tanks. According to industry standards they SHOULD be fine. But if they are going to explode they'll probably explode when getting filled. So many shops won't fill them. I don't blame the shops. If these things explode around me, I'm probably dead. For the amount of money I get paid to fill tanks, I'm not going to take ANY chance. You can figure out how old the tanks are from the stamps on them. If any stamp is older than mm*90 then they probably subject to SLC.

 

[TheHuth]

Based on what I'm reading, the Al-63 was made in March 1985. That makes sense since they charged me $10 more for the inspection on this one. He mentioned something about potential cracking in the threading and would require an additional test. I took pics of both tanks, but the markings arent legible for the second cylinder. He didnt charge me the extra fee for the Al-80, so I think its newer.

So question. Is it worth risking having one of these older Aluminum tanks? Sounds a bit risky to me. Even if I threw away $100, its not worth my life.

I'm pretty excited to have the gear. I got my regs last week (Aqualung legend with sherwood alt) and my bcd is on its way (Aqualung Seahawk). I'm close to having everything. Just need weights and an exposure suit.