Should I avoid this set of doubles or manifold?

[MSDT Mark]

I have an opportunity to pick up a set of twin AL 50s for $100. These are U.S. Divers made in 1974 (6351) alloy. They look like new inside and out. They are complete with the SS band and manifold. The U.S. Divers manifold appears to be made of aluminum. I understand that the cylinders need hydro, eddy tested and visual inspection.

These would be for smaller divers to play around with.

Should I avoid this set of doubles or the manifold?

 

[captndale]

Assuming the tanks pass hydro, visual and eddy current testing they are safe to use. The only question is whether or not you can get them filled. Many shops will no longer fill 6351 tanks. Manifolds made in the 70's generally had only one regulator outlet. Modern manifolds have two outlets so that two regulators can be mounted on them, each with its own seperate valve.

This setup could be a good buy if what you are looking for is a setup that is compact and holds 100 CF. It will be a comfortable, low profile system but it will not provide the redundancy that is assumed in modern doubles systems. You could, of course, replace the manifold with a modern one and have a nice, low volume set of doubles with redundancy.

 

[Nemrod]

First, the aluminum tanks are worthless as they are from the alloy (6351) that MANY will NOT fill anymore, not the current 6061.

The manifold is not aluminum, it is satin chrome on brass or nickle chrome on brass. They did not make aluminum manifolds to my knowledge. This is not the type of isolating manifold currently used today for tech diving. Short double tanks were used in that time and earlier to provide good trim, not increased capacity--pre BC era. In the 50s and 60, double 38s, 45s and 50s were very desireable tanks, these were all steel--the same steel chroMo alloy as used today. The early aluminum tanks from the 70s and early 80s are of a aluminum alloy (6351) that has proven prone to stress cracking and they are not good but for scrap.

Never throw aluminum away into a landfill, recycle it, it is a gift from the gods, it is energy intensive to make, always recycle aluminum.

N

 

[MSDT Mark]

First, the aluminum tanks are worthless as they are from the alloy (6351) that MANY will NOT fill anymore, not the current 6061.

The manifold is not aluminum, it is satin chrome on brass or nickle chrome on brass. They did not make aluminum manifolds to my knowledge. This is not the type of isolating manifold currently used today for tech diving. Short double tanks were used in that time and earlier to provide good trim, not increased capacity--pre BC era. In the 50s and 60, double 38s, 45s and 50s were very desireable tanks, these were all steel--the same steel chroMo alloy as used today. The early aluminum tanks from the 70s and early 80s are of a aluminum alloy (6351) that has proven prone to stress cracking and they are not good but for scrap.

Never throw aluminum away into a landfill, recycle it, it is a gift from the gods, it is energy intensive to make, always recycle aluminum.

N

Nemrod, I fully understand and respect your post concerning 6351 alloy cylinders. I am a bit paranoid by nature myself. The dive industry has many differing opinions on the use of 6351 alloy cylinders. I do believe in being on the conservative side of safety.

However to keep things in perspective over 5.9 million cylinders have been made from 6351 alloy. Only 8 scuba cylinders have ruptured from Sustained Load Cracks (SLC). All the ruptures were during filling, all were leaking and all had a long crack which could have been noticed by a trained cylinder inspection and taken out of service prior to the explosion.

Luxfer has stated the following concerning 6351 alloy cylinders: As long as the cylinder is passing its visual inspection and eddy test the cylinder is fine to keep in service.

The odds of being hit by lightning in the U.S is 1 in 700,000 annually. Your odds of being struck in your lifetime is 1 in 3,000.

 

[Nemrod]

Mark, I am not paranoid. I fully understand everything your saying--BUT---many shops, especially in Florida, will not fill them no matter what you or I say. Jesus could walk across the water and X ray vison them himself and they would not care--they will not fill them--period. In fact, many will not fill 6061 tanks that are over 10 years old. There is no point in arguing with them about the 6351 issue, they either don't care or don't care to understand. I had a perfect 6351 cylinder, eddy current and fresh hydro and VIP--nope---wil not fill it. So, all I am saying is that dpendent upon whether you have your own compressor or where you live, they are essentially worthless. I would not take them if they gave them to me because I am fatigued with arguing about filling them with dive shop gurus that know everything and will NOT FILL THEM--PERIOD.

N

 

[rjack321]

I agree with N. I wouldn't bother with 6351 tanks. Too many shops have made the risk management decision that they aren't worth it to fill. The tanks are worth about $20 each at a scrap yard though And the single outlet manifold could be used on a set of 72s for a nice vintage rig. I don't really dive that way, but its popular in some places.

 

[WD8CDH]

If he will deduct the price of the hydro and eddy current testing if the tanks fail, go for it. If the tanks fail, the manifold and backpack would still be worth it.

 

[Nick_W]

Just from personal experience I've never had any problems getting a tank filled outside of Florida and I have at least five that are made of this "bad" alloy. So, if you do a lot of diving outside of Florida, it may be worth it to you. Maybe one day those shops in Florida will realize that what they're doing just makes no sense at all and start filling these perfectly good tanks again.

 

[DA Aquamaster]

Florida is a tank fill oddity.

1. They overfill LP steel tanks inexcess of 140% in north Florida
2. Yet they cringe at overfilling steel 72's made to the same 3AA standard by the same percentage, or for that matter at all.
3. I have seen operators in south Florida fill newer 6061-T6 alloy tanks with a 3000 psi service rating to 4000 psi
4. Yet they will not fill properly inspected and re-certified 6351-T6 alloy tanks at all and some will not fill 6061-T6 tanks over 10 years old.

Its' a case of some Florida fill operators playing fast and loose with the regs and safety margins when it suits them and their preferred cutomers, and then making up new rules to try to keep those practices "safe". Personally, I agree that if I lived in an area where AL tanks were filled to 4000 psi by some operators, I would not fill one over 10 years old either, but that has nothing to do with 6351-T6 alloy.

The rest of the country is a different story and we are far enough past the uncertainty about sustained load cracking in 6351-T6 tanks that any hesitancy to fill them in the rest of the country is more or less chicken little thinking - although in defense of some shops, that thinking may be from their insurance carrier rather than the shop. Its' not right, but it's hard to change the mentality of the herd even when you have a gazillion 6351-T6 tanks being filled each year with no SLC related failures since proper inspection techniques were implemented 6 or 7 years ago. Check with your local dive shop and if they will fill them, buy them.

$100 for twin AL 50's in good shape is a good deal. The single outlet manifold is dated by modern technical diving standards but it still does what it was designed to do in the same way a K-valve still works just fine on a single 100 cu ft tank.

Weight wise they are not all that effiicient as they are in the same range as a 100 cu ft Aluminum tank and are much heavier in comparison to a 3442 psi 100 cu ft tank, which weighs about the same as an AL80 but is not much taller than a single AL 50. But double 50's trim much better than the long and large diameter AL 100 and are stable in the water.

One downside from a fill perspective is that you may pay for two tank fills (as you have doubles) for a maximum 100 cu ft of gas along side a diver with a 100 cu ft steel tank who will get the same amount of gas for half the price.

 

[paulwall]

I have an opportunity to pick up a set of twin AL 50s for $100. These are U.S. Divers made in 1974 (6351) alloy. They look like new inside and out. They are complete with the SS band and manifold. The U.S. Divers manifold appears to be made of aluminum. I understand that the cylinders need hydro, eddy tested and visual inspection.

These would be for smaller divers to play around with.

Should I avoid this set of doubles or the manifold?

If it's a solid-bar manifold, I might be interested. I have a sherwood Selpac dual-outlet manifold, and I love it.

Many shops in New Orleans won't fill pre-1990 Aluminum tanks under any circumstances. One shop won't fill my pre-1990 Steel tanks, and they were open when the tanks were made!

Fickle unregulated industry.