OLD TANKS
There are many different policies and beliefs when it comes to old scuba tanks. Some people will claim that all old tanks aren't safe, some only say old aluminum tanks aren't safe. Some shops have policies in place where they'll only fill tanks less than 20 years old, while some shops will fill any tank.
This all comes from a deep-rooted belief that old tanks are not safe. But why? Where did this belief start, and why is there so much variation in policies regarding old tanks? Well, unfortunately most of the issues surrounding old tanks comes from misinformation, lack of training, and an attempt to “make things easy”.
Most old tanks are perfectly safe to use, however a select few incidents of tanks rupturing have shaped how the dive industry views all old tanks. Even the thought of a tank rupturing is an absolute nightmare for most businesses and divers. With the risk of personal injury, as well as damage to the location, the issue can be quite terrifying. However, without proper investigation as to why the aforementioned tanks ruptured, as well as the passage of time has led to the misconception we have now.
The real “problem” was (and still is) with aluminum alloy 6351. Alloy 6351 was used by Luxfer, Walter Kidde and Cliff Impact to make SCUBA and SCBA (fire fighter) tanks. After being manufactured for a few years, it was discovered that some of these tanks began to develop Sustained Load Cracking (SLC). SLC is a peculiarity that sometimes develops in alloy 6351 in high pressure cylinders (like scuba cylinders) that have remained filled for extended periods of time. As the name suggests, SLC causes cracks in the alloy, making it unstable and can cause a tank to rupture when being refilled.
This is without a doubt a grave issue. However, a few key bits of information remain. First of all, Water Kidde and Cliff Impact were such small companies that their tanks are almost never seen in the dive industry today. Luxfer is still one of the main scuba tank manufacturing companies in North America, and when they learned about SLC they stopped using alloy 6351. By May 31st, 1988, Luxfer had completely stopped using alloy 6351, replacing it with alloy 6061 which is not susceptible to SLC. There has never been a recall on tanks made with alloy 6351, and Luxfer voluntarily changed the alloy that they use, and on their website offer a full list of which type of tank was switched when so you can easily look up to see if your tank had alloy 6351 or 6061 used on it. Brands like Catalina never used alloy 6351, so have never had the SLC issue.
The next key thing to note is, as said before, there has never been a recall on tanks made with alloy 6351. Why, you may ask? Because SLC is a very slow process. Government findings on tanks that had ruptured showed that the cracks that caused the tank to rupture had slowly formed over 8-10 years. Because of this, both DOT (the American governing body, Department of Transportation) and TC
(the Canadian governing body, Transport Canada) have made it a requirement that tanks using alloy 6351 be eddy current tested at the time of hydrostatic test, and once they have been eddy current tested be stamped with “VE” to prove it. Hydrostatic tests are required every five years in North America, and therefore if a tank has any SLC it will be discovered before it causes a rupture. It is also highly recommended that cylinders using alloy 6351 be eddy current tested at the time of visual inspection as well, which in the dive industry is every year.
Eddy current testing is not unique to the scuba industry. It is a non-destructive method of testing that uses electromagnetism to detect flaws. It is used extensively in the aerospace industry, as well as the petrochemical industry. However, in the scuba industry there ends up being a serious lack of training on eddy current testing (and cylinders in general as this post demonstrates). When used correctly, the eddy current tests will easily find even very tiny cracks. Without proper training however, eddy current tests can reveal a lot of false positives. Unfortunately, it has been
found that some dive shops have used eddy current tests on steel tanks and tanks made from alloy 6061. Steel tanks and alloy 6061 tanks do not get SLC. Without the training to know that, the training to avoid false positives is also missing.
So, knowing all this, why are there still policies regarding all old tanks, or 20+ year old tanks? Well, ease for one. Without the proper training on the different alloys tanks used, it can be hard for a shop employee to explain the issue with older tanks. Also, again if you’re not incredibly familiar with tank markings, which sad to say many divers and dive shop employees are not, it can sometimes be hard to tell what alloy a tank is made of. For example, Luxfer used to make tanks for US Divers. These tanks are not stamped “Luxfer” as the manufacturer, but instead “US Divers”. Without the knowledge of what every stamp means, and who made what tank, just looking at the manufacturers name on the tank can be very misleading. To prevent any accidents, especially with newer employees, it can be easier and safer to just make a blanket policy of “no tanks older than XX”.
After a tank for 20+ years, you have very arguably got your money’s worth out of that tank. Also, just because a tank does have SLC, doesn’t mean it is good. Like all things, with age and use metal wears down. After many years of use, especially if the tank has been well used, the threads on the tank neck can wear down. This is one of the many reasons tanks are visually inspected every year, but unfortunately threads being worn can be missed, especially since generally all the threads end up being worn, so for the casual inspector, they all look the same and therefore do not tend to raise alarm bells.
At the end of the day, if a shop has a policy of not filling old tanks, it means that they don’t have the knowledge, training, equipment or capacity to ensure safety while handling old tanks. If the shop is lacking in any of these departments, it isn’t a good idea to get an old tank serviced or filled there anyway. Human safety always comes before trying to save a few bucks on upgrading your gear. If you can find a shop that is able to safely deal with old tanks, great, but if not you can always recycle your old tanks and at the time of this writing, in BC, Canada at least, you can get about $20 per tank for the scrap aluminum.