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Scuba tank testing – hydrostatic and visual inspection

by B.B. Pelletier

I’m back in the office and I thank all of you who helped out with the answers for the past week! I will go through the questions and add my two cents to a few, but most can ride just the way they are.

Today, I want to look at scuba tanks and their testing requirements.

Controlled by the U.S. Department of Transportation
Pressure vessels are regulated by the DOT for the safety of transport vehicles, their passengers and the environments through which they pass. Every scuba tank model must pass an initial DOT certification, then EACH manufactured scuba tank must pass a test before it can be sold. This is 100 percent inspection that even racing tires are not subjected to! It is somewhat equivalent to space specifications, so this is an extremely well-controlled industry!

Hydrostatic testing
After the initial test, the results are stamped into the body of the scuba tank, along with the date (year and month) it passed. After that, the tank must be retested hydrostatically every five years. The test for each tank is on file at the DOT, and the testing station must contact them to obtain the correct test specification. A common hydrostatic test is to pressurize the scuba tank to 5/3 of its working pressure and to measure the flexing of the tank walls. Five-thirds of a 3,000 psi working pressure means that the tank will be pressurized to 5,000 psi. This is done by replacing the valve with a special hydraulic testing connection and filling the scuba tank with water under pressure – NOT air! Air would be too explosive if the tank were to let go during the test, as it sometimes will. Because water cannot be compressed, the tank cannot explode if it fails.

Testing is done inside an armored vessel. The one my hydro station uses is sunk into the shop floor for added protection. The tank is also under water, and they measure the amount that a water column rises as the tank is pressurized. This measures the amount of tank expansion. The water inside does not expand, but the aluminum or steel tank casing certainly does! They are looking for tanks that DO NOT expand as much under pressure, indicating that their walls have been work-hardened over the years. Like a piece of steel that is repeatedly flexed, the tank walls get hard to the point that they may suddenly fail with a snap. This is EXTREMELY rare outside the testing station. When it DOES happen, it’s almost always fatal to people standing close by.

You know it when they fail!
When a tank fails during hydro, it’s usually the valve threads that let go. It sounds like a loud “whump” if you are in the building when one fails. The other thing that sometimes happens, but not as often, is that the bottom of the tank bursts open. This is due to corrosion. If the tank fails to flex enough during the test, it can never be filled with air again, and no dive shop will do so. It will not be stamped with a current test stamp (one within the past five years) and is “out of hydro” as far as any dive shop is concerned. One possible use for a failed tank is to cut off the bottom with a carbide saw (a lengthy process!) and hang it far downrange on a rifle range to be used as a gong.

Will a dive shop ever cheat?
Yes, it does happen. The few fatalities that occur usually happen at dive shops when shop operators fill out-of-hydro scuba tanks. So, in a cruel way, this is a Darwinian control over cheating! If you follow the rules, you are as safe as an airline passenger, and several times safer than any car passenger. I’ve been in three auto accidents in my life, but no airplane or scuba tank accidents – thank God!

The annual visual inspection
The annual visual inspection is performed by a dive shop. They let the air out of the tank and removes the valve to have a look inside. They’re looking for signs of corrosion and will refuse to put a visual inspection sticker on the tank if they find any. These stickers are paper or plastic and they stick to the tank. Every dive shop I have used has refused to fill a tank that has an out-of date inspection sticker. But they can do the inspection right there – the tank does not have to be sent to an outside party. They open the valve a little to slowly let the air out. This prevents condensation from rapid cooling. It takes 12 hours or more for a tank to bleed down – especially since airgunners seldom let their tanks drop below 2,000 psi. Let the dive shop bleed your tank for you; they know what they’re doing!

Do the guns need an inspection?
This is a great question! What about an airgun with a fixed reservoir? It never gets tested. AirForce guns have a removable reservoir, and they DO get tested by the DOT during the manufacturing process.

Some manufacturers of 10-meter target guns with removable tanks instruct owners to discard their tanks after 20 years. There is no regulation governing that, it’s simply their recommendation. No gun with a fixed reservoir has any recommendation whatsoever. Because the U.S. DOT does not regulate pressure vessels smaller than two inches in outside diameter, they don’t have to do anything. Other countries have different regulations, so check with your nation’s regulatory agency.

Airguns ARE much safer
They are safer for two reasons. First, their reservoirs are usually so small that they have no practical end to their life span. That’s why the DOT doesn’t bother with them. The second reason is that PCPs seldom fall below 2,000 psi, so they don’t work-harden nearly as much. The few that do go lower are usually not filled to 3,000 psi. So the air reservoirs on PCP guns are not work-hardened to the same extent that scuba tanks are when they are used by divers who take them to empty every time. The DOT has to regulate scuba tanks for their harshest intended purpose, so there will be no provisions for them when they are used only for filling airguns.

Problems outside the box
There are still some issues that are not addressed in this discussion. Filling scuba tanks from your own compressor is one. Filling guns from a hand pump is another. And there are more. The onus for safe operation ultimately falls to the owner of the gun. If you play by the rules and use common sense, you will be safe and can enjoy a wonderful hobby. If you cheat and cut corners – well, there are nicknames for such people – “Lefty,” “Stumpy” and “The Late….”

18 thoughts on “Scuba tank testing – hydrostatic and visual inspection”

  1. BB,
    What about carbon fibre tanks? Are they tested at the same intervals? Do most dive shops have the ability to fill to 4,500 psi.? I live on Maui, where all the dive shops/boats constantly are getting new alum 80 tanks and sell off the older ones cheap. They replace their tanks mostly because the appearance. They still have years of life.


  2. Jason,

    I do not know the testing requirements (if any) for carbon fiber tanks. I do know that they are not yet used as scuba tanks – at least not by the general public. Also, due to the nature of their manufacture, they don’t stamp the test date into the much thinner CF tank shell in the same way they do scuba tanks.

    I’ll find out more and perhaps blog it.


  3. BB
    Just wanted to say Thank you for all you do. I check in with you most every day but have never left a comment. I know you’re a competative shooter and was hoping you would comment on the Olympic Biathalon – I know it’s firearms but I was impressed by the skill it must take to shoot accurately after skiing. I’ve also become interested in aperature sights and hope you can explore them in a future blog. Thanks again.

    Springer John

  4. Springer John,

    I know Biathlon is shot with .22s in the Olympics, but there is an airgun sport, too. They combine it with running. The cadets at West Point are doing it, and I know of others like the Boy Scounts who do a derrivative called bike-athlon. There are even special 5-shot semiauto air rifles made for it.

    As for aperture sights, that sounds like a good topic! I’ll do it soon.



  5. Most dive shops seem to still only be equiped for aluminum tanks (3000-ish PSI). The only place I can think of that fills tanks capable of 4500 PSI on a regular basis (for their own use) are Fire Departments.

  6. Altitude,

    PCPs and CO2 guns shoot faster at altitude. Spring guns shoot slower.

    A scuba tank can be taken into outer space and it won’t explode. After all, sea-level air pressure is only about 14.7 lbs. per square inch, so that’s all you are removing in a perfect vacuum.


  7. I’m having difficulty in finding a facility that’ll perform a visual inspection and hydrostatic testing on my 3 Scuba aluminum cylinders in the Jackson, MS metro area. Do you perhaps have a list of facilities that do perform both tests, please? Thank you.

  8. Thank you for your prompt reply. I have visited two dive shops (including the one you listed) in Jackson, MS and they both refused to accept the cylinders for testing due to the fact that the manufacturer (Luxfer), some years ago placed a recall on those cylinders made out of a certain aluminum alloy. The manufacturer has sent me an email indicating that my three Scuba cylinders are OK for service provided that they pass both (1) the visual inspection and (2) the hydrostatic test. I guess the dive shops’ refusal to accept the cylinders for testing is a way of forcing me to buy new ones. I won’t give up until I can get the cylinders inspected/tested and any one of them fails and must be condemned, and be placed out of service. Do you perhaps have a list of non-dive shops in this area that’ll conduct the visual inspection and hydrostatic testing? Thank you.

    • John,

      First, welcome to the blog. I forgot to do that the first time.

      Here is what you need to do. Find a local dealer of industrial gasses. A welding shop can tell you where one is if you can’t find it. They either test all pressure cylinders, or they know who does. Your two dive shops are sending their cylinders to that place.

      The dive shops are worried about liability. Better to be wrong and lose one sale than to be clever and perhaps lose a lawsuit that bankrupts your business. I’m saying they have no great interest in running the facts down as you have.

      You can have the cylinders tested yourself. After the cylinders have the new date stamp, the dive shops should accept them like all other cylinders.


  9. I’m heading in that direction. Thank you for your recommendations. I don’t have much to contribute to your blog as I’m an underwater photographer: gave up spearfishing almost 40 years ago! Much obliged, thank you, Sir.

  10. I thought you might be interested in reading what another adviser has sent me about this same subject:

    “We can test them but will not fill them. In addition to the hydrotest ($24) you must also have an eddy current neck thread test ($30) in order to get the required VE stamp for this vintage AL80. With the history of the T6-6351 alloy, I would not store them full. If you consider the cost of shipping both ways and the cost of annual VE testing, it would be more economical to scrap them and buy new ($195 or less), or buy used ($100 to $120) newer than 1/1988.”

    I’m inclined towards giving up the testing route. Thanks for your interest.

    • John and B.B.,
      I happened upon your post/blog as my dive shop is beginning to be more involved with the local paintball, airsoft, and airgun community. The problems you described in getting your cylinders tested was horrible, I agree that the dive shops may have been trying to get you to buy new cylinders.
      Most dive shops dive shops don’t hydro-test their own cylinders but send them to the local tester just as they would do with yours, it may cost less to find the local hydro-test facility and take them there yourself. The hydro-tester can do the visual eddy (VE) current test, as it is required for more types of cylinders than just SCUBA cylinders and is only REQUIRED at time of hydro-test. The VE testing is prudent to have done yearly with the required visual inspection as the valve needs to be removed anyway, although, not every visual inspector will have the VE equipment or training to do it properly.
      For the yearly visual inspection, I would also recommend you to find out who trained/certified the local dive shop personnel. If they or their inspection sticker states certified to “dive (or SCUBA) industry standards” I would be suspect as the “dive” industry doesn’t make the standards for cylinder testing, the DOT, Compressed Gas Association (CGA), and OSHA set the standards.

      John, your other adviser is correct about not storing cylinders at full pressure. Even the cylinder manufacturers recommend storing cylinders long term (greater than 6 months) with a positive low pressure +20psi and not at full pressure. This is true for any cylinder not just SCUBA.

      Other considerations for all cylinders is that a internal visual inspection is required when the cylinder is hydro-tested and not all hydro-testers perform that inspection.

      The cost of owning cylinders versus renting really depends on how often your using them.

      B.B. thanks for the blog, I hope that my input is useful and I look forward to keeping up with your blog.

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