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!
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...."