Photo of scuba tanks on a pier with words "scuba tips scuba tank markings"
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Scuba Tank Markings (According to a scuba instructor)

ByJason Botticelli Hours

As some of you may not know, scuba is an acronym and stands for self-contained underwater breathing apparatus. It is our tanks that make it “self-contained.” We are able to wear our tanks and travel freely about underwater untethered to a boat. Before our scuba tanks, divers wore brass helmets with air hoses running to an air source on a boat or barge above.

old black and white photo of hardhat diver getting ready to get in the water.
Prior to scuba tanks, divers were “hardhat” divers with a helmet and air hose running to an air source.

On our scuba tanks are stamped markings and stickers standing for various things. So let’s take a look as to what those things are so that the next time you go diving you can check to see you have a properly maintained tank.

Stamped Markings

In North America there are two agencies that regulate our scuba tanks. Those agencies are Transport Canada (“TC”) and The Department of Transportation (“DOT”).

Photo of Canadian flag with words "Transport Canada"
In North America Transport Canada and The Department of Transportation regulate scuba tanks.

Stamped near the top neck of the tank you will see TC and/or DOT showing the regulating agency. you will also see a number and letters representing the metal type of the tank. for example AL means you have an aluminum tank. AA means you have a steel tank.

Photo of scuba tank markings for DOT and tank metal type.
DOT stands for Dept. of Transportation and 3AL means its an Aluminum tank.

Next you will find the tanks working pressure, or psi. For example the tank pictured below has a working pressure of 3,000psi. This is normal for aluminum 80cuft tanks.

Photo of scuba tank marking showing 3000psi.
This tank’s working pressure is 3,000psi.

Other markings you will find will include the serial number, name of manufacturer and hydrostatic test date with tester’s mark. In the image below, the serial number is P379973, Manufacturer’s name is USD and hydrostatic test date is 11 81, which represents that the tank was tested in November 1981. The arrow in between the 11 and 81 is a special marking for that particular testing location, called the tester’s mark.

photo of scuba tank markings for serial number, name of manufacturer and hydrostatic test date and tester's mark.
You will also find the tanks serial number, name of manufacturer and hydrostatic test with tester’s mark.

Some old steel tanks may have a little plus sign after the year of the hydrostatic testing. This represents that the tank is approved for 10% overfill.

Hydrostatic testing And visual inspection

There is a great deal of energy in a filled scuba tank, about the equivalent of 11 sticks of dynamite. So they must be well maintained, tested and inspected.

Hydrostatic Testing

The hydrostatic testing must be done every five years to test for metal fatigue. The tank is drained, valve removed and filled with water, pressurizing it to a high pressure to measure the expansion of the cylinder walls.

The tanks are tested to 5 1/3 of their working pressure. So if the working pressure of the tank is 3,000psi, then 1,000 is 1/3 of the working pressure and 5 1/3 is 5,000psi. The tank will fail if it’s permanent expansion is greater than 10% of the total expansion. If the tank passes inspection, it will then get the stamp with the hydrostatic test date and tester’s mark.

Photo of hydrostatic test markings on a scuba tank.
The hydrostatic test stamp will show the month, symbol of the testing agency and year of the testing. (Photo courtesy of deepwaterhappy.com)

Visual Inspection

Just like your car, your tank needs to be visually inspected every year. This is to check for dents, cracks and corrosion. The tank is drained of its air and the valve removed in order to inspect inside and outside. If it passes, a sticker will be placed on the tank, much like when your car passes inspection.

Photo of scuba tank's visual inspection sticker.
If your tank passes its annual visual inspection it will be given a new sticker showing month and year of the inspection.

Helpful tips

Do not bother with aluminum tanks that have their first hydrostatic test prior to 1990. A bad alloy was used in some of these tanks and they have been found to develop stress cracks. Reputable dive shops will not fill these tanks.

Photo of scuba tank with markings.
A bad alloy was used in some tanks that have their first hydrostatic test date prior to 1990.

If your tank is found to have some corrosion on the inside it does not necessarily mean it is ready for the garbage. Some tanks can be cleaned up using an abrasive material, usually aluminum oxide, and tumbling the tank to remove the corrosion.

Wrapping Things Up

If you take good care of your scuba tanks, your grandchildren may be using them someday. Be safe and make sure to have your tank visually inspected once a year and hydrostatically tested every five years. Treat your tanks with respect as they harness a great deal of energy and when traveling look for the proper markings to make sure you have a good safe tank.

Tanks Scuba Jay! You’re welcome diver!!

If You’re Looking For other Scuba Diving Tips, Check Out The Below.

“How boring would the world be if everywhere and everyone were the same. Safe travels and good adventures.” Scuba Jay

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