How the heck do I decipher the symbols on my scuba tank?

I remember being a newer diver, shopping for my first set of scuba tanks, and being completely baffled by the various numbers and symbols on the tanks I looked at. Yes, they did tank instruction in my Open Water classes. Yes, there was so much info in OPW class I was overwhelmed. So, I didn’t remember much about reading the tank information at all. At first glance it is quite confusing, but with a little help you too can be an old hand at decoding your scuba tank markings. This blog should help quite a bit. And, if you still have questions after you finish reading feel free to email me at and I will help you out…no charge!

When the scuba tank is manufactured, a series of coded numbers and letters are permanently stamped into the rounded neck portion of the tank, as required by US and Canadian law. These codes reveal quite a bit of information about where the tank was made, what it is designed to do, and how it can be safely used to scuba dive. When you are considering a tank purchase, this information is invaluable. The codes will help you decided when a tank is safe and appropriate for your use, and when not.

First, let’s take a look at the basic markings that are common to all tanks, and then we will go through the markings one by one:

DOT/CTC    3AL    3000    

P12345       LUXFER       6 (diamond) 94+    

The first series of letters on the left on the top row indicate which agency regulates the safe handling and transportation, and indicates the tank country of origin. “DOT” – The main agency that governs scuba cylinder transport and handling is the Department of Transportation (DOT).  The old name for the DOT is ICC. “CTC” –  Canada’s version of the DOT is The Canada Transportation Commission. You want to make sure the tank you are buying says “DOT/CTC” meaning it was manufactured and approved for use in the USA according to USA government standards for safety. I don’t suppose there are a lot of tanks from other countries in the USA, but you never know.

The middle code states the type of metal alloy the tank is made from. Many different metal alloys are used to make cylinders. Some of these are:

molybdenum steel – 3A, 3AA

aluminum alloy – 3AL (present day)

SP6498 (to 1972)

E6498 (to 1982)


As I have mentioned in previous blogs on buying used scuba tanks, there were weaker alloys used before the 1990s in aluminum tanks. These tanks may need an additional routine test called an “Eddy Current” test. You can read about this issue in my previous blog. The previous blog also addresses how the tanks respond during diving with respect to the different alloys. You will definitely want to know whether you are looking at a steel or aluminum tank as you go about your tank purchase.

The last code on the top row states the working tank pressure. Scuba tank pressure is measured in pounds per square in (PSI). The PSI helps determine the amount of air the tank can safely hold, thus affecting the length of your bottom time during a dive. Scuba tank working pressure for recreational use is typically one of the following (the “psi” is not printed on the tank):

Low Pressure (LP) – 2250 psi, 2400 psi

High Pressure (HP) – 3000 psi, 3500 psi

Each tank has a unique serial number. The number is stamped on the tank at the time of manufacture and serves to permanently identify the tank. This number is the first series of numbers on the bottom row.

The middle set of letters on the bottom row of coding is the manufacturer name or ID.  Luxfer and Catalina, for example, put their entire name on that tank. While other manufacturers use abbreviations instead of the company name.

The last set of letters and symbols are the “born on” date. In the case of the above coding example, the month of “birth” is 6 (June), and the year is 1996. This is the date the tank was hydrostatically tested and put into service. The small diamond shape  icon in between the “6” and “96” is the inspection code symbol.

The last marking in this series is a “+” icon, which indicates the tank may safely be overfilled by 10% of the entire fill rating.

The final markings on the bottom row indicate the working capacity of the tank in cubic feet. “S80” would mean the tank holds 80 cubic feet of air. This important because the more cubic feet of air you take to depth, the longer your bottom time potential.

Finally, on this label, you have the following: An aluminum tank made in the USA, 3000 pounds per square inch pressure (PSI), serial number P12345, Manufactured by LUXFER in June of 1993, that may be overfilled by 10%.

Additional numbers stamped into tank aftermarket are Hydro testing dates: 06  98, would mean the hydro was done in June of 1998.

The last thing you would be looking for is the Visual Inspection sticker. The visual inspection month and date is punched out on this sticker, which is usually placed toward the bottom or boot of the tank.  a visual inspection sticker should be on all tanks in service, and a visual inspection should be done yearly on a scuba tank. You can’t get a fill at your LDS if this sticker is expired. The good news is your LDS can take care of this for you relatively inexpensively and quickly!

If you are buying a tank, check all of the markings on the tank carefully. The markings tell you a lot about the tank history and care. If you buy a tank with a >5 yr old hydro date you are running the risk of buying a scrap tank, for example. If you are a younger diver, or you speed thru your air supply at depth, you will want to be sure there is enough cubic feet and PSI in the tank to meet your diving needs. When you look closely and know what you are looking at the coding makes perfect sense! And now, it is no longer a secret code to you.

Authored by Lisa J Henry

“Be the sea”