For many divers, the markings around the neck(top) of the tank may well be like reading hieroglyphics. The only real numbers most divers need to know is the tank pressure that the tank will hold, the annual inspection date, and when the periodic tank inspection is due. I will talk about all these as well as the metal types of the of most popular tanks. All dive shop owners will be able to help you interrupt the markings as well as taking care of your tank. Please visit your local dive shop for more information. The marking are not hard to learn and I will attempt to take the mystery out of the markings.
First set of letters either DOT or DOT/CTC. These stand for US Department of transportation and the Canadian Transportation Commission. If you travel to Canada and take tanks across the boarder, make sure it has this CTC marking on it. Most tanks sold in the US have the DOT stamp on it.
Next set of letters tell what type metal the tank is made from. The 3A denotes carbon steel. Used for early tanks. This tank more prone to corrosion than chrome steel of aluminum.
The 3AA stands for chrome-molybdenum steel. It appears on virtually all steel tanks today.
Aluminum cylinders may bear the designations SP6498, E6498, or most times it is 3AL. First two designations identify permit numbers under which aluminum cylinders are manufactured. But the 3AL is the usual mark you will see on tanks manufactured in the US after July 1, 1982.
The next set of numbers, usually four, is the working pressure of the tank. This is expressed in pounds-per-square inch. The cylinder should not be filled past this point. The single exception to this is when you see a + sign at the end of the second row or next to this pressure. That plus sign is usually assigned to steel tank and means you can fill by 10% increase. Most aluminum tanks go to 3000 psi, a few go to 3300 psi. You need to look at this to see what your tank is rated at. Of course the dive shop you take you tanks to will look at this number.
Some tanks have a second row while others may have more. All tanks have an unique number assigned to them to identify the tank, this is the serial number. May represent the size and lot number.
This usually follows the serial number and describes the dealer who made the tank. PST is Pressed Steel Company, Luxfer, and Kidde are the common.
The cylinders initial date follows the manufacturer name. It consist of numbers representing month and year that the cylinder was tested. These numbers will be separated by the hydrostatic tester's initials or a special symbol that the tested had registered with the DOT. Each tank needs to be re-tested every 5 years. Each 5 year test date will appear somewhere at the top around the neck.
This is a brief explanation of tank markings. Like I mentioned above, ask you local dive shop to help you determine what your specific markings mean.
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