Understanding the Markings on Your SCUBA Tank
Have you ever wondered what all those markings on your SCUBA tank mean? Many people don't know it, but many of the markings that go on SCUBA cylinders are required by law. In the United States, the Department of Transportation regulates SCUBA cylinders. In Canada, it's Transport Canada that regulates SCUBA cylinders. Every country has their own regulatory authority so these two are just examples.
On most SCUBA tanks the engravings on the shoulder will indicate the following:
- The serial number of the cylinder
- The material the cylinder is made out of (steel or aluminum)
- Tank capacity (how many cubic feet or liters does the tank hold)
- Service Pressure (what amount of pressure the tank can be filled to)
- Original Hydrostatic date (this tells you the age of the cylinder)
- Department of Transportation or other regulatory markings certifying that the cylinder has passed certain minimum requirements set by the government
Below is an example of markings you’d find engraved on a Luxfer aluminum 80 and next to it an explanation of what the markings mean.
There are many different possible engravings a cylinder can have depending on the manufacturer's it’s specifications and what part of the world the tank was produced. Because of this, the following chart contains common engravings found on different SCUBA cylinders and what the engravings mean.
|184||Cylinders service pressure expressed in BAR|
|207||Cylinders service pressure expressed in BAR|
|230||Cylinders service pressure expressed in BAR|
|237||Cylinders service pressure expressed in BAR|
|2400||Cylinders service pressure expressed in PSI|
|3000||Cylinders service pressure expressed in PSI|
|3300||Cylinders service pressure expressed in PSI|
|3442||Cylinders service pressure expressed in PSI|
|"+"||indicates cylinder can be overfilled by 10% when placed next to hydro date|
|01A20||An example of a hydrostatic exam date (numbers will reflect the date)|
|3AL||U.S. specification for 3-gauge aluminum|
|3ALM||Canadian specification for 3-gauge aluminum|
|A||Independent Inspector Mark|
|DOT||Department of Transportation (Cylinder passed U.S. specifications)|
|HP||Indicated High pressure (3300 to 3500 PSI)|
|LP||Indicates low pressure (2250 to 2400 PSI)|
|LP95||Model Number (also Indicates "low pressure" 95 cubic feet)|
|M9542||DOT Manufacturer facility number|
|P869232||An example of a serial number|
|S040||Model Number (also Indicates 40 cubic feet)|
|S080||Model Number (also indicates 80 cubic feet)|
|SU7694||Canadian special permit number|
|TC||Transport Canada (Cylinder passed Canadian specifications)|
|TP5250||Test procedure for hydrostatic retest (PSI)|
|XS-80||Cylinder manufacturer and model number (80 cubic feet)|
The above list doesn’t show every possible marking. Also, things like model numbers, dates, serial numbers, etc. will vary from tank to tank. In the list above those markings are shown as examples since the actual marking will change with every tank.
Also, it's important to know that even though the model number will often indicate the cylinder capacity, you should still look up the manufacturer specs when it comes to this. As an example, a Luxfer S080 is typically marketed and referred to as an 80 cubic foot. This number, however, is rounded up. The cylinder actually contains 77.4 cubic feet. You can see the specifications for the most common tanks here.
Hydrostatic Testing Stamps
Another common engraving found on SCUBA cylinders is the hydrostatic test date. Many people simply refer to this as the hydro date. The original hydrostatic exam is usually found on the shoulder of the tank together with the other manufacturer engravings and tells you when the cylinder was originally manufactured. SCUBA tanks need to be hydrostatically tested every 5 years. Subsequent stamps are usually somewhere in the shoulder area, but you may need to look around. I've seen stamps in pretty much every location you can think of on a SCUBA cylinder.
Visual Inspection Stickers
Visual inspection stickers are another marking you'll commonly find on SCUBA tanks. While obviously not an engraving visual inspection stickers provide useful information about the cylinder. They tell you when the last inspection was performed and whether or not the cylinder has been cleaned for use with Nitrox.
Nitrox SCUBA Tanks
It's very common to see tanks designated for use with Nitrox. This is usually done with a sticker rather than an engraving. Nitrox tanks are labeled for two reasons. First, so that everyone knows that the tank contains a gas other than air. This way no one will accidentally dive with a tank that they do not know the contents of. Nitrox requires the use of different decompression tables than air and the added oxygen also presents different risks that divers need to be aware of. Second, because Nitrox tanks are often filled with pure oxygen and then topped off with air as part of a partial pressure fill, the tanks need to be cleaned for oxygen. Technically, it's the visual inspection sticker that tells the technician filling the tank that it's clean, but the big Nitrox sticker is also helpful.
Any SCUBA cylinder can be adapted for diving with Nitrox. Most new SCUBA tanks come ready to be filled with Nitrox from the factory. However, if you are purchasing a used cylinder, you may need to have your local dive shop clean the tank and for use with Nitrox along with changing the O-rings to Viton O-rings so that they are compatible with Nitrox. Once this process is completed, the dive shop will indicate that it is safe for Nitrox on the visual inspection sticker. Additionally, many shops will place a Nitrox sticker on the tank to indicate that the cylinder may contain a gas other than air.
Recommended SCUBA Cylinders
Choosing a cylinder that’s right for you involves looking at many factors. Among those factors are:
- Your breathing rate,
- Body size
- Planned diving depth
- The mix you’re using
- Your body's buoyancy characteristics
- How you trim out in the water with your current configuration
- How much gas you’d like to have in reserve at the end of the dive
These are all factors that go into deciding what SCUBA tank will be best for you. We have an article that goes in-depth into this topic here.
For more information on specific cylinders, we recommend you check with the cylinder manufacturers directly. The “additional resources” tab below contains links to the cylinder manufacturers listed above. You can find information that goes beyond the SCUBA tank size chart there.