The following information applies to ALL Aluminum Compressed Air tanks and Aluminum CO2 Tanks
All aluminum tanks (whether compressed air or CO2) will have this information STAMPED into the tank, not necessarily printed on a label. THe Stamps may
be hard to find, but they ARE there. For the purpose of this thread, I have taken white paint and filled in the stamping so that the (otherwise impossible to see) information is visible for my camera.


These images were taken sequential so that you could see how the stamping is generally placed on Aluminum tanks (common on 48-3000, 72-3000, 9oz, 12oz, 20oz, 24oz tanks)
AS you can see, I've highlighted sections of the tank information, just as I did on the composite tanks above.
The same color in all 3 photos will highlight the same information.
RED: DOT-3AL stamp - Designates the tank as ALUMINUM. NOT STEEL
GREEN: 1800 - This is the WORKING PRESSURE. This is the maximum rated operational pressure of the tank. The hydrotest pressure is x5/3 (2 1/3) this pressure. Just because the test pressure is that high, does NOT make it safe to fill the tank above this (in the case of HPA tanks!)
AQUA: Manufacturer's Serial number
YELLOW: Manufacturer's Symbol (Number) - ALL tank manufacturers, (whether making aluminum, steel, composite, etc.) are assigned a manufacturer's Symbol or ID number. THe number will range from M1000-M5999. ALways starts with M, and is always 4 digits. This number can be referenced here:
DOT Manufacturer Symbols to find out who REALLY made that tank you're carrying. This number is also present on some composite HPA tanks.
BLUE: Hydro Date - This is the BORN ON date of your tank; when it was manufactured. ALL "3AL" ALUMINUM TANKS ARE 5 YEAR TEST CYCLES.
(Yes, there is the exception, but that will be covered at the end)
PINK CIRCLE: Manufacturer's Stamp: Not every tank will have this. Some companies put a trademark stamp on their tanks as well.
MAGENTA: Tare Weight/Volume of tank: On CO2 tanks, this will have the CO2 weight that can be put into the tank. for HPA bottle, this will have the cubic inch capacity of the tank.
WHITE: TC-SU Stamp - This is the TRansport Canada number that says this tank is authorized for use in Canada. I believe that Aluminum tanks are also 5 year test cycles in Canada as well.
Now for some thoroughly confusing information.

This is a 3.5oz ALuminum CO2 tank. Notice how the information on the tank is DIFFERENT? The reason for that is because this tank is exempt from hydrotesting rules.
BLUE: DOT Special Permit Number 7737 - This permit number designates a 3E hydrotesting exempt tank. (this is a rarity, as the general rule of thumb is that 3E tanks must be made from chrome-moly steel. This is an aluminum tank, however) IF your tank is metal, and DOES NOT have "3AL" or "3HT" stamped on it, PLEASE look up the special permit online to check the restrictions/ conditions of use of your tank. IF your special permit is NOT listed in the USDOT PHMSA website, then it has expired, and your tank is NO LONGER VALID FOR USE.
MAGENTA: 1800 - This is the WORKING PRESSURE. This is the maximum rated operational pressure of the tank. The hydrotest pressure is x5/3 (2 1/3) this pressure. Just because the test pressure is that high, does NOT make it safe to fill the tank above this (in the case of HPA tanks!)
YELLOW: Hydro Date - This is the BORN ON date of your tank; when it was manufactured. This tank is one of those exceptions to the 5 year test rule - it does not need to be tested, ever.
AQUA: TC-SU Stamp - This is the TRansport Canada number that says this tank is authorized for use in Canada.
You may notice this tank also says "ALUMINUM ALLOY" on it. This is a requirement of the special permit, being an aluminum tank classified under 3E regulations.
Now for another oddity in CO2 tanks - Chrome-Moly Steel...


.
BLUE: DOT Special Permit Number 8096 - This permit number designates a 3E hydrotesting exempt tank manufactured from CHrome-moly steel. This special permit number is no longer available from the USDOT PHMSA for reference because it is expired, making it illegal to service this tank.
MAGENTA: 1800 - This is the WORKING PRESSURE. This is the maximum rated operational pressure of the tank. The hydrotest pressure is x5/3 (2 1/3) this pressure. Just because the test pressure is that high, does NOT make it safe to fill the tank above this (in the case of HPA tanks!)
WHITE: Manufacturer's Symbol (Number) - Same as info for 3AL tanks above.
YELLOW: Hydro Date - This is the BORN ON date of your tank; when it was manufactured. This tank is one of those exceptions to the 5 year test rule - it never needed testing.
Hydrotesting information
On to some wonderful additions to the hydrotesting rules. AS you can see, I have some 3E-hydrotest exempt tanks shown. Here's the special info about these tanks: ALL 3E tanks must be Chrome-MOLY steel UNLESS specified as such by the special permit issued by the USDOT PHMSA. This does NOT mean that all chrome-moly tanks are 3E-hydrotest exempt.
For METAL tanks, hydro test dates must be STAMPED into the tank to be valid. If the hydrotest information is put on a sticky label of any kind for a metal tank, it is NOT a valid recertification. The ONLY exception is for Fire Extinguisher tanks; meaning designed and USED as a fire extinguisher.
Prior Retest dates ARE NOT legal to remove unless all of the following conditions are met:
1: There is no further room for retest dates to be added:
2: The original manufacturing test date is not removed.
3: The owner's permission is obtained FIRST.
4: Minimum sidewall thickness is maintained (for metal cylinders)
The re-test date cannot be stamped into the sidewall of the tank, unless permitted by the ORIGINAL tank specification. 3E cylinders do allow for this.
The wonderful -2"x2'- rule: Any cylinder not exceeding 2 inches outside diameter and less than 2 feet in length is excepted from volumetric expansion test. NOTE: 49 CFR 180.209 does not say ONLY tanks adhering to this rule are exempt. It says tanks that meet the 2 x 2 rule are INCLUDED in the exemption from requiring retest. 3E tanks commonly exceed these dimensions.
---
Now, this information mainly covers 3AL type aluminum tanks, and 3E Chrome-Moly steel tanks. There is another metal tank still found in use. These tanks have a "3HT" stamped on them. These chrome-moly steel tanks ARE NOT FOR USE IN PAINTBALL. They are ONLY certified for AVIATION USE. They were sold mainly by Crossfire quite a few years ago.
---
INFORMATION LINKS:
Special Permits Index - Look up your tank's exemption number, print it out, and keep it with your gear. These are updated frequently, and will TELL YOU HOW OFTEN YOUR TANK MUST BE RETESTED. PLEASE
READ.
They will also tell you WHO actually manufactured the tank.
Transport Canada's TDG Permit list - Same list as above, but for TC-SU Canadian stamps. If your tank doesn't have a TC-SU number, it CANNOT be used in Canada. Period. (The USDOT PHMSA
does recognize TC-SU stamps and testing)
Authorized Cylinder Retesters List - Find a LOCAL hydrotester; it's always an option. Maybe save some shipping time and not worry about the USPS losing your $200+ tank. Hydrotesters MUST have HIGH PRESSURE or INDUSTRIAL (HIGH AND LOW PRESSURE) listed beside their name in order to be used. Low pressure or visual inspections only DO NOT QUALIFY.
If you have ANY other questions, feel free to PM me.