This page can be used as a reference for tank safety information.
Tank Re-testing:
All tanks must be periodically retested depending on the material and size. Steel tanks must be hydrotested every five years. All fiber-wrapped tanks must be hydrotested every three or five years, depending on the DOT-E regulation (see below). If you are unsure, assume the tank requires testing every three years. The only exception is for tanks that are less than two inches in diameter and less than two feet in length, which never require retesting.
Steel and aluminum tanks never expire; the tank is good as long as it pasts the hydrotesting. Fiber-wrapped tanks, however, are only good for 15 years maximum. After that, the tank must be retired.
The only exception is steel tanks marked 3HT, which must be hydrotested every three years, and expire after 24 years.
This applies to both CO2 and HPA tanks.
DOT-Exeption / DOT-Special Permit (DOT-E / DOT-SP):
All fiber-wrapped tanks are marked with a specific DOT exception regulation on the label. Recently this was switched to special permit and marked with DOT-SP, but the same regulations apply. This regulation is a code that denotes information on the tank's testing status, depending on who manufacturered it.
These tanks are three-year:
E-07277 (manufacturered by SCI)
E-09634 (Luxfer)
E-12479 (Luxfer)
E-11005 (Carleton)
Here are a list of "extended" tanks. These tanks have a cutoff date for hydrotesting. If the tank was last tested before that date, it is still on the three-year hydrotest schedule. If the tank was last tested after the cutoff date, it will then be on the five-year hydrotest schedule.
E-10915 (Luxfer) - cutoff date 05-11-2001
E-10945 (SCI) - cutoff date 07-01-2001
E-11194 (Carleton) - cutoff date 07-01-2001
Carleton tanks are used by Crossfire, Pure Energy, Smart Parts, and others. Older Pure Energy tanks used Luxfer tanks. SCI tanks can be found on WGP, ACI Bulldog, some Crossfire, and others.
Tank hydrodates are used for a reason. Filling it after the expried date is dangerous and illegal. You must ALWAYS obey all tank hydrostatic regulations.
Several example labels are provided below...
Some sample retest label tags look like this:
Different testers use different style tags so yours may not look like these above.
When reinstalling a tank onto a reg, do not use loctite. The tank pressure is sealed using the 907/90 o-ring on the tank thread stem. Loctite is not necessary because the tank will be held in position by hundreds of pounds of force put on the threads. Using loctite only makes it more difficult to remove the tank in the future.
Links:
Hydrotesting info
DOT-E exceptions and special permit codes - information relating to testing dates, arranged per special permit.
Testing date information
Label information
Hydrotesting sites in your state
CO2 tank safety info from the CPSC
DOT-E 12695 recall