"Hydro" is short for Hydrostatic Testing. After so many years (listed on each tank) a propellant tank must be retested for leaks and material strength. If it fails, it is condemned and cannot be refilled, if it passes, you may use it until the next test date, which will be engraved onto the bottle. Hydro testing can be done by either physically going to and ordering, or mailing your tank to the manufacturer.
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