The National Institute of Standards and Technology states that test measure and prover calibration may be established at 1-year intervals and extended or reduced on historical evidence up to the limit determined by your regulatory agency, or ISO regulations  but should not exceed 3 years due to possible leaking valves or seals which are often difficult to observe under field conditions.

 Vessels can only be tested if they are:

1)       Clean (no oil residues)

2)       Have no visible damage

3)       The drain valve (if applicable) is in good working order