"API Manual of Petroleum Measurement Standards”

“It is recognized that tanks do change with time and service and that volume changes may not be readily identified by visual inspection or the above verification procedures. Because of that, it is considered justifiable practice to re-calibrate tanks on a periodic basis to reassure good measurement accuracy. A re-calibration at 15 year intervals for tanks in custody transfer service and 15 - 20 year interval for others, is reasonable”.