Mass. Gen. Laws ch. 146 § 39
The air tank or other receptacle shall be prepared for internal inspection by the owner or user. The inspection plugs, handhole plates and manhole plates if provided shall be removed. The interior shall be cleaned of rust, scale, oil sludge and any other deleterious material. Ultrasonic thickness determination shall be permitted in lieu of, or in conjunction with, internal inspection for air tanks or other receptacles of 36 inches diameter or less. Thickness measurements shall be made in at least 8 areas: 2 on each head and 2 on both the top and bottom portions of the shell. Thickness determinations indicating significant reduction in material thickness over a general area shall be shown on the inspection report, as well as calculations for the reduction in allowable working pressure. The authorized inspector's employer shall be responsible for the inspector's or the ultrasonic examiner's competency in the use of the ultrasonic thickness gauge and the examiner's signed report shall be attached to the authorized inspector's inspection report. A hydrostatic test shall be applied if required by the authorized inspector. The pressure applied during the test shall be equal to one and one half times the pressure allowed on the air tank or other receptacle. A hammer test may also be applied if there is no pressure on the tank or receptacle.
Mass. Gen. Laws ch. 146, § 39