27 CFR 19.183: Scale tanks.

(a) Except as otherwise provided in paragraph (b) of this section, if the proprietor uses a tank to determine the distilled spirits tax imposed by 26 U.S.C. 5001, the tank must be mounted on scales and the contents of the tank must be determined by weight. The scale tank also must be equipped with a suitable device so that the volume of the contents can be quickly and accurately determined.

(b) The requirement to mount tanks on scales does not apply to tanks having a capacity of 55 gallons or less. Such tanks may be moved onto an accurately calibrated scale when a tax determination gauge needs to be made.