27 CFR § 19.709 - Gauging.
(a) Gauging equipment and methods. A proprietor of an alcohol fuel plant must perform periodic gauges of the distilled spirits and fuel alcohol at the alcohol fuel plant. The procedures for the gauging of spirits set forth in part 30 of this chapter also apply under this subpart. In addition, the following rules for the gauging of distilled spirits and fuel alcohol under this subpart also apply:
(4) To determine quantity by volume, the proprietor may use a tank or receptacle with a calibrated sight glass installed, a calibrated dipstick, conversion charts, an accurate mass flow meter, or other devices approved by the appropriate TTB officer;
(b) Verification by TTB. TTB officers may at any time verify the accuracy of the gauging equipment used.
(1) Upon completing the production of distilled spirits;
(2) On the receipt of spirits at the plant;
(3) Prior to the addition of materials to render the spirits unfit for beverage use;
(4) Before withdrawal from plant premises or other disposition of spirits (including fuel alcohol); and
(5) When spirits are inventoried.