46 CFR § 39.2003 - Cargo gauging system—TB/ALL.
(a) A cargo tank of the tank vessel connected to a vapor collection system must be equipped with a permanent or portable cargo gauging device that—
(1) Is a closed type as defined in 46 CFR 151.15.10(c) that does not require opening the tank to the atmosphere during cargo transfer;
(2) Allows the operator to determine the level of liquid in the tank for the full range of liquid levels in the tank;
(3) Has an indicator for the level of liquid in the tank that is located where cargo transfer is controlled; and
(4) If portable, is installed on the tank during the entire transfer operation.
(b) Each cargo tank of a tank barge must have a high-level indicating device, unless the barge complies with 46 CFR 39.2009(a). The high-level indicating device must—
(1) Indicate visually the level of liquid in the cargo tank when the liquid level is within a range of 1 meter (3.28 feet) of the top of the tank;
(2) Show a permanent mark to indicate the maximum liquid level permitted under 46 CFR 39.3001(e) at even keel conditions; and
(3) Be visible from all cargo control areas.