27 CFR 30.21: Requirements.

(a) General. The proof of distilled spirits shall be determined by the use of gauging instruments as prescribed in this part.

(b) Proprietors. Proprietors shall use only accurate hydrometers and thermometers that show subdivisions or graduations of proof and temperature which are at least as delimitated as the instruments described in § 30.22.

(c) Appropriate TTB officers. Appropriate TTB officers shall use only hydrometers and thermometers furnished by the Government. However, where this part requires the use of a specific gravity hydrometer, TTB officers shall use precision grade specific gravity hydrometers conforming to the provisions of § 30.24, furnished by the proprietor. However, the appropriate TTB officer may authorize the use of other instruments approved by the appropriate TTB officer as being equally satisfactory for determination of specific gravity and for gauging. From time to time appropriate TTB officers shall verify the accuracy of hydrometers and thermometers used by proprietors.