27 C.F.R. § 19.188 – Measuring devices and proofing instruments.

(a) General. A proprietor of a distilled spirits plant must have accurate instruments and equipment at the plant for determining the proof and volume of spirits.
(b) Instruments. The hydrometers and thermometers that a proprietor uses to gauge spirits must show subdivisions or graduations of proof and temperature as specified in part 30 of this chapter. Proprietors must frequently test their hydrometers and thermometers to ensure their accuracy. If an instrument appears to be in error, the proprietor may not use the instrument until it is tested and certified as accurate by the manufacturer or another qualified person.
(c) Meters. A proprietor may use an accurate mass flow meter to measure the volume of bulk spirits. A mass flow meter used for tax determination of bulk spirits must be certified by the manufacturer or other qualified person as accurate within a tolerance of plus or minus 0.1 percent. A mass flow meter used for all other required gauges of bulk spirits must be certified by the manufacturer or other qualified person as accurate within a tolerance of plus or minus 0.5 percent. The proprietor must make corrections for the temperature of the spirits being measured in conjunction with the volumetric measurement of spirits by mass flow meter. The proprietor must also test mass flow meters at least every 6 months to ensure that they are accurate within the required tolerances.
(26 U.S.C. 5204)