The following is from ttb.gov
ATF Ruling 91-2
The Bureau of Alcohol, Tobacco and Firearms (ATF) has been asked whether Anton-Paar density meter DMA-58 may, in addition to Mettler-Paar density meters DMA-55, DMA-46, and DPR-2000, be used in lieu of a hydrometer and thermometer to determine the proof of distilled spirits.
By way of background, ATF Rul. 82-3, A.T.F.B. 82-1 15, authorized the use of the DMA-55 for determining the proof of high solid distilled spirits products which contained solids in excess of 600 milligrams per 100 milliliters at gauge proof. ATF Ruling 82-3 was superseded by ATF Rul. 85-12, A.T.F.B. 1985-3 44, which held that the DMA-55 could be used for all proof determinations, including tax determinations. ATF Ruling 85-12 also held that the DMA-46 could be used to determine the proof of all distilled spirits, but that the instrument was too unreliable to be used for tax determination purposes. ATF Ruling 85-12 was amplified by ATF Rul. 86-2, A.T.F.B. 1986-1 95, which authorized the use of the DPR-2000 in determining the proof of all distilled spirits for tax determination purposes.
27 C.F.R. 30.31 prescribes a number of procedures to be used in determining the proof of spirits. These procedures vary depending on the extent to which the proof is obscured due to the presence of interferences such as dissolved solids. All of the procedures described in section 30.31 utilize a hydrometer and thermometer, unless the Director authorizes or recognizes a different approved method to determine the proof of distilled spirits.
Anton-Paar submitted technical information regarding the specifications and performance data of the DMA-58. This information, which was evaluated by the ATF laboratory, demonstrates that the DMA-58 provides accuracy comparable to the hydrometer and thermometer method. As a result of these studies, ATF has concluded that the DMA-58 is reliable to the extent that it can be used in lieu of the hydrometer and thermometer to determine the proof of all distilled spirits for tax determination purposes.
Held, Anton-Paar density meter DMA-58, in addition to Mettler-Paar density meters DMA-55, DMA-46, and DPR-2000, is recognized as an approved method under 27 C.F.R. section 30.31 to determine the proof of distilled spirits whenever a proof determination is required, with the exception that the DMA-46 density meter may not be used to determine the proof of spirits for tax determination purposes.
Provided, these density meters are not approved for use in determining the proof of denatured spirits solutions in those instances where a hydrometer and thermometer do not provide an acceptable method of proof determination under the regulations.
Provided further, use of the density meters does not eliminate the need to add obscuration to the apparent proof determined by such meters when spirits contain more than 400 milligrams, but not in excess of 600 milligrams of solids per 100 milliliters at gauge proof (27 C.F.R. section 30.31().
Provided furthermore, that to make a true proof determination of spirits with such a high solids content, the spirits are first distilled in a small laboratory still and restored to the original volume and temperature by the addition of pure water to the distillate (27 C.F.R. section 30.31©).
ATF Rulings 85-12, A.T.F.B. 1985-3 44 and 86-2, A.T.F.B. 1986-1 95 are superseded.
27 CFR 30.31: (Also 30.32, 19.91, 19.383, 19.454, 19,517, and 19.537)