[Code of Federal Regulations]
[Title 27, Volume 1]
[Revised as of April 1, 2005]
From the U.S. Government Printing Office via GPO Access
[CITE: 27CFR30.23]

[Page 810]
 
            TITLE 27--ALCOHOL, TOBACCO PRODUCTS AND FIREARMS
 
 CHAPTER I--ALCOHOL AND TOBACCO TAX AND TRADE BUREAU, DEPARTMENT OF THE 
                                TREASURY
 
PART 30_GAUGING MANUAL--Table of Contents
 
                      Subpart C_Gauging Instruments
 
Sec. 30.23  Use of precision hydrometers and thermometers.

    Care should be exercised to obtain accurate hydrometer and 
thermometer readings. In order to accomplish this result, the following 
precautions should be observed. Bulk spirits should be thoroughly 
agitated so that the test samples will be representative of the entire 
quantity. The hydrometers should be kept clean and free of any oily 
substance. Immediately before readings are taken, the glass cylinder 
containing the thermometer should be rinsed several times with the 
spirits which are to be gauged so as to bring both the cylinder and the 
thermometer to the temperature of the spirits (if time permits, it is 
desirable to bring both the spirits and the instruments to room 
temperature). If the outer surface of the cylinder becomes wet, it 
should be wiped dry to avoid the cooling effect of rapid evaporation. 
During the readings the cylinder should be protected from drafts or 
other conditions which might affect its temperature or that of the 
spirits which it contains. The hands should not be placed on the 
cylinder in such a manner as to warm the liquid contained therein. The 
hydrometer should be inserted in the liquid and the hydrometer bulb 
raised and lowered from top to bottom 5 or 6 times to obtain an even 
temperature distribution over its surface, and, while the hydrometer 
bulb remains in the liquid, the stem should be dried and the hydrometer 
allowed to come to rest without wetting more than a few tenths degrees 
of the exposed stem. Special care should be taken to ascertain the exact 
point at which the level of the surface liquid intersects the scale of 
proof in the stem of the hydrometer. The hydrometer and thermometer 
should be immediately read, as nearly simultaneously as possible. In 
reading the hydrometer, a sighting should be made slightly below the 
plane of the surface of the liquid and the line of sight should then be 
raised slowly, being kept perpendicular to the hydrometer stem, until 
the appearance of the surface changes from an ellipse to a straight 
line. The point where this line intersects the hydrometer scale is the 
correct reading of the hydrometer. When the correct readings of the 
hydrometer and the thermometer have been determined, the true percent of 
proof shall be ascertained from Table 1. Another sample of the spirits 
should then be taken and be tested in the same manner so as to verify 
the proof originally ascertained. Hydrometer readings should be made to 
the nearest 0.05 degree and thermometer readings should be made to the 
nearest 0.1 degree, and instrument correction factors, if any, should be 
applied. It is necessary to interpolate in Table 1 for fractional 
hydrometer and thermometer readings.

    Example. A hydrometer reads 192.85[deg] at 72.10 [deg]F. The 
correction factors for the hydrometer and the thermometer, respectively 
are minus 0.03[deg] and plus 0.05[deg]. The corrected reading, then, is 
192.82[deg] at 72.15 [deg]F.

From Table 1:
193.0[deg] at 72.0 [deg]F.                                   =  190.2[de
                                                                      g]
192.0[deg] at 72.0 [deg]F.                                   =  189.1[de
                                                                      g]
                                                           -----
    Difference                                               =  1.1[deg]
192.0[deg] at 72.0 [deg]F.                                   =  189.1[de
                                                                      g]
192.0[deg] at 73.0 [deg]F.                                   =  188.9[de
                                                                      g]
                                                           -----
    Difference                                               =  0.2[deg]


    The hydrometer difference (1.1[deg]) multiplied by the fractional 
degree of the hydrometer reading (0.82[deg])=0.902.
    The temperature difference (0.2[deg]) multiplied by the fractional 
degree of the temperature reading (0.15[deg])=0.03[deg].
Proof at 60[deg] F.=189.1+0.902-0.03=189.972[deg]=190.0[deg].
    As shown, the final proof is rounded to the nearest tenth of a 
degree of proof. In such cases, if the hundredths decimal is less than 
five, it will be dropped; if it is five or over, a unit will be added.

(Sec. 201, Pub. L. 85-859, 72 Stat. 1358, as amended (26 U.S.C. 5204))

[T.D. ATF-198, 50 FR 8535, Mar. 1, 1985, as amended by T.D. ATF-381, 61 
FR 37004, July 16, 1996]