This specification covers glass hydrometers of various scale graduation systems, as required by the ASTM test methods in which they are used. Hydrometers shall be of the constant-mass, variable-displacement type. Hydrometers shall be made of glass, except for the scale, ballasting material, and the thermometric liquid of thermohydrometers. Material used for ballast shall be secured to the lower part of the body, and no loose material of any sort may be inside a hydrometer. The stem shall be uniform in cross section, with no perceptible irregularities. The preferred shapes for the bodies of hydrometers are presented. Graduation lines and inscriptions shall be in a permanent black marking material, such as India ink. All hydrometers shall be graduated to read correctly where the plane of the level liquid surface intersects the stem. The thermometer shall be of the mercury-in-glass type, unless otherwise specified. Hydrometers shall be inspected, tested, and standardized in accordance with the requirements prescribed.