ASTM E126-13a Inspection, Calibration, and Verification of ASTM Hydrometers

ASTM E126-13a Inspection, Calibration, and Verification of ASTM Hydrometers

The purpose of this ASTM E126-13a test method is to establish a common method by which manufacturers, calibration laboratories, and users of hydrometers are used to inspect, verify, or calibrate them. The objective is to provide a standard method that is simple, easily understood, and to produce reliable results.


    ASTM E126-13a test method describes the principles, apparatus, and procedures for the inspection, calibration, and verification of ASTM glass hydrometers. This test method applies to ASTM hydrometers used for other general hydrometers of the constant-mass, variable-displacement type.

    Test Procedure:

    The hydrometer (Figure 1) is carefully inspected for; there are no cracks, fissures, deep scratches, rough areas, or other obvious damage to the glass. A hydrometer is rejected if any of these defects are present. The hydrometer is checked for strain in the glass, especially at the stem/body junction with the use of a polariscope. A hydrometer is rejected if any of these defects are present. It is particularly important for thermo hydrometers.

    The paper scale is inspected within the hydrometer stem. The paper scale is straight and without a twist. The linear dimensions and diameters are checked for compliance with specification requirements by comparing the hydrometer with the appropriate device.

    A hydrometer is calibrated at a minimum of three calibration points, spaced approximately

    Equally across its range, nominally high, low, and mid-scale. For example, a hydrometer with a range of 9° to 21° API shall be calibrated at (approximately) 10, 15, and 20 API.

    Figure 1: A hydrometer

    Specimen size:



    The interval between graduations of density and relative density (specific gravity) hydrometers is smaller near the bottom of the scale. The proper spacing shall be obtained from the following formula:

    l = L × d2/d× (d – d1)/ (d2 – d1)


    l = distance from the top line to any line, d, between the top and the bottom,

    L = distance between the top and the bottom graduations of the scale,

    d2 = density value, or relative density (specific gravity), of the bottom line, and

    d1 = density value, or relative density (specific gravity), of the top line.


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