ISO 3675 Crude petroleum and liquid petroleum products - Laboratory determination of density - Hydrometer method
10 Procedure
10.1 Transfer a portion of the sample at test temperature (see 7.2.1 and note 2 in 7.2.1) to the clean hydrometer cylinder (5.1) at a stabilized temperature, without splashing, avoiding the entrainment of air bubbles, and reducing to a minimum the evaporation of the lower boiling constituents of volatile materials.
10.2 Remove any bubbles that have collected on the surface of the sample, by touching them with a piece of clean filter paper.
10.3 Place the cylinder containing the sample in a vertical position in a location free from air currents and where the temperature of the surrounding medium will not change by more than 2 °C during the time taken to carry out the test. When testing at temperatures differing by more than +/- 2 °C from ambient, a constant-temperature bath (5.3) shall be used to avoid excessive temperature changes (see 10.13).
10.4 Stir the test portion with the appropriate thermometer (see table 2), or, if using a resistance thermometer, with a stirring rod (5.5), using a combination of vertical and rotational motions, to ensure uniform temperature and density throughout the cylinder. Record the temperature to the nearest 0.1 °C. Remove the thermometer, and if appropriate the stirring rod, from the hydrometer cylinder.
10.5 Lower the appropriate hydrometer (5.2) into the liquid and release it when in a position of equilibrium, taking care to avoid wetting the stem above the level at which it floats freely. Observe the meniscus shape when the hydrometer is pressed below the point of equilibrium by 1 mm or 2 mm and allowed to return to equilibrium. If the meniscus shape changes, clean the hydrometer stem. Repeat this procedure until the meniscus shape remains constant.
10.6 For opaque viscous liquids, allow the hydrometer to settle slowly into the liquid.
10.7 For transparent low viscosity liquids, depress the hydrometer approximately two scale divisions into the liquid and release it. The remainder of the stem of the hydrometer, which is above the level of the liquid, shall be kept dry, since unnecessary liquid on the stem affects the reading obtained.
10.8 Impart a slight spin to the hydrometer on releasing it, to assist in bringing it to rest floating freely away from the walls of the cylinder. Allow sufficient time for the hydrometer to come to rest and for any bubbles that form to come to the surface (see 10.2). Remove any bubbles from the hydrometer before taking a reading.
10.9 When using a cylinder made from plastics, dissipate any static charge by wiping the outside of the cylinder with a damp cloth.
NOTE - Static charges often build up when using such cylinders and may prevent the hydrometer from floating freely.
10.10 When the hydrometer has come to rest, floating freely away from the walls of the cylinder, read the hydrometer scale to the nearest one fifth of a scale division, in accordance with 10.11 or 10.12.
10.11 For transparent liquids, record the hydrometer reading at the point on the hydrometer scale at which the principal surface of the liquid cuts the scale by placing the eye slightly below the level of the liquid and slowly raising it until the surface, first seen as a distorted ellipse, appears to become a straight line cutting the hydrometer scale, (see figure 1).
10.12 For opaque liquids, record the hydrometer reading at the point on the hydrometer scale to which the sample rises by observing with the eye slightly above the plane of the surface of the liquid, (see figure 2).
NOTE - When testing completely opaque samples using metal hydrometer cylinders, accurate reading of the hydrometer can only be assured if the level of the sample is within 5 mm of the top of the cylinder.
10.13 Immediately after recording the hydrometer scale reading, carefully lift the hydrometer out of the liquid and stir the sample vertically with the thermometer. Record the temperature of the test portion to the nearest 0.1 °C. If this temperature differs from the reading taken at the start of the test by more than 0.5 °C, repeat the hydrometer observations, and then the thermometer observations, until the temperature becomes stable within +/- 0.5 °C. If a stable temperature cannot be obtained, place the hydrometer cylinder and its contents in a constant temperature bath and repeat the procedure from 10.3.
10.14 If the test temperature is greater than 38 °C, allow all hydrometers, of the lead shot in wax type, to drain and cool in a vertical position.
