D1298_9 (12b)

Bring the sample to the test temperature, which shall be such that the sample is sufficiently fluid, but not so high as to cause the loss of light compounds, nor so low as to result in the appearance of waxy solids in the test portion. The density, relative density, or API gravity determined by the hydrometer is most accurate at or near the reference temperature. The volume and density, the relative density, and the API corrections in the volume correction tables are based on the average expansions of a number of typical materials. Since the same coefficients were used in compiling each set of tables, corrections made over the same temperature interval minimize errors arising from possible differences between the coefficient of the material under test and the standard coefficients. This effect becomes more important as temperatures diverge from the reference temperature. The hydrometer reading is obtained at a temperature appropriate to the physiochemical characteristics of the material under test. This temperature is preferably close to the reference temperature, or when the value is used in conjunction with bulk oil measurements within 3 degrees Celsius of the bulk temperature. See section 5.3. For crude petroleum, bring the sample close to the reference temperature, or, if wax is present, to 9 degrees Celsius above it's expected pour point, or 3 degrees Celsius above its expected cloud point, or WAT, whichever is higher. For crude petroleum, an indication of the WAT can be found using IP 389 with the modification of using 50 microliters plus or minus 5 microliters of sample. The precision of WAT for crude petroleum using this technique has not been determined. Bring the hydrometer cylinder and thermometer to within approximately 5 degrees Celsius of the test temperature. Transfer the sample to the clean, temperature-stabilized hydrometer cylinder without splashing to avoid the formation of air bubbles and minimize evaporation of the lower boiling point constituents of more volatile samples. Warning-- extremely flammable. Vapors may cause flash fire. Transfer highly volatile samples by siphoning or water displacement. Warning-- do not siphon by mouth, as it could result in ingestion of sample. Samples containing alcohol or other water soluble materials should be placed into the cylinder by siphoning. Remove any air bubbles formed after they have collected on the surface of the test portion by touching them with a piece of clean filter paper or other suitable means before inserting the hydrometer. Place the cylinder containing the test portion in a vertical position in a location free from air currents and where the temperature of the surrounding medium does not change more than 2 degrees Celsius during the time taken to complete the test. When the temperature of the test portion differs by more than 2 degrees Celsius from ambient, use a constant temperature bath to maintain an even temperature throughout the test duration. Insert the appropriate thermometer, or temperature measuring device, and stir the test portion with a stirring rod using a combination of vertical and rotational motions to ensure uniform temperature and density throughout the hydrometer cylinder. Record the temperature of the sample to the nearest 0.1 degree Celsius, and remove the thermometer or temperature measuring device and stirring rod from the hydrometer cylinder. If a liquid and glass thermometer is used, this is commonly used as the stirring rod. Lower the appropriate hydrometer into the liquid, and release when in a position of equilibrium, taking care to avoid wetting the stem above the level at which it floats freely. For low viscosity transparent or translucent liquids, observe the meniscus shape when the hydrometer is pressed below the point of equilibrium about 1 millimeter to 2 millimeters and allowed to return to equilibrium. If the meniscus changes, clean the hydrometer stem and repeat until the meniscus shape remains constant. For opaque viscous liquids, allow the hydrometer to settle slowly into the liquid. For low viscosity transparent or translucent liquids, depress the hydrometer about two scale divisions into the liquid and then release it, imparting a slight spin to the hydrometer on release to assist in bringing it to rest, floating freely from the walls of the hydrometer cylinder. Ensure that the remainder of the hydrometer stem, which is above the liquid level, is not wetted, as liquid on the stem affects the reading obtained. Allow sufficient time for the hydrometer to come to rest, and for all air bubbles to come to the surface. Remove any air bubbles before taking a reading. See section 9.2.12. If the hydrometer cylinder is made of plastic, dissipate any static charges by wiping the outside with a damp cloth. Static charges often build up on plastic cylinders and may prevent the hydrometer from floating freely. When the hydrometer has come to rest, floating freely away from the walls of the cylinder, read the hydrometer scale reading to the nearest 1/5 or 1/10 of a full scale division, depending upon the number of subdivisions, that is 5 or 10 respectively, that exist between a full scale division in accordance with section 9.2.12.1 or 9.2.12.2. For transparent liquids, record the hydrometer reading as 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. For opaque liquids, record the hydrometer reading at the point on the hydrometer scale to which the sample rises above its main surface by observing with the eyes slightly above the plane of the surface of the liquid. See figure 2. This reading requires a meniscus correction. This correction can be determined by one of two methods. One, observe the height above the main surface of the liquid to which the sample rises on the hydrometer scale when the hydrometer in question is immersed in a transparent liquid having a surface tension similar to that of a sample under test. Or two, uses the nominal value shown in Table 1. Document this value as the meniscus correction. When testing opaque liquids using a metal hydrometer cylinder, accurate readings of the hydrometer scale can only be ensured if the liquid's surface is within five millimeters of the top of the cylinder. Immediately after recording the hydrometer scale reading, carefully lift the hydrometer out of the liquid. Insert the thermometer or temperature measuring device, and stir the test portion vertically with the stirring rod. Record the temperature of the test portion to the nearest 0.1 degrees Celsius. If this temperature differs from the previous reading-- see section 9.2.6-- by more than 0.05 degrees Celsius, repeat the hydrometer observations and thermometer observations until the temperature becomes stable, within 0.05 degrees Celsius. If a stable temperature cannot be obtained, place the hydrometer cylinder in a constant temperature bath and repeat the procedure from section 9.1. If the test temperature is higher than 38 degrees Celsius, allow all hydrometers of the lead, shot, and wax type to drain and cool in a vertical position.