ASTM D6822 for Density, Relative Density and API Gravity of Petroleum Products
ASTM D6822 Standard Test Method for Density, Relative Density, and API Gravity of Crude Petroleum and Liquid Petroleum Products by Thermohydrometer Method
7. Procedure
7.1 Effect of Test Temperature:
7.1.1 The density or API gravity determined by the thermohydrometer method is most accurate at or near the reference temperature of 15°C or 60°F. Other temperatures within the range of the enclosed thermometer may be used, if consistent with the type of sample and the necessary limiting conditions shown in Table 3.
7.1.2 If the test temperature is significantly different from the reference temperature of 15°C or 60°F, the expansion or contraction of the glass may affect the calibration of the thermohydrometer. A hydrometer correction factor (HYC) may be applied to the measured density value to provide a corrected reading. Historically, the following equations have been used within volume correction factor tables.
ρcorrected = ρtest x HYC
where:
HYC = hydrometer thermal correction factor,
ρcorrected = test density, corrected for the effects of temperature on the thermohydrometer, and
ρtest = test density, as observed by reading the thermohydrometer.
HYC = 1 - 0.00001278(t - 60) - 0.0000000062(t - 60)2
where:
t = observed temperature (°F) or
HYC = 1 - 0.000023(t - 15) - 0.00000002(t - 15)2
where:
t = observed temperature (°C).
7.1.2.1 All parties affected by the measurement should agree upon the need for and the method of correction.
7.1.3 When the thermohydrometer value is used to select factors for correcting volumes to standard temperatures, the thermohydrometer reading preferably should be made at a temperature within more or less 3°C (more or less 5°F) of the temperature at which the bulk volume of the oil was measured (see Note 1). However, when appreciable amounts of light fractions may be lost during determination at the bulk oil temperature, the limits given in Table 3 shall be applied.
NOTE 1 - Volume and density correction tables are based on average expansion for a number of typical materials. Since the same coefficients were used in computing both sets of tables, corrections made over the same temperature interval minimize errors arising from possible differences between the coefficients of the material under test and the standard coefficients. This effect becomes more important as temperatures diverge significantly from 15°C or 60°F.
7.2 Density Measurement:
7.2.1 Adjust the temperature of the sample in accordance with Table 3. For field testing, test temperatures other than those listed in Table 3 may be used, however, accuracy may be sacrificed. The hydrometer cylinder shall be at approximately the same temperature as the sample to be tested.
7.2.2 Transfer the sample into the clean hydrometer cylinder without splashing, so as to avoid the formation of air bubbles and to reduce, to a minimum, the evaporation of the lower boiling constituents of the more volatile samples (Warning - Extremely flammable. Vapors may cause a flash fire). For the more volatile samples, transfer to the hydrometer cylinder by siphoning (Warning - Siphoning by mouth could result in ingestion of sample). Use a rubber aspirator bulb to siphon the more volatile samples. Remove any air bubbles formed, after they have collected on the surface of the sample, by touching them with a piece of clean absorbent paper before inserting the thermohydrometer. For field testing, the thermohydrometer may be inserted directly into a sampling thief. Place the cylinder containing the sample in a vertical position in a location free from air currents. Take precautions to prevent the temperature of the sample from changing appreciably during the time necessary to complete the test.
7.2.2.1 During this period, the temperature of the surrounding medium should not change more than 3°C (5°F).
7.2.3 Lower and raise the thermohydrometer no more than two scale divisions in the sample cylinder to minimize vapor loss and in such a manner that the stem will not be wetted higher than the approximate floating position.
7.2.3.1 Keep the rest of the stem dry, as unnecessary liquid on the stem changes the effective weight of the instrument, and so affects the reading obtained.
7.2.3.2 Gently lower the thermohydrometer into the center of the hydrometer cylinder. When the thermohydrometer has settled, ensure it is not resting on the bottom of the cylinder by depressing it about two scale divisions into the liquid. Give the thermohydrometer a slight spin, allowing it to float freely away from the walls of the hydrometer cylinder.
7.2.3.3 Allow enough time for the thermohydrometer to come to rest, all air bubbles to come to the surface, and the thermohydrometer temperature to stabilize, usually 3 to 5 min. This is particularly necessary in the case of more viscous samples. Use a temperature bath if control of the sample temperature is required.
7.2.4 Read the thermohydrometer to the nearest scale division (see 7.2.7 for details). The correct reading is that point on the thermohydrometer scale at which the surface of the liquid cuts the scale. To make a reading for transparent liquids in a transparent hydrometer cylinder, determine this point 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 thermohydrometer scale. See Fig. 2 for details on reading the meniscus.
7.2.5 To make a reading with opaque liquids, observe the point on the thermohydrometer scale to which the sample rises above its main surface, placing the eye slightly above the plane surface of the liquid. This reading requires meniscus correction (see Note 2). Determine this correction for the particular thermohydrometer in use by observing the height above the main surface of the liquid to which the sample rises on the thermohydrometer scale when the thermohydrometer in question is immersed in a transparent liquid having a surface tension similar to that of a sample under test. Refer to Fig. 3 for details of reading the meniscus.
NOTE 2 - When determination of the actual meniscus correction is not practical, industry practice has been to add 0.5 kg/m3 to the density reading or to subtract 0.1°API from the API reading.
7.2.6 Read and record the thermohydrometer scale reading with meniscus correction to the nearest 0.5 kg/m3 or 0.1°API and the thermometer reading to the nearest 0.5°C (1.0°F).
7.2.7 It may be difficult to ensure that the temperature of the thermohydrometer and liquid has stabilized. To provide this assurance, two successive determinations of density or gravity may be made with the same liquid and each determination corrected to 15°C (60°F). The two successive corrected values should be within 0.5 kg/m3 or 0.1°API to be acceptable. If this repeatability cannot be obtained, the temperature may not have stabilized or loss of light hydrocarbons may be occurring.
7.2.8 Gradually withdraw and wipe the thermohydrometer to expose the thermometer scale until a reading can be made. Always make sure the thermometer bulb remains in the liquid when reading the temperature. Read the temperature to the nearest 0.5°C (1.0°F).
