ISO 12185 Crude petroleum and petroleum products - Determination of density - Oscillating U-tube method
11 Test procedure
11.1 Check that the density-meter reading when the cell is filled with ambient air is within +/- 1 of the least significant digit compared to the reference value achieved during calibration (10.2). If it is not, reclean and dry the cell and repeat the check. If the reading still differs recalibrate the density meter.
11.2 Introduce the test portion of the sample into the cell using a suitable syringe or autosampler, filling the cell according to the manufacturer's instructions.
When testing waxy distillates or waxy crude oils or residual fuels oils, warm the syringe or autosampler to a temperature 3 °C above the cloud point or WAT, or 20 °C above the pour point of the material being tested.
11.3 When using an autosampler, either run samples in duplicate or introduce check samples, in order that errors due to bubble formation may be detected and the system performance monitored.
11.4 Do not apply suction to samples prone to light-end loss at any stage. Either the sample shall be poured into the syringe and then injected into the cell, or the autosampler shall be one which uses vial pressure to force the sample into the instrument.
11.5 When making a manual injection, switch on the cell illumination before injecting, check the cell for bubbles and fill in accordance with the manufacturer's instructions. If bubbles are detected, empty and refill the cell and recheck for bubbles.
Turn off the illumination immediately after examination of the cell, as heat generated by the light affects cell temperature.
11.6 When the density meter displays a reading steady to within 0.1 kg/m3 for density, or to five significant figures for the oscillation period, note and record the indicated figure and the cell temperature to the nearest 0.1 °C.
NOTE 11 A consistent drift in the oscillation period or density reading normally indicates that the cell has not reached equilibrium temperature.
Random variations in readings normally indicate that air or gas bubbles are present in the cell. In this case, the cell should be recharged with a fresh sample. If the variations in reading are due to bubbles, it may be necessary to conduct the test at a lower temperature to ensure that the sample remains in a single phase.
If large water droplets are present in the sample due to inefficient mixing prior to sample introduction, the indicated density or oscillating period will tend to be erratic.
11.7 When measuring the density of viscous liquids, it is sometimes possible to obtain a stable reading even when gas or air bubbles are present. With these liquids, a slight overpressure shall be applied to the cell once the first density has been read, and the density redetermined. If the liquid is in single phase, the shift in density due to the additional pressure will be minimal. If, on the other hand, gas or air bubbles are present, a much larger shift in density will be observed as they are compressed. In such cases the cell shall be emptied and refilled with a fresh sample.
11.8 If the sample contains finely suspended water droplets, the density shall be observed as soon as thermal equilibrium has been reached.
NOTE 12 If samples containing finely suspended water droplets are left in the cell for a lengthy period, the water droplets will slowly coalesce and migrate to the cell antinodes, giving an apparent change in density.
11.9 Clean and dry the cell in accordance with the procedure given in 9.2.
12 Calculation
12.1 If the density meter displays an oscillation period, calculate sample density from the observed cell oscillation period in accordance with the manufacturer's instructions.
12.2 If the density is required at a reference temperature other than that at which it was determined, convert the density to the density at reference temperature using either ISO 91-1 for 15 °C or ISO 91-2 for 20 °C, after making an adjustment for the hydrometer correction (see annex A).
NOTE 13 ISO 91-1 and ISO 91-2 refer the user to the IP/ASTM/API Petroleum Measurement Tables and the IP Petroleum Measurement Paper Nos. 2 and 3.
