ASTM D7042 Test Method for Dynamic Viscosity and Density of Liquids by Stabinger Viscometer (and the Calculation of Kinematic Viscosity)
6. Apparatus
6.1 Stabinger Viscometer
6.1.1 Viscosity Measurement - The Stabinger viscometer uses a rotational coaxial cylinder measuring system. The outer cylinder (tube) is driven by a motor at a constant and known rotational speed. The low-density inner cylinder (rotor) is held in the axis of rotation by the centrifugal forces of the higher density sample and in its longitudinal position by the magnet and the soft iron ring. Consequently, the system works free of bearing friction as found in rotational viscometers. A permanent magnet in the inner cylinder induces eddy currents in the surrounding copper casing. The rotational speed of the inner cylinder establishes itself as the result of the equilibrium between the driving torque of the viscous forces and the retarding eddy current torque. This rotational speed is measured by an electronic system (Hall effect sensor) by counting the frequency of the rotating magnetic field (see Fig. 1 and Fig. 2, No. 2).

6.1.2 Density Measurement - The digital density analyzer uses a U-shaped oscillating sample tube and a system for electronic excitation and frequency counting (see Fig. 2, No. 3).

6.1.3 Temperature Control - The copper block surrounds both the viscosity and the density measuring cell in a way that both cells are held at the same temperature. A thermoelectric heating and cooling system (see Fig. 2, No. 1) ensures the temperature stability of the copper block within +/- 0.005 °C from the set temperature at the position of the viscosity cell over the whole temperature range. The uncertainty (k = 2; 95 % confidence level) of the temperature calibration shall be no more than +/- 0.03 °C over the range from 15 °C to 100 °C. Outside this range the calibration uncertainty shall be no more than +/- 0.05 °C.

6.1.4 The thermal equilibration time depends on the heat capacity and conductivity of the liquid and on the difference between injection temperature and test temperature. Adequate temperature equilibration of the test specimen is automatically determined when successive viscosity values are constant within +/- 0.07 % over 1 min and successive density values are constant within +/- 0.00003 g/cm3 over 60 s.

NOTE 1 - The Stabinger Viscometer, manufactured by Anton Paar GmbH, fulfills the stated requirements when operated in the most precise mode of operation.

6.2 Syringes, commercially available, at least 5 mL in volume, with a Luer tip. All construction materials for syringes shall be fully compatible with all sample liquids and cleaning agents, which contact them.

6.3 Flow-Through or Pressure Adapter, for use as an alternative means of introducing the test specimen into the measuring cells either by pressure or by suction, provided that sufficient care and control is used to avoid any bubble formation in the test specimen. All construction materials for adaptors shall be fully compatible with all sample liquids and cleaning agents, which contact them.

6.4 Hot Filling Adapter, for use with manual syringe filling for the purpose of preventing the precipitation of waxy components dissolved in sample and lowering sample viscosity for easier sample introduction and cleaning routines.

6.5 Autosampler, for use in automated injection analyses. The autosampler shall be designed to ensure the integrity ofthe test specimen prior to and during the analysis and be equipped to transfer a representative portion of test specimen into the measuring cells. The autosampler shall transfer the test specimen from the sample vial to the measuring cells of the apparatus without interfering with the integrity of the test specimen. The autosampler shall be able to mimic the procedure for sample handling as set forth in 11.1 and 11.2. The autosampler may have heating capability as a means to prevent the precipitation of waxy components dissolved in the sample and lower the viscosity of the sample for filling the measuring cells.

6.6 Screen, with an aperture of 75 µm, to remove particles from the sample.

6.7 Magnet, strong enough to remove iron fillings from the sample. Magnetic stirring rods are suitable.

6.8 Ultrasonic Bath, Unheated (optional), with an operating frequency between 25 kHz to 60 kHz and a typical power output of ≤100 W, of suitable dimensions to hold container(s) placed inside of bath, for use in effectively dissipating and removing air or gas bubbles that can be entrained in viscous sample types prior to analysis. It is permissible to use ultrasonic baths with operating frequencies and power outputs outside this range, however it is the responsibility of the laboratory to conduct a data comparison study to confirm that results determined with and without the use of such ultrasonic baths does not materially impact results.

7. Reagents and Materials
7.1 Sample Solvent, completely miscible with the sample.
7.1.1 For most samples, a volatile petroleum spirit or naphtha is suitable. If the solvent dries up without residues in an applicable time frame, the use of a separate drying solvent is not required.

7.1.2 For residual fuels, a prewash with an aromatic solvent such as toluene or xylene may be necessary to remove asphaltic material.

7.2 Drying Solvent, a volatile solvent miscible with the sample solvent (see 7.1).
7.2.1 Highly concentrated ethanol (96 % or higher) is suitable.

7.3 Dry Air or Nitrogen, for blowing the measuring cells.
7.3.1 If the measuring cell temperature is below or near the dew point temperature of the ambient air, the use of an appropriate desiccator is required.

8. Sampling, Test Specimens, and Test Units
8.1 General Considerations and Guidelines:
8.1.1 Sampling is defined as all the steps required to obtain an aliquot of the contents of any pipe, tank, or other system, and to place the sample into the laboratory test container. The laboratory test container and sample volume shall be of sufficient capacity to mix the sample and obtain a homogeneous sample for analysis.

8.1.2 For some sample types, such as viscous lube oils that are prone to having entrained air or gas bubbles present in the sample, the use of an ultrasonic bath (see 6.8) without the heater turned on (if so equipped), has been found effective in dissipating bubbles typically within 1 min.

8.1.3 Particles - For samples that are likely to contain particles (for example, used oils or crude oils) pass the sample through a 75 µm screen to remove the particles. For the removal of iron filings the use of a magnet is appropriate. Waxy samples must be heated to dissolve the wax crystals prior to filtration and a preheated filter shall be used.

8.1.4 Test Specimen - A portion or volume of sample obtained from the laboratory sample and delivered to the measuring cells. The test specimen is obtained as follows:
8.1.4.1 Mix the sample, if required, to homogenize. Mixing at room temperature in an open container can result in the loss of volatile material; mixing in closed, pressurized containers, or at sub-ambient temperatures is recommended.

8.1.4.2 Draw the test specimen from a properly mixed laboratory sample using an appropriate syringe. Alternatively, if the proper attachments and connecting tubes are used, the test specimen may be delivered directly to the measuring cells using a flow through or pressure adapter (see 6.3) or autosampler (see 6.5) from the mixing container. For waxy or other samples with a high pour point, before drawing the test specimen, heat the laboratory sample to the desired test temperature, which has to be high enough to dissolve the wax crystals.

8.2 Instructions for Residual Fuel Oils:
8.2.1 (Warning - Exercise care as vigorous boil-over can occur when opaque liquids which contain high levels of water are heated to high temperatures. Wear appropriate personal protective equipment for handling hot materials.) Place the required number of disposable syringes or samples vials to be used for the batch analysis into a sample preheat apparatus (such as an oven, bath, or heating block) held between 60 °C and 65 °C. If using manual syringe filling, a hot filling adapter must be installed on the apparatus and the injection adapters must also be pre-warmed along with the syringes.

8.2.2 Place the first batch of residual fuel samples to be analyzed for the day in their original containers in a sample preheat apparatus that is between 60 °C and 65 °C for 1 h. Ensure the cap on each container is tightly closed. For samples of a very waxy nature or oils of high kinematic viscosity, it may be necessary to increase the heating temperature above 60 °C to achieve proper mixing. The sample should be sufficiently fluid for ease of stirring and shaking.

8.2.3 Vigorously stir each sample for approximately 20 s with a glass or steel rod of sufficient length to reach the bottom of the container.

8.2.4 Remove the stirring rod and inspect for sludge or wax adhering to the rod. If there is sludge or wax adhering to the rod, continue stirring until the sample is homogene.

8.2.5 Recap each container tightly and shake vigorously for 1 min. Then loosen the cap, retighten to finger tight, then back off 3/4 to a full turn and place back into the sample preheat apparatus.

8.2.6 Upon completion of 8.2.5 for all samples in the batch, increase the sample pre-heat apparatus temperature to between 100 °C and 105 °C and heat for 30 min.

8.2.7 Remove each container from the sample pre-heat apparatus, close tightly, and shake vigorously for 60 s.

8.2.8 If a heated auto sampler is used, follow instructions in 8.2.8.1 below. If a sample handler is used, follow instructions in 8.2.8.2 below.
8.2.8.1 Ensure the sample vial magazine is held at a temperature between 60 °C and 80 °C. Load each sample into its own preheated sample vial from 8.2.1, insert the vials into the sample vial magazine, and wait for 10 min to 15 min before commencing measurement.

8.2.8.2 Load a preheated sample vial from 8.2.1 with the first sample to be tested in the batch and place the sample vial into the sampler handler that is held between 60 °C and 80 °C. Loosen the cap of the other containers, retighten to finger tight, then back off 3/4 to full turn and place the containers back into the sample preheat apparatus that is reset to hold temperature between 60 °C and 80 °C.

8.2.9 Complete the measurements according to 11.1 or 11.3. Analysis of all samples in the batch must be completed within 1 h from completion of 8.2.7.