ASTM D7110 Standard Test Method for Determining the Viscosity-Temperature Relationship of Used and Soot-Containing Engine Oils at Low Temperatures
4. Summary of Test Method
4.1 Used and sooted engine oils are analyzed using a special rotational viscometer with analog or digital output to a computer program. A specially made glass stator/metal rotor cell is attached to the viscometer and subjected to a programmed temperature change for both calibration and sample analysis. Following calibration of the rotor-stator set, an approximately 20 mL test sample of a test lubricating oil is poured into the stator and preheated for 1.5 h to 2.0 h at 90 °C in an oven or water bath. Shortly after completing the preheating step, the room-temperature rotor is put into the stator containing the heated oil and coupled to a torque-sensing viscometer head using an adapter to automatically center the rotor in the stator during test. A programmable low-temperature bath is used to cool the cell at a specified rate of 3 °C/h from -5 °C to the temperature at which the maximum torque recordable is exceeded when using a speed of 0.3 r/min for the rotor. After the desired information has been collected, the computer program generates the desired viscometric and rheological values from the recorded data.
5. Significance and Use
5.1 Significance of Low Temperature, Low Shear Rate, Engine Oil Rheology - The low-temperature, low-shear viscometric behavior of an engine oil, whether new, used, or sooted, determines whether the oil will flow to the sump inlet screen, then to the oil pump, then to the sites in the engine requiring lubrication in sufficient quantity to prevent engine damage immediately or ultimately after cold temperature starting. Two forms of flow problems have been identified, flow-limited and air-binding behavior. The first form of flow restriction, flow-limited behavior, is associated with the oil's viscosity; the second, air-binding behavior, is associated with gelation.
5.2 Significance of the Test Method - The temperature-scanning technique employed by this test method was designed to determine the susceptibility of the engine oil to flow-limited and air-binding response to slow cooling conditions by providing continuous information on the rheological condition of the oil over the temperature range of use. In this way, both viscometric and gelation response are obtained in one test.
NOTE 1 - This test method is one of three related to pumpability related problems. Measurement of low-temperature viscosity by the two other pumpability test methods, D3829 and D4684, hold the sample in a quiescent state and generate the apparent viscosity of the sample at shear rates ranging up to 15 s(-1) and shear stresses up to 525 Pa at a previously selected temperature. Such difference in test parameters (shear rate, shear stress, sample motion, temperature scanning, and so forth) can lead to differences in the measured apparent viscosity among these methods with some test oils, particularly when other rheological factors associated with gelation are present. In addition, the three methods differ considerably in cooling rates.
5.3 Gelation Index and Gelation Index Temperature - This test method has been further developed to yield parameters called the Gelation Index and Gelation Index Temperature. The first parameter is a measure of the maximum rate of torque increase caused by the rheological response of the oil as the oil is cooled slowly. The second parameter is the temperature at which the Gelation Index occurs.
