5.1 When an engine oil is cooled, the rate and duration of cooling can affect its yield stress and viscosity. In this laboratory test, used engine oil is slowly cooled through a temperature range where wax crystallization is known to occur, followed by relatively rapid cooling to the final test temperature. As in other low temperature rheological tests such as Test Methods D3829, D4684, and D5133, a preheating condition is required to ensure that all residual waxes are solubilized in the oil prior to the cooldown (that is, remove thermal memory). However, it is also known that highly sooted used diesel engine oils can experience a soot agglomerization phenomenon when heated under quiescent conditions. The current method uses a separate preheat and agitation step to break up any soot agglomerization that may have occurred prior to cooldown. The viscosity of highly sooted diesel engine oils as measured in this test method have been correlated to pressurization times in a motored engine test (1).4
5.2 Cooling Profiles:
5.2.1 For oils to be tested at –208201;°C and –258201;°C, Table X1.1 applies. The cooling profile described in Table X1.1 is based on the viscosity properties of the ASTM Pumpability Reference Oils (PRO). This series of oils includes oils with normal low-temperature flow properties and oils that have been associated with low-temperature pumpability problems (2-7).
1.1 This test method covers the measurement of the yield stress and viscosity of engine oils after cooling at controlled rates over a period of 438201;h or 45 h to a final test temperature of –208201;°C or –258201;°C. The precision is stated for test temperatures –208201;°C and –258201;°C. The viscosity measurements are made at a shear stress of 525 Pa over a shear rate of 0.48201;s-1 to 15 s-1. This test method is suitable for measurement of viscosities ranging from 4000 mPa·s to gt;4008201;000 mPa·s, and is suitable for yield stress measurements of 7 Pa to gt;350 Pa.
1.2 This test method is applicable for used diesel oils. The applicability and precision to other used or unused engine oils or to petroleum products other than engine oils has not been determined.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3.1 Exception—This test method uses the SI based unit of milliPascal second (mPa·s) for viscosity which is equivalent to centiPoise (cP).
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