1. Scope
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 43 or 45 h to a final test temperature of -20 or -25°C. The viscosity measurements are made at a shear stress of 525 Pa over a shear rate of 0.4 to 15 s(-1). This test method is suitable for measurement of viscosities ranging from 4000 mPa·s to >400 000 mPa·s, and is suitable for yield stress measurements of 7 Pa to >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 This test method uses the millipascal second (mPa·s) as the unit of viscosity. For information, the equivalent centipoise unit is shown in parentheses.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
D3829 Test Method for Predicting the Borderline Pumping Temperature of Engine Oil
D4684 Test Method for Determination of Yield Stress and Apparent Viscosity of Engine Oils at Low Temperature
D5133 Test Method for Low Temperature, Low Shear Rate, Viscosity/Temperature Dependence of Lubricating Oils Using a Temperature-Scanning Technique
3. Terminology
3.1 Definitions:
3.1.1 apparent viscosity - the determined viscosity obtained by use of this test method.
3.1.2 Newtonian oil or fluid - an oil or fluid that at a given temperature exhibits a constant viscosity at all shear rates or shear stresses.
3.1.3 non-Newtonian oil or fluid - an oil or fluid that at a given temperature exhibits a viscosity that varies with changing shear stress or shear rate.
3.1.4 shear rate - the velocity gradient in fluid flow. For a Newtonian fluid in a concentric cylinder rotary viscometer in which the shear stress is measured at the inner cylinder surface (such as the apparatus described in 6.1), and ignoring any end effects, the shear rate is given as follows:
where:
Gr = shear rate at the surface of the rotor in reciprocal seconds, s(-1),
Ω = angular velocity, rad/s,
Rs = stator radius, mm,
Rr = rotor radius, mm, and
t = time for one revolution of the rotor, s.
For the specific apparatus described in 6.1,
Gr = 63/t
3.1.5 shear stress - the motivating force per unit area for fluid flow. For the rotary viscometer being described, the rotor surface is the area under shear or the shear area.
where:
Tr = torque applied to rotor, N·m,
M = applied mass, g,
Ro = radius of the shaft, mm,
Rt = radius of the string, mm,
Sr = shear stress at the rotor surface, Pa, and
h = height of the rotor, mm.
For the dimensions given in 6.1.1,
3.1.6 viscosity - the ratio between the applied shear stress and rate of shear, sometimes called the coefficient of dynamic viscosity. This value is thus a measure of the resistance to flow of the liquid. The SI unit of viscosity is the pascal second Pa·s. A centipoise (cP) is one millipascal second mPa·s.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 calibration oils - those oils that establish the instrument's reference framework of apparent viscosity versus speed, from which the apparent viscosities of test oils are determined. Calibration oils, which are essentially Newtonian fluids, are available commercially and have an approximate viscosity of 30 Pa·s (30 000 cP) at -20°C.
3.2.2 test oil - any oil for which the apparent viscosity and yield stress are to be determined by this test method.
3.2.3 used oil - an oil which has been used in an operating engine.
3.2.4 yield stress - the shear stress required to initiate flow.
3.2.4.1 Discussion - For all Newtonian fluids and some non-Newtonian fluids, the yield stress is zero. An oil can have a yield stress that is a function of its low-temperature cooling rate, soak time, and temperature. Yield stress measurement by this test method determines only whether the test oil has a yield stress of at least 35 Pa; a yield stress below 35 Pa is considered to be insignificant for engine oils.