ASTM D4684 standard test method for determination of yield stress and apparent viscosity of engine oils at low temperature
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 exceeding 45 h to a final test temperature between -10 and -40°C. The viscosity measurements are made at a shear stress of 525 Pa over a shear rate of 0.4 to 15 s(-1).
1.2 This test method is applicable for unused oils, sometimes referred to as fresh oils, designed for both light duty and heavy duty engine applications. It also has been shown to be suitable for used diesel oils. The applicability 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. Terminology
2.1 Definitions:
2.1.1 apparent viscosity - the determined viscosity obtained by use of this test method.
2.1.2 Newtonian oil or fluid - an oil or fluid that at a given temperature exhibits a constant viscosity at all shear rates or shear stresses.
2.1.3 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.
2.1.4 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 this apparatus, described in 5.1), 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),
V = angular velocity, rad/s,
Rs = stator radius, mm,
Rr = rotor radius, mm, and
t = time in seconds for one revolution of the rotor.
For the specific apparatus being described in 5.1.1,
Gr = 63/t
2.1.5 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 5.1.1,
2.1.6 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].
2.2 Definitions of Terms Specific to This Standard:
2.2.1 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.
2.2.2 test oil - any oil for which the apparent viscosity and yield stress are to be determined by this test method.
2.2.3 unused oil - an oil which has not been used in an operating engine.
2.2.4 used oil - an oil which has been used in an operating engine.
2.2.5 yield stress - the shear stress required to initiate flow. For all Newtonian fluids and some non-Newtonian fluids, the yield stress is zero. An engine 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.