ASTM D5481 Standard Test Method for Measuring Apparent Viscosity at High-Temperature and High-Shear Rate by Multicell Capillary Viscometer
1. Scope
1.1 This test method covers the laboratory determination of high-temperature high-shear (HTHS) viscosity of engine oils at a temperature of 150°C using a multicell capillary viscometer containing pressure, temperature, and timing instrumentation. The shear rate for this test method corresponds to an apparent shear rate at the wall of 1.4 million reciprocal seconds (1.4 X 10(6)s(−1)). This shear rate has been found to decrease the discrepancy between this test method and other high-temperature high-shear test methods3 used for engine oil specifications. Viscosities are determined directly from calibrations that have been established with Newtonian oils with viscosities from 2 to 5 mPa•s at 150°C.

1.2 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:
D4683 Test Method for Measuring Apparent Viscosity at High Shear Rate and High Temperature by Tapered Bearing Simulator
D4741 Test Method for Measuring Viscosity at High Temperature and High Shear Rate by Tapered Plug Viscometer

3. Terminology
3.1 Definitions:
3.1.1 apparent shear rate at the wall - shear rate at the wall of the capillary calculated for a Newtonian fluid, as follows:
Sa = 4V/πR3t
where:
Sa = apparent shear rate at the wall, s(-1),
V = volume, mm3,
R = capillary radius, mm, and
t = measured flow time, s.
3.1.1.1 Discussion - The actual shear rate at the wall will differ for a non-Newtonian fluid.

3.1.2 apparent viscosity - the determined viscosity obtained by this test method.

3.1.3 density - mass per unit volume.
3.1.3.1 Discussion - In the SI, the unit of density is the kilogram per metre cubed (kg/m3); the gram per cubic centimetre (g/cm3) is often used. One kg/m3 is 10(−3) g/cm3.

3.1.4 kinematic viscosity - the ratio of the viscosity to the density of the fluid.
3.1.4.1 Discussion - Kinematic viscosity is a measure of a fluid's resistance to flow under the force of gravity. In the SI, the unit of kinematic viscosity is the metre squared per second (m2/s); for practical use, a submultiple (millimetre squared per second, mm2/s) is more convenient. The centistoke (cSt) is 1 mm2/s and is often used.

3.1.5 Newtonian oil or fluid - an oil or fluid that exhibits a constant viscosity at all shear rates or shear stresses.

3.1.6 non-Newtonian oil or fluid - an oil or fluid that exhibits a viscosity that varies with changing shear rate or shear stress.

3.1.7 shear rate - the spatial gradient of velocity in laminar flow; the derivative of velocity with respect to distance in a direction perpendicular to the direction of flow.
3.1.7.1 Discussion - The derived unit of shear rate is velocity divided by length. With the time in seconds and with consistent units of length, shear rate becomes reciprocal seconds, or s(−1).

3.1.8 shear stress - force per area of fluid in the direction of flow.
3.1.8.1 Discussion - In a capillary viscometer, the significant shear stress is the shear stress at the wall, that is, the total force acting on the cross section of the capillary divided by the area of the inside surface of the capillary. The shear stress at the wall does not depend on the fluid properties (that is, Newtonian or non-Newtonian). The SI unit for shear stress is the pascal (Pa). Mathematically, the shear stress at the wall of a capillary viscometer is as follows:
Z = PR/2L
where:
Z = shear stress, Pa,
P = pressure drop, Pa,
R = capillary radius, and
L = capillary length in consistent units.

3.1.9 viscosity - the ratio between shear stress and shear rate at the same location.
3.1.9.1 Discussion - Viscosity is sometimes called the coefficient of viscosity, or the dynamic viscosity. It is a measure of a fluid's resistance to flow. In the SI, the unit of viscosity is a pascal second (Pa•s); for practical use a submultiple (millipascal second, mPa•s) is more convenient. The centipoise (cP) is 1 mPa•s and is often used.

3.2 Definitions of Terms Specific to This Standard:
3.2.1 calibrations oils - those oils used for establishing the instrument's reference framework of apparent viscosity versus pressure drop from which the apparent viscosities of the test oils are determined.
3.2.1.1 Discussion - Calibration oils, which are Newtonian fluids, are available commercially or can be blended by the user.

3.2.2 test oil - any oil for which the apparent viscosity is to be determined by the test method.

3.2.3 viscometric cell - that part of the viscometer comprising all parts which may be wet by the test sample, including exit tube, working capillary, fill tube, pressure/exhaust connection, plug valve, and fill reservoir.