ASTM D4624 Measuring Apparent Viscosity by Capillary Viscometer
ASTM D4624 Standard Test Method for Measuring Apparent Viscosity by Capillary Viscometer at High-Temperature and High-Shear Rates
4. Summary of Test Method
4.1 Viscosity is determined from measurements of the relationship between pressure drop and flow rate through a capillary under the desired conditions. Two types of capillary viscometers are recognized by this test method:
4.1.1 Constant Pressure Viscometer, in which the pressure is set to a desired shear stress and the resulting flow rate is measured.
4.1.2 Constant Flow-Rate Viscometer, in which the flow rate is set to give a desired shear rate and the resulting pressure drop through the capillary is measured.
4.2 For viscometers of both types, the capillary is first calibrated by establishing the relationship between viscosity and flow rate or between viscosity and pressure drop for a series of Newtonian oils by the same procedure and under the same conditions used for test oils.
4.3 This test method determines the viscosity at 150°C and a shear rate of 10(6)s(-1). For constant pressure viscometers, the viscosity at 10(6)s(-1) is determined by interpolation from viscosities measured at several shear stresses. Constant flow-rate viscometers usually can be set for a flow rate that will be at or very close to a shear rate of 10(6)s(-1). Operating procedures are highly dependent on the design of each particular viscometer.
5. Significance and Use
5.1 Many equipment manufacturers and lubricant specifications require a minimum high-temperature high-shear viscosity at 150°C and 10(6)s(-1). The apparent shear rate at the wall for this test method is 10(6)s(-1). For non-Newtonian oils, the shear rate at the wall will vary from this value. For oils tested so far, this test method has shown higher results for non-Newtonian oils than have rotational viscometer methods (Test Methods D4683 and D4741) run at 10(6)s(-1). The difference has been on the order of 0.1 mPa•s. Experiments by the Subcommittee 7 high-temperature high-shear working group suggest the flow rate in this test method should be increased by 50 % for the results to correspond with the rotational methods.
5.2 The conditions of shear rate and temperature of this test method are thought to be representative of those in the bearings of automotive engines in severe service.
5.3 This test method was evaluated in an ASTM cooperative program.
