ASTM D5621 Sonic Shear Stability of Hydraulic Fluids
When a hydraulic pump is operated, the hydraulic fluid experiences varying temperatures, pressures and other stresses. This may cause changes in the fluid's viscosity, potentially reducing the effectiveness and safe operating ranges. Polymers are added to improve the viscosity index of these fluids. This test determines the percentage of viscosity changes in polymer-containing fluids exposed to sonic shearing vibrations.
The initial viscosity is determined. The sample is then placed in the test beaker, brought to the test temperature and irradiated in a sonic oscillator for the test time. The viscosity of the irradiated sample is taken. The report lists the initial viscosity, the final viscosity and the percent viscosity change in centistokes.