ISO 20844:2004 specifies a method to assess the resistance to shear stresses applied to mineral oils, synthetic oils and other fluids containing polymers, when passed through a specified diesel injector nozzle. The shear stability is measured by the change in viscosity of the fluid under test, brought about by the polymer degradation during stress. Under normal circumstances, ISO 20844:2004 is applied to hydraulic fluids of categories HR and HV as defined in ISO 6743-4 and specified in ISO 11158, but it may also be applied to fire-resistant hydraulic fluids within categories HFA, HFB, HFC and HFD, with modified conditions as specified in ISO 12922.
No formal correlation has been established between the viscosity loss, or the absence of viscosity loss, obtained using the procedures described in ISO 20844:2004 and that of oils and fluids in actual service. However, ISO 20844:2004 provides standardized conditions for the evaluation of polymer stability under minimized thermal and oxidative stresses. It is normally used by manufacturers of fluids and additives, and users, as a means of ranking existing and potential formulations.
Changes to properties other than viscosity are specified in some specifications, but these are not covered by the procedures specified in ISO 20844:2004.