This part of the standard specifies a method for determination of the ageing behaviour of hydraulic fluids of category HFC as defined in ISO 6743-4 and specified in ISO 12922. The ageing is accelerated by the presence of oxygen, water and metal catalysts at elevated temperature, and the degradation of the fluid is followed by changes in pH value and insolubles content. Other parts of the standard specify similar procedures for the determination of ageing behaviour of mineral oils and specified classes of fire-resistant fluids in hydraulic and other applications.