GB/T 3781.9-2025 is the ninth part of the acetylene carbon black series of standards, which specifies the determination method of resistivity. The standard specifies the test principle, required instruments, operation steps and result calculation and representation, and is suitable for the resistivity detection of all kinds of acetylene carbon black. The standard can unify the detection method and ensure the accuracy of the data. It provides a reliable technical basis for the production quality control of acetylene carbon black, the evaluation of electrical conductivity and the formulation design of conductive products such as rubber and plastics.