This standard specifies the test method for determining the maximum pore size of filter paper and board using the "bubble point method." The core principle is that when gradually increasing air pressure is applied to one side of a sample saturated with a wetting agent, gas will first displace the liquid from the largest interconnected pores in the sample, forming bubbles. The pressure at this moment (i.e., the "bubble point pressure") can be converted using a specific physical formula to derive the maximum pore size. The test must be performed using a dedicated automatic maximum pore size analyzer (i.e., a capillary flow porometer or bubble point tester). This method is primarily applicable to filter paper and filter boards with a thickness ranging from 0.10 mm to 3.50 mm that can be fully wetted by the wetting agent.