GB/T 1540-2002 "Paper and board—Determination of water absorption (Cobb method)" specifies the method for determining the water absorption of paper and paperboard using the Cobb method, applicable to various types of paper and paperboard. The specimen is placed horizontally and in contact with absorbent paper, and the water absorption is calculated by measuring the amount of water absorbed by the specimen within a specified time. Parameters such as specimen dimensions, testing time, and contact pressure are clearly defined, providing an intuitive reflection of the water absorption performance of paper under contact water absorption conditions. This serves as a reference for applications such as packaging and printing.