GB/T 461.3-2005 "Paper and board - Determination of water absorptiveness (Immersion method)" specifies the method for determining the water absorptiveness of paper and board using the immersion method, which is applicable to all types of paper and board. The test specimen is vertically immersed in distilled water, and the amount of water absorbed by the specimen within a specified time is recorded to calculate the water absorption per unit area or the height of water absorption. The standard defines the specimen dimensions, immersion temperature, test duration, and result calculation method, reflecting the water absorption performance of paper and board under static immersion conditions.