Because when the weighing bottle is taken out of the drying oven, the hot dry coal sample has extremely strong moisture absorption. When the temperature drops sharply, moist air is sucked in due to the slight negative pressure generated in the weighing bottle, causing the dried coal sample to gain weight and the moisture measurement result to be lower. Therefore, it is stipulated that the weighing bottle should be immediately capped when taken out of the drying oven, placed in a dryer and cooled to room temperature before weighing (about 20 minutes).