2. Move the tray containing the coal sample to a pre adjusted temperature drying oven, control the temperature to not exceed 50 ℃, weigh once an hour, and wait until the coal sample quality changes by no more than 0.1% after continuous drying for 1 hour, reaching the air drying state. 3. Remove the coal sample from the drying oven, cool it slightly, and then put it into a coal sample bottle. The amount of coal sample loaded should not exceed 3/4 of the volume of the coal sample bottle, so that it can be mixed during use.