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Pressure inside the drums lowers as the temperature falls causing the outside pressure to push in. Seal when colder or if you need to do it when it's warmer add a LITTLE dry ice before you seal it. If it is 100 degrees I calculate you should add ~10.4 oz of dry ice. As the CO2 warms up it returns to its gaseous state and increases its pressure. just after you add the dry ice and seal it, stay back for 30 minutes or so to make sure it wont explode.