On one hand, the insulating filler is filled between the electric heating wire and the metal tube shell. The filler has better thermal insulation performance under the condition of dry and dense. However, they all have different degrees of hygroscopicity. The exposed filler material at both ends of the heating tube will greatly reduce the electrical insulation performance once it is absorbed by moisture. When the end of the heating tube is not sealed, it has a great influence of the insulation performance.
On the other hand, when magnesium oxide is absorbed by moisture, a part of magnesium oxide (especially the part with poor or imperfect crystalline state) will have hydration. Magnesium oxide that is hydrated will increase its molecular size and agglomerate. Then the electric heating tube cannot recover the insulation when it is energized.
The first point is that insulation turns poor when wet, the second point is that hydration affects the recovery of the insulation resistance of the heating tube. For these two points, the electric heating tube needs to be sealed at the port.