to its own weight the door latch 13 as shown in fig6 a enters into the door lock and holds onto the step of the front panel 2 at the same time the circular hole on the door lock spring 18 coincides with the hole on the door lock casing 17 allowing the stainless steel pin 21 to descend and pass through the guide hole 23 on the door lock casing 17 and accurately into the door lock spring 18 as shown in fig8 the floating pontoon 25 rises to its highest point with the rise of the water level in the floating pontoon cabinet 24 while the stainless steel pin 21 descends at the same time to its lowest point as shown in fig6 b until it is entirely in the door lock casing 17 and the door lock spring 18 and firmly fixed to them at this moment with the door lock spring 18 firmly fixed the door latch 13 can only hold onto the step of the front panel 2 as there is no room for its backward movement therefore the door cannot be opened and the safety function of the door lock is activated as water continues to flow in and fill the entire floating pontoon casing 24 and then the drum cabin 34 the water level continues to rise the accurate positioning of the heterotypic silicon rubber seal 10 enables its hollow round portion to press against the internal door cover 11 and the front panel 2 effecting thereby an air tight sealing of the drum cabin 34 a ventilation tube 36 on top of the drum cabin 34 ensures the pressure inside the drum cabin.