Keeping SF6 gas inside circuit-breaker tanks during the filling process is nearly impossible without using a selfsealing coupling. This special 3-way valve features a DN-20 tank fillport and a special regulation valve to allow leak-free checking of the Gas Density Monitor, and a self-sealing valve for the Gas Density Monitor. The valve is normally in the "open" position so that SF6 gas flows to the Gas Density Monitor. However, it can be moved to the "close" position using a special key to turn the valve. The exact position of the valve can easily be seen because the red arrow of the screw points toward the visual band where "open" or "close" is indicated. Once the screw reaches the first part of the "close" band, it is 100% closed. However, further tightening deeper into the "close" band is also possible.