Hermetic coaxial connectors are connectors that are created as a means of carrying microwave signals into a tank or other item. This type of connector uses coax cables to create a plug of sorts that helps to prevent any gasses or liquids from escaping around the outside of the connection.

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This type of connector can be used with glass, metal, resin and more and helps to create a solid and sealed connection from the outside of one area, into the area. These can also be used with liquids, gasses, or air and can be placed in the sides of tanks to which you might need to gain access while it is in use or while it is pressurized without losing pressure or without opening the tank.

This is a connector in which one part of the connector is outside the tank or container and the other is inside the tank with the space between sealed fully to prevent any air or other substances from getting out. 

A hermetic seal means that it is airtight and that no air or gas can pass through. A hermetic coaxial connector is one that may need to be specially built to fit the purpose that it is going to be used for.