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The hazards from SF6 are located in 2 locations:
The accelerator tank is filled with SF6 at 480 to 650 kPa (70 to 90 PSI). There is a blower, some filters, some dryers, and some piping connected to the tank to keep the SF6 circulated, clean, and dry. It is possible for any of this piping or the tank itself to leak SF6. Since SF6 is significantly heavier than air, it will flow to the lowest point, and gather there, until mixing with the air and dissipating. The pit under the accelerator is the lowest point in the lab. The O2 monitor can be seen from the top of the stairs leading into the pit.
The storage tank and compressor for the SF6 are located in the loading dock. The amount of SF6 stored in the tank is generally much less than what is stored in the accelerator tank. When the tank is open to atmosphere, all of the SF6 is in the storage tank on the dock.