The Potter OSYSU-2 is used to monitor the open position of an OS&Y (outside screw and yoke) type gate valve. This device contains two sets of SPDT (Form C) contacts. These switches mount conveniently to most OS&Y valves ranging in size from 2" to 12" (50mm to 300mm). They will mount on some valves as small as 1/2" (12,5mm). The cover is held in place by two tamper resistant screws that require a special tool to remove. The tool is furnished with each device. More information can be found on the Potter Electrical Signal Company Website