In terms of tank filling procedures, what does a positive shut off device do?

A positive shut off device is designed to prevent overfilling a tank during the filling process. This device automatically stops the flow of liquid when it reaches a predetermined level, ensuring that the tank does not exceed its capacity and reducing the risk of spills and hazardous situations. This is particularly important in environments where fuel, chemicals, or other liquids are stored, as overfilling can lead to contamination, environmental damage, and safety hazards.

The other options focus on aspects of tank management that, while important, are not the primary function of a positive shut off device. Leak detection pertains to identifying issues with the integrity of the tank, vapor emission limits relate to controlling the release of gases during the filling process, and installation assistance does not connect directly to the operational purpose of preventing overfill. Thus, the correct focus on preventing overfilling highlights the essential safety functionality that a positive shut off device provides.