What device indicates that an LP gas tank is full during filling?

The correct choice is the option that specifies a fill gauge or shut-off valve on the tank. During the process of filling an LP gas tank, a fill gauge is specifically designed to provide a visual indication of the gas levels within the tank. As the LP gas fills the tank, the gauge will show when the tank reaches capacity.

Moreover, the shut-off valve serves an important function as it is typically designed to automatically close when the tank is full to prevent overfilling, which can lead to venting and other safety hazards. Knowing when a tank is full is crucial for safety and operational efficiency, making the fill gauge and shut-off valve essential components during the filling process.

In contrast, while other options might provide certain information or be relevant in different contexts, they do not specifically indicate when an LP gas tank is full while being filled. A pressure relief valve, for instance, primarily serves to relieve excess pressure, while a digital meter measures flow rates or total volume but does not typically indicate fullness directly during filling. A weight scale can determine the overall weight of the tank but does not provide a simple visual cue to the operator regarding the fullness level during the filling process.