Which of the following devices increases the safety of propane-filled cylinders specifically during filling?

The correct choice, overfilling prevention devices, plays a crucial role in enhancing the safety of propane-filled cylinders during the filling process. These devices are specifically designed to ensure that the cylinder is not filled beyond its safe capacity, preventing overfilling, which can lead to excessive pressure within the cylinder and increase the risk of leaks or ruptures.

Overfilling prevention devices often include mechanisms like float valves or devices that automatically shut off the flow of propane when the cylinder reaches the appropriate fill level. This is essential not only for regulatory compliance but also for ensuring safe handling and usage of propane.

While pressure gauges, leak detectors, and thermometers contribute to the overall safety and management of propane systems, they do not specifically address the concerns associated with filling cylinders. Pressure gauges help monitor existing pressures, leak detectors identify potential gas escape, and thermometers measure temperature. However, these functions do not prevent the act of overfilling itself, highlighting why overfilling prevention devices are critical in this scenario.