An overfilling prevention device (OPD) on a cylinder is a __________ means to determine when a cylinder is properly filled.

An overfilling prevention device (OPD) is classified as a secondary means to determine when a cylinder is properly filled. The primary method for checking the fill level is typically based on weight, which involves measuring the weight of the filled cylinder and comparing it to the cylinder's specifications. This ensures that the cylinder is not filled beyond its rated capacity.

The OPD serves as an additional safety feature that prevents overfilling, particularly when the primary method may not be practical or could be overlooked during filling operations. While the OPD is a critical safety component, it is designed to function alongside primary measurement methods rather than replace them. Thus, it acts as an auxiliary layer of protection to ensure safety during the refilling process, which is why it is correctly identified as a secondary means of determining proper cylinder fill levels.

Other types of measures, such as quantitative means, typically refer to numerical value assessments rather than safety devices that inherently function as a backup to direct measurements.