Which factor must be monitored during cylinder filling according to safety regulations?

Monitoring the cylinder liquid level during the filling process is essential for safety regulations. It ensures that the cylinder is not overfilled, which can lead to dangerous situations such as excessive pressure build-up due to the expansion of the liquid when the temperature increases. Proper filling practices dictate that cylinders should be filled only to the specified liquid level, which usually means leaving an appropriate space at the top of the cylinder to accommodate this expansion.

While cylinder temperature, weight, and manufacturing date are important factors in various contexts, they do not directly impact the immediate safety of the filling process as significantly as monitoring the liquid level. Temperature is relevant as it can affect pressure, but it does not dictate how much liquid should be in the cylinder at the time of filling. Cylinder weight can ensure the correct amount of propane is delivered but is not the primary concern during the actual filling process. The manufacturing date is important for determining the life and safety of the cylinder but does not affect the filling itself. Hence, monitoring the cylinder liquid level is crucial for maintaining safety regulations during the filling process.