A pressure regulator is a device that:

A pressure regulator is specifically designed to reduce the pressure of gas from the inlet to a desired outlet level. This is crucial in many applications where maintaining a consistent and safe pressure is necessary for operational efficiency and safety. The role of a pressure regulator is to adjust the incoming higher pressure to a controlled, lower pressure that can be safely used in a system or process. It ensures that the equipment downstream operates optimally without the risk of over-pressurization, which could lead to system failures or safety hazards.

In this context, the other options focus on functions that are not characteristic of a standard pressure regulator. For instance, while some devices measure gas flow, that is not the fundamental function of a pressure regulator. Similarly, a regulator does not increase gas pressure; rather, it serves to reduce it. Lastly, while maintaining pressure can be part of the functionality of certain types of regulators, the primary purpose remains the reduction of pressure from the inlet to the outlet. Therefore, the correct answer reflects the essential role of a pressure regulator in managing gas pressure safely and effectively.