What is the correct procedure for entering high-pressure systems?

The correct procedure for entering high-pressure systems emphasizes the importance of controlling the flow and pressure differential within the system to ensure safety. Engaging both the first and second valves upstream and downstream is crucial because it allows for a staged approach to pressure management. This method helps in isolating the section of the system you're about to enter, significantly reducing the risk of sudden pressure changes that could lead to hazardous situations.

By actuating the first valve, you can begin to control the pressure entering the work area. The second valve acts as a secondary protection measure, providing an extra layer of security to manage flow and maintain a stable environment for the diver entering the system. This procedure not only helps in maintaining safety but also facilitates better control over the conditions within the high-pressure environment.

The other choices do not incorporate the necessary safety measures for dealing with high-pressure systems. Simply entering via the last valve or only using the main valve could lead to unexpected pressure surges or unsafe conditions. Opening all valves indiscriminately poses a high risk, as it can create an uncontrolled atmosphere. Thus, engaging both the first and second valves is the most effective and responsible procedure when working with high-pressure systems.