After confirming CV #1 is functioning properly, what should you do next?

Opening the High Control one full turn is the correct action to take after confirming that CV #1 is functioning properly because it ensures that the system maintains appropriate pressure and flow controls as intended. This step is crucial for the operation of a backflow prevention device, as it directly affects the system's ability to manage the pressure differential effectively.

By adjusting the High Control valve, you are allowing the system to respond to pressure changes and maintain proper flow rates. This helps to ensure that the entire backflow prevention assembly operates efficiently, preventing potential backflow incidents.

Other actions, such as closing all valves tightly, would not allow for necessary flow adjustments needed for the system to operate effectively. Turning off the pressure gauge might lead to a lack of pressure monitoring, which is essential for ensuring safety and functionality. Opening the outlet valve slightly could lead to improper pressure readings or system imbalances, making it a less effective choice in this scenario.