What method is NOT allowed for supervising open valves on a backflow prevention device?

The method of supervision for open valves on a backflow prevention device is critical for ensuring the proper functioning and safety of the system. The correct answer indicates that visual supervision is not an acceptable method.

Visual supervision may not provide the level of assurance needed to verify that valves are functioning correctly over time, as it relies heavily on human observation and can lead to inconsistencies based on visibility and the observer's attention. In contrast, methods like sealing and locking provide a physical barrier to prevent unauthorized access or tampering with the valves. Monitoring, often through electronic systems, allows for real-time assessment and alerts in the event of valve discrepancies.

Therefore, the reliance on visual checks alone does not meet the rigorous standards needed for the oversight of backflow prevention devices, highlighting the importance of more reliable methods that generate certainty in system integrity.