All monitoring devices must be able to trigger an alarm or automated response when actual measurements vary by 10% or more in either direction from the design set points. The signal or alarm is most often relayed to a building management system that balances supply and return air volumes, monitors and controls minimum fresh air volumes, and provides a reliable reference point for commissioning of VAV systems. The alarm can be audible or visual and be relayed to building facility staff or directly to the occupants, to alert them to open windows.