What is damper authority and how is it calculated?

Damper authority is the ratio of pressure drop across the fully open damper to the total system pressure drop at design airflow, analogous to valve authority in hydronic systems. An authority of 0.3 or higher is recommended for stable modulating control; below 0.1, the damper has almost no control effect until nearly closed. The calculation is: Authority = ΔP_damper_full_open / (ΔP_damper_full_open + ΔP_rest_of_system). Opposed-blade dampers maintain better authority characteristics than parallel-blade dampers at partial opening due to their more linear inherent characteristic.