Automatic dampers control airflow in HVAC systems. Applications include controlling mixed air temperature in economizer cycles and airflow to the space in variable air volume systems. Selecting and sizing dampers to provide linear control action aids proper system operation. With linear control, a given change in damper position produces a proportional change in air quantity. If control is not linear, a given change in control signal might produce a consistent change in damper position but a different change in air quantity. The result is unstable or inaccurate control.