A new air volume damper was just added to the EffectiV HVAC product line, the SKC-C Constant Air Volume Damper for Circular Duct.
SKC-C Constant Air Volume Damper For Circular Duct greatly facilitates the balancing of ventilation systems. It maintains constant air volume at varying pressures caused by connection and disconnection of system parts, clogging of filters and ducts, wind effects, window opening, etc.
Available for 4″, 5″, 6″, 8″, 10″, 12″ and 16″ round ducts.
See the product page for more information: http://effectiv-hvac.com/product/skc-c/