Round, 12-inch static pressure regulator damper (SPRD). Single-blade, steel barometric damper used to bypass excess air when majority of zone dampers are closed. Counter-balanced weighted arm to control bypass air for zoned systems. Air pressure in duct system increases as zone dampers close, pushing the SPRD open automatically. Arm and weight both adjustable to control amount of air bypassed. Recommended for low-pressure systems with less than 0.5-inch water column (wc). For larger systems and tighter pressure control, use MARD damper and static pressure controller (SPC). Additional weights available.