Construced of galvanized steel sheet 0.9 mm. thick , for frame , jambs and blades , high strength structure , blades can be adjusted manually from full open to full close in opposed pattern for high effictiveness in air volume control performance . Lockable crank arm for manually adjusting the blades , and scale plate for indicating the blade position settings , are also completely equipped , one type (VD). Applications for mounting to duct line , building sidewall or shaft for control volume of supplied , exhausted , fresh or pressurized air manually .