Constructed of galvanized steel sheet 0.9 mm . Thick , for frame and jambs , and galvanized steel 0.55 thick for blades , high strength structre . Blades will be opened when air flows to same direction as opening of blades by air pressure , and eill be closed when air not flows to reverse direction by gravity . So instllation must always by vertical ( upright ) . One type ( GD ) . Applications for mounting to duct line , building sidewall or shaft for control air flow to only one direction , preventing air flow reversal automatically ( same as check valve )