For BPTUs, casings are made from 0.9mm galvanized steel sheet with circular inlet spigot and rectangular outlet connection and damper blades are made from 1.5mm 7 galvanized steel sheet. The damper blades are double-walled galvanized steel plate sandwiching a closed cell neoprene gasket lined with acoustic mineral insulation and black tissue facing which meets UL 181 standard. The damper blade has a precision die cast zinc alloy shaft which rotates in self-lubricating brass bushes which facilitates extremely low friction in operations.