Marketing InformationN.F.P.A. 13, 13D and 13R requires the use of an expansion chamber on anti-freeze systems with backflow preventers. The TrimFit ET25 is a rubber bladder filled expansion chamber designed for use on anti-freeze based fire sprinkler systems. It has been tested and U.L. listed to be compatible with both propylene glycol and glycerin based anti-freeze. The expansion chamber regulates pressure caused by thermal expansion and contraction within the closed system.