While training in Halo while in service,I used this device on all my jumps,and although I pulled at altitude OK, I could always hear it go off almost immediatly after I pulled.They were set to go off at a given altitude.One of the students in my class never pulled his ripchord,he always let the FXC do it for him.(not recommended) I never did see or hear of one malfunctioning. We had them on our mains.