Abstract: A safety shut-off system especially adapted for propane delivery trucks and the like utilizes a pressure differential switch to immediately close the outlet valve of the cargo tank in the event that the differential between an initial reference pressure in the system and the current delivery pressure becomes high enough. When the delivery process is started, both sides of the pressure differential switch are simultaneously exposed to the delivery pressure so that the switch remains in a standby condition. As the delivery pressure becomes stabilized, a timer closes a trapping valve on the high pressure side of the pressure differential switch, trapping a quantity of fluid against the high pressure side of the switch to comprise a reference pressure.