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KEEP THIS MANUAL WITH THE S-2A. Introduction The S-2A vapor-monitoring system utilizes advanced technology for detection of propane (LPG) fuel vapors as well as CNG (Compressed Natural Gas). The S-2A allows the connection of two (2) sensors and has both visual and audible alarms. Test switches are provided for full checking of all operations. System operation and correct sensor operation are continually monitored and indicators are provided for fault detection. The S-2A has an internal relay for