Abstract: Apparatus is provided for continually monitoring the flash point temperature of a liquid pertoleum product flowing through a pipeline. The apparatus is located adjacent the pipeline for periodically drawing product samples therefrom and for testing the samples to determine the respective flash point temperatures. The apparatus includes a microprocessor, which is programmed to control the operation of the apparatus, a testing chamber and a Wheatstone bridge circuit for measuring the Lower Explosive Limit (LEL) of a combustible gas-air mixture which is drawn through the testing chamber. The gas-air mixture ignites on the surface of an active resistive filament, thereby increasing its temperature and resistance, which causes a current to flow within the magnitude bridge circuit. The magnitude of the current is directly proportional to the change of resistance of the active filament and the LEL of the vapor-air mixture.