Abstract: A backfill prevention system for a gas flow conduit, comprising a gas flow monitor circuit which measures the rate and direction of gas flow through a gas flow conduit and converts the measured data into a voltage signal. A valve control circuit operably connected to a valve or valves in the gas flow conduit receives the voltage signal from the gas flow monitor circuit and closes the valve or valves in the event that the voltage signal indicates backflow of a gas through the gas flow conduit. The valve control conduit may further be provided with a first light emitting diode (LED) which is illuminated during normal flow of the gas through the conduit, and a second LED which is illuminated in the event of gas backflow through the conduit. The system is typically used in conjunction with a mass flow controller in the conduit.