Flue Gas Recirculation Systems are used to measure temperature, capacitance, frequency and duty cycle measurements. It reduces nitrogen oxides (NOx) emissions (up to 60 percent) in industrial boilers by recirculating a portion of the boiler flue gas (up to 20 percent) into the main combustion chamber. It also reduces the peak combustion temperature and lowers the percentage of oxygen in the combustion air/flue gas mixture.