The O2 sensor is used in Bacharach portable combustion analyzers to measure the percentage content of oxygen levels in flue gases. The range of the oxygen sensor is 0 - 20.9%. O2 measurement is used in conjunction with the chosen fuel to calculate the CO2 levels in flue gases. Bacharach portable combustion analyzers only calculate CO2 when oxygen levels are below 16%. Bacharach uses O2 and other readings in determining efficiency rather than CO2 for a more accurate measurement. Measuring oxygen content in the excess air lets you know if you have too much air or too much fuel, allowing you to tune your furnace or boiler to maximum efficiency. Measuring O2 directly can save fuel and save time during a combustion tune-up test.