Combustion analyzers and flue gas analyzers measure the gas content of flue gas in order to monitor the combustion efficiency of fuel-burning equipment. Combustion efficiency indicates how well fuel-burning equipment burns a fuel such as coal, oil, propane, and natural gas. Combustion efficiency calculations assume complete fuel stoichiometric combustion and account for fuel chemistry, the net temperature of stack gases, and the percentage of oxygen or total combustibles by volume after the combustion process. Combustion analyzers and flue gas analyzers measure these stack gases, which contain water vapor when hot and liquid condensate when cooled. Stack gas consists mainly of nitrogen, with oxygen, carbon dioxide and trace total combustible gases, and they should be measured before dilution occurs.