Industrial applications in which these sensors may be used include: air-to-fuel ratio (such as the mass ratio of air to fuel present during combustion) in applications such as fired heaters, power plant steam generators, and large gas-fired turbines; gas leak detection for gas meters; HVAC filter maintenance, crucial to keeping HVAC ductwork clean; analytical instrumentation which includes equipment used in the analysis of solid materials, gases or chemical compounds in many industrial, scientific, environmental and security applications; fuel cells that chemically-generate energy and require controlled amounts of air or gas to optimize operation; gas meters used to measure the volume of natural gas and propane used at residential, commercial and industrial buildings that consume fuel gas; and VAV (variable air volume) systems that vary the amount of conditioned air in different parts of a building to meet heating and cooling needs.