reference gas with a known mixture of the gases in question and then through the "detector" chambers containing the pure forms of the gases in question. When a "detector" chamber absorbs some of the infrared radiation, it heats up and expands. This causes a rise in pressure within the sealed vessel that can be detected either with a pressure transducer or with a similar device. The combination of output voltages from the detector chambers from the sample gas can then be compared to the output voltages from the reference chamber. The latest Infrared Gas Analyzers Like earlier infrared gas analyzers, modern analyzers also use nondispersive infrared technology to detect a certain gas by detecting the absorption of infrared wavelengths that is characteristic of that gas. Infrared energy is emitted from a heated filament. By optically filtering the energy, the radiation spectrum is limited to the absorption band of the gas being measured. A detector measures the energy after the infrared energy has passed through the gas to be measured. This is compared to the energy at reference condition of no absorption. Many analyzers are wall-mounted devices intended for long-term, unattended gas monitoring. There are now analysers that measure a range of gases and are highly portable to be suitable for a wider range of geoscience applications. Fast response high-precision analyzers are widely used to measure gas emissions and ecosystem fluxes using eddy covariance method when used together with fast-response sonic anemometer. In some analyzers, the reliability of measurements is enhanced by calibrating the analyzer at the reference condition and a known span concentration. If the air would interfere with measurements, the chamber that houses the energy source is filled with a gas that has no detectable concentration of the gas being measured. Depending on the gas being measured, fresh air, chemically stripped air or nitrogen may be used. See also Nondispersive infrared sensor Eddy covariance References Auble, D.L.; Meyers, T.P. (1992). "An open path, fast response infrared absorption gas analyzer for H2O and CO2". Boundary-Layer Meteorology 59(3):243–256. Measuring instruments