In-situ gas analyzer

A gas analyzer which utilizes an infrared beam which passes through or has emanated from a gas stream. The analyzer has a plurality of gas cells which contain gas of the type whose concentration is being measured in various concentrations and at various total pressures. A beam of energy is forwarded to the analyzer, and is passed selectively to the various cells. In one embodiment, the cells are mounted where a rotatable deflector can selectively deflect and receive the beam to and from two fixed beam segments so that sequential readings at an infrared detector can provide measurements of concentrations, and calibration. In another embodiment, the cells are selectively moved into the beam path. The beam is formed and forwarded by a Cassegrainean system or by a combination of reflectors including a retro-reflector. An infrared calibration beam which does not pass through the gas stream is utilized for calibration.