The 821S has ten identical capillaries and a precision balance regulator. The span gas can be selected to flow through a number of those capillaries with the diluent flowing through the remainder, to produce a ratio. Many types of gas analyser are inherently non-linear due to the physical nature of the analysing technique. Photometric analysers (NDIR, NDUV, FTIR / spectrophotometers) need to be linearised before they leave the factory and then at regular intervals during service. Other more linear analysers will also need to be checked for linearity periodically whilst in service. This should be part of the QA/QC regime. Also known as a gas blender or gas mixer.

Inside the Heated Sample Line: Why Temperature Integrity Matters in Gas Analysis
In the realm of precise gas analysis, particularly within continuous emissions monitoring systems (CEMS) and process control applications, the integrity of the sample from the point of extraction to the analyser is paramount. A critical, yet often underestimated, component in maintaining this integrity is the heated sample line. Failure to