The measurement of oxygen inside a rotating kiln provides a good indication of combustion efficiency. This oxygen measurement can also provide an inferred indication of calcining rates and the production of thermal NOx. Maintaining a consistent oxygen level can help prevent variations in product quality. Rosemount(TM) in situ oxygen analyzers place the sensing cell at the end of a long probe, and use diffusion pathways to allow flue gas to flow from the process to the oxygen cell.