In the SITRANS TO500 multipoint temperature transmitter light with a wavelength from 1 500 to 1 600 nm is generated by means of a continuously adjustable laser and decoupled to the measuring lance. Fiber Bragg gratings (FBG) are mounted at defined measurement points on the measurement lances. Each FBG reflects light of a defined wavelength. The wavelength reflected by the FBG varies depending on the temperature. The reflection at the FBGs is thus a measurement of the temperature at the corresponding measuring point. A maximum of 48 FBGs per channel can be evaluated, depending on the temperature range.

A gas cell with fixed absorption line serves as a reference in the SITRANS TO500 and the wavelength determination is continuously adjusted by it.