Abstract: An ATR sensor has a housing and sensor components in the housing including an electromagnetic radiation source for emitting a predefined wavelength range, a reflection body permeable for radiation and contactable with a fluid to be evaluated, and a detector for the reflected radiation as well as other members for conducting the measurement and for operation. The sensor is particularly suited for measuring a CO2 content of a fluid. Before sealing the housing in a gas-tight manner, in particular for avoiding a measurement value drift and for keeping the radiation paths free of CO2, a CO2 getter material is introduced into the housing and/or into a receiving space of an additional housing that communicates with the interior space of the housing before sealing the interior space and/or the receiving space.