A fluid sensor for performing a reference measurement includes a support structure having a top main surface region; a thermal emitter on the top main surface region of the support structure; a first waveguide section and a first thermal radiation detector on the top main surface region of the support structure; and a cover structure on at least one part of the first waveguide section. The first waveguide section guides a first portion of the thermal radiation emitted by the thermal emitter to the first thermal radiation detector. The first thermal radiation detector detects the guided first portion of the thermal radiation for performing the reference measurement.