Combination sensor for pressure sensing and determining the composition of a gas

One example discloses a combination sensor, comprising: a pressure sensor having an actuator which has a first resonant frequency; a cavity, coupled to the pressure sensor and able to receive a substance; wherein the cavity, in an absence of the substance, has a second resonant frequency in response to excitation by the actuator; wherein the cavity, in a presence of the substance, has a third resonant frequency in response to excitation by the actuator; wherein the first resonant frequency differs from the second and third resonant frequencies; and a sensor circuit which outputs a substance detected signal in response to the third resonant frequency in the cavity.