A thermal sensor device for determining a concentration of a target gas in a gas sample that further comprises a disturbance gas, the thermal sensor device comprising: a first measurement structure comprising a first temperature sensor for determining a first temperature signal ( T 1), the first temperature signal ( T 1) being indicative of a temperature of the first measurement structure; a second measurement structure comprising a second temperature sensor for determining a second temperature signal ( T 2), the second temperature signal ( T 2) being indicative of a temperature of the second measurement structure; at least one heater element (11, 12; 31; 51) operable to cause heat transfer to the first and second measurement structures (1, 2) through the gas sample; and processing circuitry configured to provide heating power ( P 1, P 2; P 3; P 5) to the at least one heater element (11, 12; 31; 51) and to derive an output signal based on a response of the first and second temperature signals ( T 1, T 2) to the heating power ( P 1, P 2; P 3; P 5), the output signal being indicative of a concentration of the target gas in the gas sample, wherein the first and second measurement structures (1, 2) have different heat dissipation capabilities, wherein the processing circuitry is configured to derive the output signal based on a weighted difference of the first and second temperature signals ( T 1, T 2), and wherein the thermal sensor device comprises a substrate having an opening or recess, characterised in that the first measurement structure is a first measurement bridge that spans the opening or recess, the second measurement structure is a second measurement bridge that spans the opening or recess, and the first and second measurement bridges differ with respect to dimensions and/or structure such that the first and second measurement bridges (1, 2) have different heat dissipation capabilities. a first measurement structure comprising a first temperature sensor for determining a first temperature signal ( T 1), the first temperature signal ( T 1) being indicative of a temperature of the first measurement structure; a second measurement structure comprising a second temperature sensor for determining a second temperature signal ( T 2), the second temperature signal ( T 2) being indicative of a temperature of the second measurement structure; at least one heater element (11, 12; 31; 51) operable to cause heat transfer to the first and second measurement structures (1, 2) through the gas sample; and processing circuitry configured to provide heating power ( P 1, P 2; P 3; P 5) to the at least one heater element (11, 12; 31; 51) and to derive an output signal based on a response of the first and second temperature signals ( T 1, T 2) to the heating power ( P 1, P 2; P 3; P 5), the output signal being indicative of a concentration of the target gas in the gas sample, wherein the first and second measurement structures (1, 2) have different heat dissipation capabilities, wherein the processing circuitry is configured to derive the output signal based on a weighted difference of the first and second temperature signals ( T 1, T 2), and wherein the thermal sensor device comprises a substrate having an opening or recess, characterised in that the first measurement structure is a first measurement bridge that spans the opening or recess, the second measurement structure is a second measurement bridge that spans the opening or recess, and the first and second measurement bridges differ with respect to dimensions and/or structure such that the first and second measurement bridges (1, 2) have different heat dissipation capabilities.