A flow and thermal conductivity sensor comprising: a semiconductor substrate comprising an etched portion; a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises at least one dielectric membrane located over the etched portion of the semiconductor substrate; and a heating element located within the dielectric membrane, a pair of temperature sensing elements (6a, 6b) located on opposing sides of the heating element, wherein the pair of temperature sensing elements are located within the dielectric membrane; wherein the dielectric membrane comprises: at least one recessed region (7a, 7b) between the heating element and an edge of the dielectric membrane, a first region between the heating element and a first temperature sensing element of the pair of temperature sensing elements, wherein the first region comprises the at least one recessed region; a second region between the heating element and a second temperature sensing element of the pair of temperature sensing elements, wherein the second region comprises a continuous portion of the dielectric membrane between the heating element and the second temperature sensing element; and wherein the at least one recessed region is arranged to be asymmetrical about an axis defined by the heating element to thereby introduce a temperature difference between the first temperature sensing element of the pair of temperature sensing elements and the second temperature sensing element of the pair of temperature sensing elements, wherein a differential signal between the pair of temperature sensing elements is configured to determine a property, composition or concentration of a fluid based on a thermal conductivity of the fluid. a semiconductor substrate comprising an etched portion; a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises at least one dielectric membrane located over the etched portion of the semiconductor substrate; and a heating element located within the dielectric membrane, a pair of temperature sensing elements (6a, 6b) located on opposing sides of the heating element, wherein the pair of temperature sensing elements are located within the dielectric membrane; wherein the dielectric membrane comprises: at least one recessed region (7a, 7b) between the heating element and an edge of the dielectric membrane, a first region between the heating element and a first temperature sensing element of the pair of temperature sensing elements, wherein the first region comprises the at least one recessed region; a second region between the heating element and a second temperature sensing element of the pair of temperature sensing elements, wherein the second region comprises a continuous portion of the dielectric membrane between the heating element and the second temperature sensing element; and at least one recessed region (7a, 7b) between the heating element and an edge of the dielectric membrane, a first region between the heating element and a first temperature sensing element of the pair of temperature sensing elements, wherein the first region comprises the at least one recessed region; a second region between the heating element and a second temperature sensing element of the pair of temperature sensing elements, wherein the second region comprises a continuous portion of the dielectric membrane between the heating element and the second temperature sensing element; and wherein the at least one recessed region is arranged to be asymmetrical about an axis defined by the heating element to thereby introduce a temperature difference between the first temperature sensing element of the pair of temperature sensing elements and the second temperature sensing element of the pair of temperature sensing elements, wherein a differential signal between the pair of temperature sensing elements is configured to determine a property, composition or concentration of a fluid based on a thermal conductivity of the fluid.