Thermal fluid sensor

A fluid sensor for sensing a concentration or composition of a fluid, the sensor comprising:

• a semiconductor substrate comprising at least one etched portion;
• a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises a first dielectric membrane located over the first etched portion of the semiconductor substrate;
• a first heating element located within the first dielectric membrane;
• a second heating element;
• a first temperature sensing element positioned with the first heating element;
• and
• a second temperature sensing element positioned with the second heating element;
• wherein the first heating element is arranged to thermally shield the first and second temperature sensing elements and the second heating element from ambient temperature changes;
• wherein the first heating element is configured to operate in a constant temperature or constant resistance mode;
• and
• wherein the second heating element is configured to operate in a constant current or constant voltage mode or constant power wherein a separation between the second temperature sensing element and the first temperature sensing element introduces a temperature difference between the second temperature sensing element and the first temperature sensing element, such that a differential signal between the first temperature sensing element and the second temperature sensing element is indicative of the concentration or composition of the fluid based on a thermal conductivity of the fluid.