Thermal sensors are highly media-dependent. If pressure, temperature, or the composition of the medium changes, then the sensor signal changes. This dependency can be utilized to determine the thermal properties, such as thermal conductivity, heat conductivity, specific thermal capacity, and density. Thus with thermal sensors, the composition of a gas can also be determined.
Gases differ in their thermal properties. If just one component within a gas mixture changes, inferences concerning its concentration can be made via the thermal variables. For measurement, a microchip was developed with self-supporting silicon micro-wires that are freely suspended as microscopic bars in the headspace to be measured. A middle wire is configured as heater, two detector wires at different distances to the middle wire are configured as temperature sensors. At sinusoidal heat output, a sinusoidal progression of the sensor signals occurs, which is highly dependent on the thermal properties of the gas that surrounds the wires. The thermal transfer takes place via the unknown heat transfers from the heater into the gas to be analyzed and from the gas into the sensor. By measuring the temperature of the heater with two identical sensors at different distances to the heater, the unknown heat transfers can be eliminated in the measuring arrangement.
The transmitted sine wave and the received sine wave are compared for the evaluation. With calibration of the signal via the phase shift between heater and the detectors, the CO2 content in air can be resolved with 0.2 vol%. Because gases are compressible and change their density through pressure and temperature, the corresponding drifts are compensated. Through evaluation of other measured variables that the sensor provides, thermal conductivity, temperature conductivity, and if the density of the gas is known, also the specific heat output, can be determined – a possible method for analyzing even unknown gas mixtures.