Abstract: The thermal conductivity in a gas blend is measured in order to determine concentration using several different temperatures for the gas but the same sensor, which includes a Silicon carrier of a few hundred micrometers thick and being provided with an electrically insulating layer; carrying a sputtered on or vapor deposited, meandershaped thin film resistance; a pit in the carrier underneath the resistance and the insulating layer, there being one or more perforations in the insulation to permit access of gas to the interior of the pit; a cover plate made also of silicon and of comparable thickness dimensions as the carrier, and also having a pit of comparable dimensions as the first mentioned pit is disposed above the carrier so that said pits are aligned and constitute a common measuring chamber; and a diffusion channel in the cover plate for feeding a gas through diffusion so that the measuring chamber is defined by the communicating pits.